cond-mat.supr-con

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cond-mat.supr-con 2026-05-13 Recognition

Damping peaks below Tc from Andreev states in c-axis YBCO/LSMO

Anomalous spin-pumping behavior of half-metallic ferromagnet/d-wave superconductor heterostructures

Interface states dominate spin pumping when proximity to the half-metal locally suppresses the d-wave order parameter.

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Spin-pumping experiments in superconductor/ferromagnet heterostructures, which probe spin-sinking by the superconductor, have revealed a variety of complex behaviors. Most studies have focused on conventional s-wave superconductors combined with metallic or insulating ferromagnets. Here, we study a d-wave superconductor paired with a half-metallic ferromagnet, in epitaxial YBa2Cu3O7-d/La0.7Sr0.3MnO3 heterostructures with two crystalline orientations: one in which YBCO is c-axis oriented, and the other in which YBCO grows along the (103) direction. Using ferromagnetic resonance (FMR), we probe the temperature-dependent Gilbert damping coefficient {\alpha}. For (103) heterostructures, {\alpha}(T) initially decreases below Tc, but then increases at lower temperatures, exceeding normal-state levels. This behavior can be understood in terms of the opening of the superconducting gap and spin transport via nodal quasiparticles, which dominate when the ab-plane of YBCO is exposed at the interface. In stark contrast, c-axis heterostructures exhibit a pronounced enhancement of {\alpha}(T) below Tc, peaking at 0.65-0.7Tc before decaying. This anomaly suggests the dominance of interface-bound Andreev states, arising from a locally suppressed superconducting order parameter due to proximity effects with the half-metallic LSMO.

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cond-mat.supr-con 2026-05-13 2 theorems

Phase slips destroy long-range order in FeSe

Phase-slip residual-order spin state in FeSe

A narrow manifold of defects in a stripe background reproduces both observed spin-fluctuation types.

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Clarifying the magnetic ground state is essential for analysing unconventional superconductivity, because microscopic magnetic order provides one of the basic starting assumptions for spin-fluctuation pairing theories. FeSe exhibits pronounced stripe- and Neel-type spin fluctuations yet lacks static long-range order, posing a long-standing puzzle. By combining PBE and r2SCAN mixed exchange-correlation calculations with spectrally weighted simulations of the static spin structure factor S(q), we show that FeSe is not governed by a single magnetic configuration but by a nearly degenerate manifold of phase-slip defects embedded in a stripe background. We term this state a residual-order spin state (ROSS): a spin state that retains local stripe-like antiferromagnetic correlations but loses long-range phase coherence because of phase slips. Multiple slip configurations are compressed into an exceptionally narrow energy window. Non-local magneto-elastic coupling redistributes domain-wall formation energy through the lattice, whereas competing magnetic interactions truncate the real-space coherence length to an optimal scale of about ten moments. Weighted superpositions of these slip states reproduce the momentum-space line shapes of both stripe- and Neel-type spin fluctuations observed by inelastic neutron scattering, providing a microscopic basis for superconductivity models built on spin fluctuations.

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cond-mat.supr-con 2026-05-13 2 theorems

LaFeAsO shows mismatched nematic Weiss temperatures

Nematicity in LaFeAsO single crystals studied by elastoresistance, high-resolution thermal expansion and shear-modulus measurements

Elastoresistivity and shear modulus yield different divergence points, unlike in BaFe2As2 and contrary to Landau theory.

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Nematicity in LaFeAsO single crystals is studied by means of high-resolution thermal expansion, shear modulus, and elastoresistivity measurements. A softening of the shear modulus $C_{\rm 66}$ towards the structural phase transition at $T_{\rm S}$ is observed. In addition, a similar Curie-Weiss-like divergence of the nematic susceptibilities is found in the temperature dependence of both $\chi^{sh}$ and $\chi^{er}$, which are deduced from the shear modulus (sh) and the elastoresistivity (er) studies, respectively. These observations provide evidence for an electronic origin of nematicity in LaFeAsO. The characteristic energy of the coupling between the lattice and the electronic degrees of freedom is deduced to $\sim$30~K. The comparison to corresponding measurements on BaFe$_2$As$_2$ single crystals reveals a very similar temperature dependence of the shear modulus but yields contrasting results for $\chi^{er}$ : In BaFe$_2$As$_2$, $\chi^{er}$ diverges similarly as the uncoupled nematicity deduced from the shear modulus data as it is expected from the underlying Landau theory. In contrast, the Weiss temperatures of $\chi^{er}$ and $\chi^{sh}$ are significantly different in LaFeAsO. This difference is at odds with the commonly anticipated theories of resistivity anisotropy and electronic nematicity in iron pnictides.

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cond-mat.supr-con 2026-05-13 2 theorems

Superconducting pairings induce Euler topology on honeycomb lattices

Euler Topology in Superconducting Honeycomb Lattices

s-wave and f-wave states produce mirror-protected helical modes at domain walls and non-Abelian Dirac-node braiding under anisotropy.

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Electronic bands in systems with space-time inversion (IST) symmetry can host nontrivial Euler topology. Here, we investigate the band topology of IST-symmetric superconducting honeycomb lattices and demonstrate that s-wave spin-singlet (SWSS) and f-wave spin-triplet (FWST) superconducting pairings give rise to valley-Euler and Euler superconductors, respectively. We find that Euler topology in both pairing states gives rise to mirror-symmetry-protected helical domain-wall modes. Furthermore, we show that Euler topology in the FWST state induces non-Abelian braiding of Dirac nodes in momentum space when anisotropic hopping is introduced. Our work establishes superconducting electronic instabilities as a natural route to realizing nontrivial Euler band topology in Dirac materials.

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cond-mat.supr-con 2026-05-13 1 theorem

Strain stabilizes superconductivity in nickelate films at ambient pressure

Experimental Progress in Ambient-Pressure Superconducting Bilayer Nickelate Films

Bilayer films now permit transport, spectroscopy, and device studies without high-pressure setups.

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Bilayer Ruddlesden-Popper nickelates display superconductivity near 80 K under high pressure, establishing a new nickelate platform for studying unconventional high-temperature superconductivity. The recent stabilization of superconducting RA3Ni2O7 (RA = rare earth or alkaline earth) films at ambient pressure has changed the experimental landscape: epitaxial strain can reproduce key structural ingredients of the high-pressure phase while making transport, spectroscopy, microscopy, and device-oriented measurements directly accessible. This Review summarizes the experimental progress on ambient-pressure superconducting bilayer nickelate films, with emphasis on synthesis routes, oxygen stoichiometry, substrate-induced strain, normal-state transport, superconducting properties, doping phase diagrams, and momentum-resolved electronic structure. We highlight several issues that remain unsettled, including the reproducibility of phase-pure ultrathin films, the microscopic origin of the two-step superconducting transition, the role of oxygen defects and substrate-derived doping, the position of the Ni 3dz2-derived {\gamma} band, and the pairing symmetry. We close by outlining experimental directions that could establish a more quantitative link among crystal structure, orbital reconstruction, and superconductivity in bilayer nickelate films.

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cond-mat.supr-con 2026-05-13 2 theorems

Superconductivity boosts CDW phase coherence in cuprates

Superconductivity Reinforces Charge-Density-Wave Phase Coherence across Cuprates

Across multiple families, phase coherence grows below Tc with no peak broadening and precise wave-vector locking, even in disordered samples

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For decades, superconductivity in high-Tc cuprates has been viewed as a competitor that suppresses charge-density-wave (CDW) order by reducing its amplitude and spatial extent. Here, we show that this picture is incomplete, as superconductivity is accompanied by a systematic enhancement of CDW phase coherence across multiple cuprate families. Using resonant soft x-ray scattering combined with a coherence-sensitive momentum-profile analysis, we uncover a BCS-like growth of phase coherence below Tc, which phenomenologically manifests as the absence of CDW peak broadening and near-perfect wave-vector locking. This enhancement remains visible even in a disorder-dominated regime created by long-term crystal aging and follows a common trend when compared with published data on Bi-, Hg-, Y-, and Nd-based cuprates. These results indicate that superconductivity reshapes CDW order in two distinct ways, suppressing its amplitude while strengthening its phase coherence, and reveal an additional phase-level interplay with lattice coupling in high-Tc cuprates.

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cond-mat.supr-con 2026-05-13 1 theorem

PtPb4 shows symmetry-broken superconductivity with Majorana-like zero modes

Discovery of a nonsymmorphic superconductor with spontaneous rotational symmetry breaking and nontrivial zero modes

Anisotropic resistivity and persistent vortex zero states in this nonsymmorphic compound suggest a new route to topological states.

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Topological superconductivity has attracted great interest due to its fundamental significance for realizing Majorana quasiparticles and fault-tolerant quantum computation. Nonsymmorphic superconductors, with symmetry-protected nontrivial electronic structures, offer a promising route to exotic topological superconducting states, yet experimental realizations remain scarce. Here we identify nonsymmorphic compound PtPb4 as a robust platform hosting superconductivity with spontaneous rotational symmetry breaking and nontrivial zero-energy modes. PtPb4 crystallizes in a frustrated Shastry-Sutherland lattice and exhibits nontrivial band topology. By combining in-plane and out-of-plane resistivity measurements, pronounced twofold anisotropy is observed in both the superconducting state and the upper critical field, evidencing spontaneous rotational symmetry breaking. Scanning tunneling microscopy/spectroscopy further reveal twofold-symmetric magnetic vortices, providing direct real-space evidence for the symmetry-broken superconducting state. Notably, a robust zero-energy vortex bound state emerges and persists without spatial splitting over extended distances, consistent with the characteristics expected for Majorana bound state. These findings uncover an exotic superconducting state in PtPb4 and establish a promising platform for exploring topological superconductivity and superconducting quantum devices.

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cond-mat.supr-con 2026-05-12 2 theorems

Embedding method tracks superconducting proximity over hundreds of nanometers

First-principles real-space embedding theory of the superconducting proximity effect

Real-space dynamical embedding with normal and anomalous self-energies predicts spectral functions in mesoscopic heterostructures without厚sl

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When a superconductor is placed in contact with a normal material, Cooper pairs penetrate the latter and induce superconductivity via the proximity effect. Despite its central role in quantum materials, superconducting devices and topological platforms, a predictive first-principles description of the proximity effect at realistic interfaces has remained computationally prohibitive so far. Here, we fill this gap by developing a Green's-function framework based on real-space dynamical embedding that enables first-principles simulations of superconducting proximity in mesoscopic systems. We show that the proximity effect admits a transparent diagrammatic formulation in terms of normal and anomalous embedding self-energies, which disentangle and quantify the distinct renormalization mechanisms generated by coupling to a superconducting bath. By combining this formalism with recursive schemes, we compute local spectral functions and proximity lengths extending over hundreds of nanometers into the bulk without resorting to thick interface slabs. We deploy the approach on tight-binding models (Qi-Hughes-Zhang and Fu-Kane-Mele), where we analyze mixed-parity superconductivity in topological insulators proximitized by $s$-wave superconductors, and on first-principles simulations of NbSe$_2$/CrBr$_3$ heterostructures based on density-functional theory and maximally-localized Wannier functions, the latter enabling direct comparison with scanning tunneling spectroscopy experiments. Our work provides a scalable and conceptually unified framework that bridges microscopic electronic structure and mesoscale proximity physics, enabling predictive atomistic simulations of superconducting interfaces.

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cond-mat.supr-con 2026-05-12 Recognition

Tuned exchange fields yield Josephson diode effect with over 40% nonreciprocity

Anomalous and diode Josephson effect in junctions with inhomogeneous ferromagnetic barrier and interfacial Rashba spin-orbit coupling

Symmetry analysis and numerical CPR calculations show how orientations of ferromagnets, Rashba interfaces and d-wave lobes control anomalous

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We theoretically investigate the anomalous and diode Josephson effects in planar two-dimensional Josephson junctions with arbitrarily oriented exchange fields in two ferromagnets within the barrier, and spin-orbit coupling at the superconductor/ferromagnet interfaces, where the superconducting electrodes can have $s$-wave or arbitrarily oriented $d$-wave order parameter lobes. We perform a systematic symmetry analysis of the junction Hamiltonian and identify the minimal conditions for breaking time-reversal and space-inversion symmetries, which are required for the emergence of anomalous and diode Josephson effects. We classify the junctions into three classes, with particular attention to those between $d_{x^2-y^2}$ and $d_{xy}$ oriented superconductors. Our symmetry analysis is supported by numerical calculations of the current-phase relation (CPR) obtained using a generalized Furusaki-Tsukada (F-T) approach. By tuning the directions of exchange fields in the ferromagnets, Rashba SOC at the interfaces and superconducting order parameter orientations, nonreciprocity can be enhanced by more than 40\%. We further analyze the phase-dependent Andreev bound states (ABS) spectrum and their contribution to charge transport, as well as their signatures in the nonreciprocal transport characteristics. By comparing the current carried by ABS with that obtained using the F-T technique, we find that the contribution from continuum states above the gap becomes pronounced in presence of zero energy crossings in the ABS spectrum, and in junctions with $d$-wave superconducting electrodes due to the narrower superconducting gap, which may become closed. In the nonreciprocal regime, the ABS spectra show an asymmetric profile with respect to phase inversion, indicating the presence of a finite current at zero phase difference and unequal critical currents in opposite directions.

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cond-mat.supr-con 2026-05-12 1 theorem

Buffer order preserves 92 K Tc in freestanding GdBCO films

Freestanding GdBa2Cu3O7 Thin Films via Optimized Buffer Layer Design: Preserving Superconducting Properties

LaAlO3/SrTiO3 bilayer maintains epitaxial quality and transition temperature after lift-off, while reversed or single-layer stacks reduce Tc

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Freestanding GdBa2Cu3O7 (GdBCO) superconducting thin films were fabricated using a water-soluble Sr3Al2O6 (SAO) sacrificial layer in combination with thermal release tape. An amorphous Al2O3 capping layer was introduced to suppress crack formation during the lift-off process. The influence of buffer-layer design inserted between the GdBCO and SAO layers was systematically investigated with respect to structural integrity and superconducting properties after lift-off. A LaAlO3/SrTiO3 bilayer buffer was found to be essential for maintaining epitaxial growth and a superconducting transition temperature (Tc) of approximately 92 K after lift-off, comparable to that of the as-grown films. In contrast, a reversed SrTiO3/LaAlO3 bilayer and single-layer buffer structures led to a suppression of Tc, highlighting the critical role of stacking sequence. These results demonstrate that optimization of the buffer-layer design is a key factor for realizing high-quality freestanding GdBCO films while maintaining their superconducting characteristics.

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cond-mat.supr-con 2026-05-12 2 theorems

Strained superconductors produce magnetism perpendicular to the field

Transverse Magnetic Response from Orbitally Polarized Cooper Pairs in Elemental Superconductors

Symmetry reduction in vanadium and niobium activates equal-orbital-moment pairs whose response appears as transverse magnetization.

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We demonstrate how crystalline symmetry lowering, as for instance through strain, allows elemental superconductors such as vanadium and niobium to realize spin-singlet orbitally polarized Cooper pairs composed of electrons with identical orbital moments. Using superconducting density functional theory, we show that lowering of trigonal symmetry to $C_s$, thus keeping only a single mirror plane, activates interorbital pairing in bulk and (111) surfaces, with a pronounced surface enhancement. In a magnetic field, the resulting orbitally polarized superconducting state leads to a novel transverse magnetic response. For in--plane field orientations that break the remaining mirror symmetry, a sizable orbital magnetization emerges perpendicular to the applied field. We show that this effect is a direct consequence of equal--orbital-moment Cooper pairing, providing an experimentally accessible signature of this state. Our results establish strained elemental superconductors as a minimal material platform for superconducting orbitronics.

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cond-mat.supr-con 2026-05-12 2 theorems

Confinement renormalizes coherence length in thin-film superconductors

Ginzburg-Landau Theory for Confined Thin-Film Superconductors

The effect, driven by density-of-states and Fermi-energy shifts, shortens the coherence length, lengthens the penetration depth, and matches

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We develop a Ginzburg--Landau theory for superconducting thin films under quantum confinement. Starting from the microscopic BCS free energy and the recently developed confinement theory of metallic thin films, explicit analytical expressions are derived for the Ginzburg--Landau coefficients, coherence length, penetration depth, electronic mean free path, and Ginzburg--Landau parameter in confined geometries. The central result is that quantum confinement directly renormalizes the intrinsic superconducting coherence length through confinement-induced modifications of the electronic density of states and Fermi energy. This effect is absent in conventional thin-film transport theories based solely on surface scattering. As a consequence, confinement simultaneously suppresses the coherence length and enhances the penetration depth, thereby driving superconductors toward progressively stronger type-II behavior with decreasing film thickness. The theory predicts a crossover regime in which confinement-induced renormalization of superconducting length scales and transport scattering become strongly intertwined. Comparison with recent penetration-depth measurements in Al thin films shows that the observed enhancement of the penetration depth originates from the interplay between confinement-induced renormalization of the coherence length and suppression of the effective mean free path by surface and disorder scattering. The results establish a direct connection between quantum confinement and superconducting electrodynamics in confined metallic films.

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cond-mat.supr-con 2026-05-12 2 theorems

Confinement shortens coherence length in superconducting thin films

Ginzburg-Landau Theory for Confined Thin-Film Superconductors

By altering density of states and Fermi energy, it drives films toward stronger type-II behavior and matches aluminum film measurements.

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We develop a Ginzburg--Landau theory for superconducting thin films under quantum confinement. Starting from the microscopic BCS free energy and the recently developed confinement theory of metallic thin films, explicit analytical expressions are derived for the Ginzburg--Landau coefficients, coherence length, penetration depth, electronic mean free path, and Ginzburg--Landau parameter in confined geometries. The central result is that quantum confinement directly renormalizes the intrinsic superconducting coherence length through confinement-induced modifications of the electronic density of states and Fermi energy. This effect is absent in conventional thin-film transport theories based solely on surface scattering. As a consequence, confinement simultaneously suppresses the coherence length and enhances the penetration depth, thereby driving superconductors toward progressively stronger type-II behavior with decreasing film thickness. The theory predicts a crossover regime in which confinement-induced renormalization of superconducting length scales and transport scattering become strongly intertwined. Comparison with recent penetration-depth measurements in Al thin films shows that the observed enhancement of the penetration depth originates from the interplay between confinement-induced renormalization of the coherence length and suppression of the effective mean free path by surface and disorder scattering. The results establish a direct connection between quantum confinement and superconducting electrodynamics in confined metallic films.

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cond-mat.supr-con 2026-05-12 Recognition

Bilayer phase resonance pinned at single-particle band splitting

Layer-antisymmetric pair-phase resonance at the bonding-antibonding splitting in the AA-stacked bilayer attractive Hubbard model

The layer-antisymmetric pole appears at 2t_h set by hybridization rather than Josephson coupling in the attractive Hubbard bilayer.

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The relative phase between the two pair condensates of a bilayer s-wave superconductor is a collective degree of freedom distinct from the usual in-phase Anderson-Bogoliubov mode. Working at the Gaussian fluctuation level for the AA-stacked attractive-Hubbard honeycomb bilayer, we show analytically that the layer-antisymmetric pair-phase channel hosts an in-gap collective pole at twice the single-particle interlayer hopping, $2t_h$, precisely the bonding-antibonding band splitting. The mechanism is algebraic: at this frequency, the antisymmetric phase bubble reduces pointwise in momentum space to the static symmetric phase bubble that enforces the in-phase Goldstone pole. The resulting resonance scale is therefore fixed by the single-particle hybridization, rather than by the interaction-driven Josephson coupling that controls the canonical Leggett mode. The identity is verified numerically by direct Bogoliubov-de Gennes calculations. The diagonal antisymmetric phase-channel kernel zero is exact within Gaussian theory at any chemical potential; the full coupled amplitude-phase pole coincides with it at half filling and tracks it closely away from half filling. The excitation is Raman-forbidden by inversion, which motivates layer-odd probes. We find that a layer-imbalance drive has finite Gaussian-level overlap with the pair-phase sector, suggesting a possible cold-atom layer-bias response feature near the sub-kilohertz scale for typical optical-lattice parameters.

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cond-mat.supr-con 2026-05-12 2 theorems

Antisymmetric pair-phase resonance pinned at bonding-antibonding energy

Layer-antisymmetric pair-phase resonance at the bonding-antibonding splitting in the AA-stacked bilayer attractive Hubbard model

The collective mode energy is fixed by single-particle interlayer hopping instead of Josephson coupling.

abstract click to expand

The relative phase between the two pair condensates of a bilayer s-wave superconductor is a collective degree of freedom distinct from the usual in-phase Anderson-Bogoliubov mode. Working at the Gaussian fluctuation level for the AA-stacked attractive-Hubbard honeycomb bilayer, we show analytically that the layer-antisymmetric pair-phase channel hosts an in-gap collective pole at twice the single-particle interlayer hopping, $2t_h$, precisely the bonding-antibonding band splitting. The mechanism is algebraic: at this frequency, the antisymmetric phase bubble reduces pointwise in momentum space to the static symmetric phase bubble that enforces the in-phase Goldstone pole. The resulting resonance scale is therefore fixed by the single-particle hybridization, rather than by the interaction-driven Josephson coupling that controls the canonical Leggett mode. The identity is verified numerically by direct Bogoliubov-de Gennes calculations. The diagonal antisymmetric phase-channel kernel zero is exact within Gaussian theory at any chemical potential; the full coupled amplitude-phase pole coincides with it at half filling and tracks it closely away from half filling. The excitation is Raman-forbidden by inversion, which motivates layer-odd probes. We find that a layer-imbalance drive has finite Gaussian-level overlap with the pair-phase sector, suggesting a possible cold-atom layer-bias response feature near the sub-kilohertz scale for typical optical-lattice parameters.

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cond-mat.supr-con 2026-05-12 2 theorems

Anisotropy creates apparent double Tc from single BKT transition

Apparent double-T_c from a single BKT transition in anisotropic phase-only models

Linear resistance curves look split under common analysis methods, yet scaling diagnostics confirm only one underlying transition.

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Transport experiments on two-dimensional superconductors often yield direction-dependent transition temperatures, raising the question of whether such a ``double-$T_c$'' reflects a true thermodynamic splitting or a transport artifact. To establish a baseline, we study a minimal anisotropic phase-only Josephson-junction array in equilibrium and under resistively shunted junction dynamics with fluctuating twist boundary conditions. The equilibrium model exhibits a single Berezinskii--Kosterlitz--Thouless (BKT) transition. Out of equilibrium, anisotropic Josephson couplings and anisotropic dissipation reshape the linear $R$--$T$ curves in a finite-size, finite-current crossover regime, so that curve-shape criteria such as Halperin--Nelson fits and fixed-resistance thresholds yield an apparent double-$T_c$. In contrast, critical-scaling criteria -- the universal exponent $\alpha=3$ and dynamic finite-size scaling -- remain consistent with the single $T_{\mathrm{BKT}}$. A robust splitting that persists in the nonlinear critical scaling, such as that recently reported at KTaO$_3$ interfaces, therefore points to physics beyond this clean anisotropic baseline.

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cond-mat.supr-con 2026-05-12 2 theorems

Circular supercurrents link distant ferromagnets in superconductors

Superconductivity Mediated Long Range Magnetic Coupling

Ferromagnetic insulators on a Rashba film produce power-law magnetic interactions that can be ferromagnetic when static.

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We study a Rashba superconductor thin film with ferromagnetic insulators (FIs) placed on top of it. We show that the ferromagnetic insulators generate circular super-currents, enabling long-range magnetic interactions (LRMI), decaying in power laws. In the static case, the long-range magnetic interaction can be ferromagnetic, in contrast to previous studies showing that superconductor mediates anti-ferromagnetic interactions decaying exponentially. Surprisingly, we find that in the dynamic case, the LRMI has a different distance dependence. Our results have potential applications in superconducting spintronics.

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cond-mat.supr-con 2026-05-11 1 theorem

Hole doping suppresses magnetism in C136 schwarzite

Hole-Doping Suppresses Competing Magnetism in High-DOS C136 Carbon Schwarzite: A Computational Route Toward Superconductivity in Negative-Curvature Carbon Networks

Magnetization falls to 4.76 Bohr magnetons per cell while density of states stays high at 44.7 states/eV/cell, identifying a route for later

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Carbon schwarzites are negative-curvature carbon networks with electronic structures distinct from graphene, fullerenes, and conventional carbon allotropes. Here we report a spin-polarized first-principles screening study of D-type C136 carbon schwarzite focused on the competition between magnetism, doping, and high-DOS metallic behavior. Neutral C136 has a robust competing magnetic branch, with total magnetization of about 11.01-11.03 Bohr magnetons per 136-atom cell. Charged-cell calculations reveal a clear electron-hole asymmetry: adding two electrons per cell increases the total magnetization to 12.11 Bohr magnetons per cell, while removing two electrons reduces it to 9.61. Further hole doping suppresses the magnetic branch monotonically, giving 8.02, 6.34, and 4.76 Bohr magnetons per cell for removal of 4, 6, and 8 electrons, respectively. The most strongly hole-doped point, h8, was examined with spin-polarized NSCF and density-of-states calculations on a 4x4x4 k-point mesh. The NSCF Fermi energy, -0.7414 eV, agrees with the SCF value, -0.7413 eV. The DOS remains high near the Fermi level: at E = -0.740 eV, the total DOS is about 44.69 states/eV/cell, with DOS_up = 33.11 and DOS_down = 11.58 states/eV/cell. Thus h8 combines substantial suppression of the competing magnetic branch with preservation of a high-DOS metallic state. We do not claim superconductivity in C136. Instead, these calculations identify hole doping as a route for suppressing a competing magnetic instability while preserving electronic conditions relevant for further superconductivity screening. Lattice stability, electron-phonon coupling, and transition-temperature estimates remain open problems.

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cond-mat.supr-con 2026-05-11 2 theorems

Model connects resonator quality factor to temperature and power

Quasiparticle Quality Factors in Superconducting Resonators: Effects of Bath Temperature and Readout Power

Power-dependent phonon term in Rothwarf-Taylor equations fits NbN data and marks shift between thermal and microwave loss regimes.

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The performance of superconducting resonators underpins a wide range of modern quantum technologies, yet their quality factor often deviates at low temperatures from standard Mattis-Bardeen predictions. This discrepancy is often attributed to nonthermal quasiparticles generated by microwave readout power, which limits the sensitivity of superconducting devices. We present a macroscopic model based on modified Rothwarf-Taylor equations that incorporates a power-dependent phonon generation term, providing an explicit relationship between quality factor, bath temperature and readout power. The model shows excellent agreement with temperature sweep measurements of NbN microstrip resonators with \b{eta}-Ta terminations over a wide dynamic range of readout power levels, accurately capturing the transition between thermally-dominated and microwave-induced loss regimes. This framework provides a predictive tool for optimizing superconducting resonators and advancing the design of high-Q devices for quantum sensing and quantum information processing.

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cond-mat.supr-con 2026-05-11 2 theorems

Width optimization preserves SNSPD efficiency in magnetic fields

Geometry-enabled magnetic resilience in superconducting nanowire single-photon detectors

Saturating intrinsic detection efficiency achieved for wide photon energy range under practical fields

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While magnetic fields and superconductors are both central to classical and quantum technologies, their combined use is often challenging, as magnetic fields significantly affect superconducting device performance. In superconducting nanowire single-photon detectors (SNSPDs), magnetic fields drastically reduce detection efficiencies, hampering their application in magnetically-active classical and quantum photonics. Here, we systematically characterize the performance of NbTiN SNSPDs under magnetic fields and show the enhancement of their intrinsic detection efficiency (IDE) at lower bias currents and its suppression at higher currents. This leads to SNSPD performance degradation through reduced or disappearing saturation plateaus. We show that the magnitude of this degradation is highly dependent on nanowire width and demonstrate width-optimized SNSPDs with saturating IDE for a wide range of photon energies under application-relevant magnetic fields. Minimizing degradation in superconducting devices under magnetic fields enables applications like detector-integrated spin-optic and atomic quantum processors, high-sensitivity magnetometry, and quantum transduction.

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cond-mat.supr-con 2026-05-11 Recognition

Altermagnets stabilize pair-density-wave superconductivity without fields

Finite temperature pair density wave superconductivity in d-wave altermagnets

Momentum-dependent spin splitting promotes finite-momentum pairing that survives thermal fluctuations over a temperature range.

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We demonstrate that altermagnetism provides a field-free mechanism for stabilizing finite-momentum superconductivity in two dimensions. Using a non-perturbative static path approximation Monte Carlo approach, we show that a d-wave altermagnet supports a robust pair-density-wave (PDW) phase that persists over a finite temperature window despite strong thermal fluctuations. The underlying mechanism originates from momentum-dependent spin splitting, which effectively enhances pairing instabilities at finite center-of-mass momentum without Zeeman fields. We identify distinct thermal scales associated with phase coherence, gap closing, and pseudogap formation, and establish characteristic spectroscopic and real-space signatures of the PDW state. Our results reveal altermagnetism as a robust route to thermally stable finite-momentum superconductivity and provide experimentally testable signatures for altermagnetic materials.

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cond-mat.supr-con 2026-05-11 2 theorems

Chain intercalation breaks Tc-spacing trade-off in TaS2

Breaking the Trade-off: Bulk 2D Ising Superconductivity with High Tc and Giant Interlayer Spacing via a Unique Chain Intercalation in (BaS)1/3TaS2

Ba-S-S-Ba chains create 12.75 angstrom gaps between bilayers while enabling enhanced transition temperature and Ising superconductivity.

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Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising platforms for low dimensional superconductivity. However, in conventional intercalated systems, achieving a high superconducting transition temperature (Tc) often comes at the expense of reduced interlayer spacing and weakened 2D character. Here, we overcome this long-standing compromise through a unique chain-like intercalation strategy. We report the synthesis and properties of a new polymorph, (BaS)1/3TaS2, in which a distinctive Ba-S-S-Ba chain structure is inserted between TaS2 bilayers. This unique configuration breaks the bulk c axis mirror symmetry while achieving exceptional interlayer decoupling, with an inter-bilayer spacing of 12.75 {\AA}-more than three times that of pristine 2H-TaS2. By suppressing interlayer electronic coupling, this structural evolution allows local inversion symmetry breaking within individual TaS2 layers to dominate. This prevents compensation of the Ising spin-orbit fields typical of centrosymmetric bulk phases, enabling robust 2D Ising superconductivity. Remarkably, the compound exhibits an enhanced Tc without sacrificing its large interlayer spacing, thereby breaking the conventional trade-off between large spacing/high anisotropy and high Tc. Comprehensive transport, magnetic, and thermodynamic measurements confirm its robust superconducting state. Our work establishes a versatile intercalation framework for designing bulk-like 2D Ising superconductors, providing a new route to reconcile competing material demands and expanding the scope of Ising superconductivity research.

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cond-mat.supr-con 2026-05-08

Spin exchange scattering shapes thin LaBi2 superconductivity

Pair-Breaking and Dimensionality in Spin-Orbit Coupled Superconductors

Field enhancement in the thin limit isolates its contribution alongside orbital and paramagnetic effects.

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The response of ultra-thin superconducting materials under parallel magnetic fields is often leveraged to obtain insight into the nature of the condensate, including features attributable to unconventional forms of pairing. Despite there being multiple competing mechanisms responsible for suppressing superconductivity, it is common for these analyses to overlook certain depairing channels. Here we report an analysis of thickness dependent superconductivity in thin films of \ce{LaBi2} using the multi-mechanism Kharitonov-Feigel'man framework . By resolving field-enhanced superconductivity in the thin-limit, we obtain an estimate the role of spin exchange scattering, in addition to paramagnetic and orbital effects. Our analyses offer insight into how fundamental quantities such as the critical temperature as well as Pauli limit are defined, recasting the landscape for how scattering times in two-dimensional superconductors can be interpreted.

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cond-mat.supr-con 2026-05-08

Zero-bias conductance saturates in deep Josephson contacts

Josephson spectroscopy study of kagome superconductors toward the deep point-contact regime

Kagome superconductor experiments attribute the effect to series resistance and caution its use in Majorana studies at low resistances.

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Josephson scanning tunneling microscopy (JSTM) has emerged as an important technique for probing the superconducting order parameter at the atomic scale. However, the Josephson current in JSTM may behave quite differently when the coupling strength varies. Here, we push the junction to the deep point-contact regime, reaching a normal-state junction resistance of only 0.15 $h/2e^2 \simeq 2~{\rm k}\Omega$. We demonstrate, using kagome superconductors, that the zero-bias conductance, a key characteristic of the Josephson current, deviates strongly from the quadratic dependence on the normal-state conductance upon entering the deep point-contact regime. Furthermore, we observe a striking saturation of the zero-bias conductance, which we show arises from the series resistance in the circuit. This also serves as a cautious reminder when interpreting zero-bias conductance saturation or quantization in studies of exotic physics such as that of Majorana zero modes if the tip-sample junction resistance is extremely small. Finally, we identify an optimum regime where JSTM can be used as an atomic-scale probe for studying pair-density wave states in materials with low superconducting transition temperature, such as AV3Sb5 kagome superconductors.

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cond-mat.supr-con 2026-05-08

Independent feeding cuts AC losses 90% in hybrid superconducting cables

Massive Mitigation of Transport AC Losses in Superconducting Hybrid CORC-TSTC Cables

Decoupling CORC and TSTC at the injection point blocks current sharing and stabilises waveforms at operating levels

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High-current superconducting cables are emerging as key enablers for next-generation power transmission systems; however, their deployment is often limited by transport AC losses. Hybrid superconducting cables combining Conductor-on-Round-Core (CORC) and Twisted Stacked-Tape Conductor (TSTC) architectures have recently been proposed as a promising route toward cables with high current capacity and compact form factors. However, their electrodynamic response under transport current operation remains poorly understood, particularly regarding how current injection conditions govern internal current redistribution. Here, we employ a fully-3D electromagnetic model, previously validated against magnetisation experiments in equivalent cables, to investigate the influence of current injection strategy on the electrodynamics of hybrid CORC-TSTC cables under self-field conditions. By comparing configurations in which the total current is either injected through a common connection between the CORC and TSTC conductors (non-insulated feeding) or supplied independently to each conductor (insulated feeding), we show that electrical coupling in non-insulated designs leads to strong current redistribution, pronounced waveform distortion and elevated AC losses once the CORC layers approach magnetic saturation. In contrast, independent current feeding suppresses inter-conductor current exchange, stabilises the current waveforms, and exhibits an outstanding reduction in transport AC losses of up to 90% at practical operating currents, compared with conventional feeding schemes. These findings reveal the central role of the current injection strategy in governing the internal electrodynamics and energy dissipation of hybrid superconducting cables, and identify the electrical decoupling of the constituent conductors at the feeding point as a simple and scalable route toward ultra-efficient power cables.

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cond-mat.supr-con 2026-05-08

Proton irradiation boosts critical current fivefold in Hg1223 crystals

Enhancement of J_c by Proton Irradiation in HgBa₂Ca₂Cu₃O₈_+_δ Single Crystals

Optimal dose of 1×10^16 cm^{-2} raises 2 K self-field Jc from 5.5 to 26 MA/cm² and keeps it above 0.1 MA/cm² at 77 K.

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Critical current density is the key parameter for the practical application of superconductivity. In this study, 3 MeV proton irradiation experiments were conducted on HgBa$_2$Ca$_2$Cu$_3$O$_8$$_+$$_\delta$ single crystals to introduce pinning centers. The critical current density is found to be strongly enhanced after the irradiation with its maximum at a dose of 1$\times$10$^{16}$/cm$^2$, where the self-field critical current density at 2 K is enhanced from 5.5 MA/cm$^2$ to 26 MA/cm$^2$. At 77 K, the self-field critical current density for all irradiated crystals is over 0.1 MA/cm$^2$. The power-law dependence of the critical current density on the magnetic field is observed after irradiation, with a large power-law exponent $\alpha$ close to 1. A monotonic magnetic field dependence of the normalized magnetic relaxation rate is observed, which could be attributed to the low irreversibility field caused by the large anisotropy in Hg1223 single crystals. Through the analysis of the pinning force density of the crystal before and after irradiation, a clear mechanism change has been observed.

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cond-mat.supr-con 2026-05-07

Nematic fluctuations produce unconventional collective-mode dispersion

Superconductivity mediated by nematic fluctuations -- the dispersion of collective modes

Pair susceptibility poles differ from BCS, altering phase and amplitude modes even for s+- symmetry.

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We analyze the spectrum of collective modes in a superconductor in which pairing is mediated by long-range nematic fluctuations. Previous experimental and theoretical studies have found that the superconducting gap in such a system is highly anisotropic and, at any finite $T<T_c$, vanishes on four arcs of the Fermi surface, even when the pairing symmetry is $s$ wave ($s^{+-}$ between hole and electron pockets). We derive the expression for the pair susceptibility $\chi(\mathbf{q},\Omega)$ at finite momentum $\mathbf{q}$ and frequency $\Omega$ deep in the superconducting phase. We analyze the spectral function, $\operatorname{Im}\chi(\mathbf{q},\Omega)$, and its pole structure in the transverse (phase) and longitudinal (amplitude) channels, and compare the results with those of a conventional $s$-wave superconductor. We find that the analytic structure of the pair susceptibility in both channels is qualitatively distinct from that in a BCS superconductor. This gives rise to a highly unconventional dispersion of phase and amplitude collective modes.

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cond-mat.supr-con 2026-05-07

Response tensor captures non-reciprocal Josephson currents under magnetic fields

Response tensor for the superconducting (Josephson) diode effect

The tensor links the dipole of critical-current angle dependence to field direction and flags nematic order inside the superconducting state

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We propose a response tensor $\mathbf{\hat \chi}$ to characterize the non-reciprocal critical current response of the superconducting (Josephson) diode effect. It describes the coupling between the dipole component of the angular distribution of the critical current and the applied magnetic field -- an analogue to the Hall response in the normal state. In quasi-2D systems with Rashba spin-orbit coupling and point group symmetries $C_{3v}$, $C_{4v}$ or $C_{6v}$, this tensor takes a fully antisymmetric form. When nematicity is present, a symmetric contribution emerges, providing an indicator of the nematic order in the superconducting state. In contrast, for systems exhibiting Dresselhaus spin-orbit coupling with the $D_{2d}$ symmetry, the tensor becomes diagonal traceless, and nematicity brings in a trace part. Our analysis not only accounts for the superconducting diode effect under external applied or intrinsic effective magnetic fields, but also predicts the symmetry conditions for realizing the diode effect when the magnetic field is aligned with the current. Beyond this, the proposed tensor provides a promising tool for detecting nematicity and potential nematic transitions deep within the superconducting phase. It may also encode additional information about the underlying electronic structure and symmetry-breaking orders, warranting further experimental investigation.

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cond-mat.supr-con 2026-05-07

Quasiperiodicity induces new transition in coupled Kitaev chains

Nonlocal transport phenomena in coupled quasiperiodic Kitaev chains

Coupled Fibonacci Kitaev chains display Majorana edge modes and lead-dependent conductance with a phase boundary set by the quasiperiodic p

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We investigate the topological phases in a coupled one-dimensional p-wave superconducting Fibonacci quasicrystal modeled by the quasiperiodic Kitaev chain. Recent studies have shown that the coupled system can host topological edge modes with Majorana fermions and enhance their topological protection, depending on the pattern of quasiperiodicity. In this work, we elucidate the topological phases of the coupled system and demonstrate the dependence of differential conductance on the lead connecting pattern employing the Keldysh formalism and the recursive Green's function method. Our findings reveal the emergence of topological phases in the coupled system, which are characterized by the presence of Majorana edge modes and the seepage of the Majorana wave function. Furthermore, we identify a new topological phase transition induced by quasiperiodicity in the coupled system.

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cond-mat.supr-con 2026-05-07

Two models differ on gap symmetry in moiré WSe2 superconductor

Superconductivity in moir\'e transition metal dichalcogenide bilayers: comparison of two distinct theoretical approaches

Negative-U Hubbard yields isotropic s-wave while t-J-U allows unconventional pairing after Coulomb renormalization.

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Superconductivity has recently been observed in moir\'e transition-metal dichalcogenide bilayers. Here, we investigate the superconducting state in twisted WSe$_2$ using two complementary theoretical approaches. The first is based on the negative $U$-Hubbard model and represents a relatively conventional pairing scenario, in which strong electron-electron repulsion does not directly affect the paired state and an isotropic $s$-$wave$ gap emerges. The second approach employs the $t$-$J$-$U$ model, allowing for unconventional gap symmetries and incorporating strong correlation effects via substantial renormalization induced by Coulomb repulsion. We compare the key properties of the superconducting states obtained within these two frameworks and discuss their implications in light of available experimental observations.

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cond-mat.supr-con 2026-05-07

Oxygen content sets Tc and Hc2 in La3Ni2O7+δ

Regulating oxygen content and superconductivity in La₃Ni₂O_{7+δ}

Systematic tuning isolates phases and directly modulates bilayer upper critical field under pressure.

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The synthesis of high-quality Ruddlesden-Popper (RP) nickelates remains challenging due to variations in oxygen content and the prevalence of intergrown RP phases. Precisely controlling the stoichiometry and characterizing the resulting physical properties are essential for understanding the mechanism of high-$T_c$ superconductivity in these materials. In this work, we synthesize a series of La$_3$Ni$_2$O$_{7+\delta}$ samples with systematically controlled oxygen content and perform comprehensive structural and compositional analyses. Precise oxygen tuning enables us to tailor the microstructure, yielding a pure bilayer phase, a mixture of bilayer and hybrid single-layer-bilayer phases, and a predominantly bilayer phase containing trilayer intergrowths. High-pressure transport measurements reveal distinct superconducting transitions with contrasting $T_c$ values, corresponding to the bilayer phase, the hybrid phase, and trilayer inclusions. Notably, we find that oxygen content not only governs the phase purity$-$i.e., the presence of intergrowth phases$-$but also directly modulates the upper critical field ($H_{c2}$) of the bilayer superconductivity. By establishing a phase diagram of $T_c$ and $H_{c2}$ as functions of oxygen content in La$_3$Ni$_2$O$_{7+\delta}$, this work advances synthetic control and provides new insights into the superconducting mechanism of RP nickelates.

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cond-mat.supr-con 2026-05-06

Majorana flat bands make uniform superconductivity unstable

Nonuniform superconducting states from Majorana flat bands

Topological superconductors instead adopt pair density waves or phase crystals that gap the zero-energy edge states, with the pair density波u

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Zero-energy flat bands within the superconducting gap can give rise to competing ordered phases. We investigate such phases in topological superconductors based on the magnetic adatom platform hosting a flat band of Majorana edge states. Our self-consistent calculations of the superconducting order parameter show the emergence of both a pair density wave with edge-localized amplitude modulations and a phase crystal characterized by edge-localized phase modulations. These two phases lower the free energy of the system by gapping out the Majorana flat band, as dictated by winding numbers, which are primarily tuned by the chemical potential. In fact, at zero temperature the uniform superconducting solution with Majorana flat band never survives and the phase diagram features a pair density wave, while the order parameter transitions into a phase crystal when amplitude modulations are insufficient to hybridize all the Majorana states. A broad intermediate region connects these two phases with comparable modulations in both amplitude and phase. At finite temperatures, the pair density wave survives up to around 80% of the bulk superconducting transition temperature, while the phase crystal only appears at lower temperatures and the intermediate region is strongly suppressed. Our findings establish the ubiquity of emergent nonuniform superconducting phases and their temperature-dependent behavior in topological superconductors.

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cond-mat.supr-con 2026-05-06

First-order transitions emerge in weak-ISOC superconductors

First-Order Transitions in Weak Ising Spin-Orbit-Coupled Superconductors

Free-energy analysis finds the true critical field while gap equations only locate supercooling, and spectra show two in-gap peaks.

abstract click to expand

Ising spin-orbit coupling (ISOC) can strongly protect superconductivity against exchange-field-induced depairing, typically leading to critical fields far exceeding the Pauli limit and continuous (second-order) phase transitions. Here, using a free-energy approach, we demonstrate that first-order transitions can emerge in superconductors with weak ISOC under large exchange fields. In this regime, conventional theoretical approaches based on the gap equation fail to determine the thermodynamic critical field and instead yield only the supercooling field. Moreover, we identify two pronounced in-gap coherence peaks in the quasiparticle spectra, which represent the weak-ISOC manifestation of the previously reported mirage-gap states. Our results establish the importance of free-energy analysis in describing the first-order phase transitions in Ising superconductors and reveal distinct spectroscopic signatures of the weak-ISOC regime.

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cond-mat.supr-con 2026-05-05

Cu-Sn method coats copper cavities with Nb3Sn for higher Q

Nb₃Sn Thin Films Using a Cu-Sn Route for Dark Matter Detection

Zero-field tests show 40% better performance than bare copper, offering a route to improved axion dark matter detectors.

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Axion dark matter searches require superconducting radio-frequency (SRF) cavities on copper (Cu) substrates with quality factors Q > 10^5 in multi-tesla magnetic fields. Copper reduces thermal noise and allows complex geometries. Nb3Sn is a strong candidate due to its superior superconducting properties. However, uniform high-Tc Nb3Sn thin films on Cu are challenging due to Sn loss and substrate strain. This work uses solid-state diffusion of Sn from high-Sn Cu-Sn alloys into Nb layers to form Nb3Sn at Cu-compatible temperatures (650-750{\deg}C), avoiding the traditional ~1100{\deg}C vapor method. Varying Cu-Sn composition yielded an optimal alloy that maintains high Sn activity. Compositional and thermal expansion analyses showed Tc is suppressed below 18 K by Cu substrate strain. Experiments on Nb and sapphire substrates isolated the strain effects. Two routes were developed: (1) Cu-Sn on Ta-coated Cu with hot Nb sputtering (Tc = 16 K), and (2) Nb on Ta/Cu with Cu-Sn evaporation and ex-situ reaction. Route 2 gave uniform Nb3Sn and was chosen for cavity coating. A hexagonal cavity combining designs from the University of Washington and Center for Axion and Precision Physics was coated using Route 2 and tested to 50 mK and 9 T. At zero field it reached Q = 77,000 (40% above bare Cu's Q = 55,000), but Q dropped sharply in field. Nb3Sn coatings on Cu cavities outperform bare Cu at zero field and provide practical routes for improved axion detectors.

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cond-mat.supr-con 2026-05-05

Berry curvature bounds Cooper pair size

Quantum Geometric Quadrupole of Cooper Pairs

Quantum geometry from bands sets a lower limit on pair length when dispersion vanishes, matching lengths seen in graphene.

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The size of Cooper pairs defines a fundamental length scale of superconductivity, conventionally set by band dispersion and the superconducting gap. This picture breaks down in flat bands, where quenched dispersion makes quantum geometry essential. Here we develop a general framework based on the Cooper pair quadrupole moment, whose trace gives the pair size. The framework holds for both dispersive and flat-band cases, and provides a unified description of the geometric origin of this length scale. In particular, when time-reversal symmetry is broken, Berry curvature enters through the phase structure of the pair wavefunction and gives an essential contribution absent from previous quantum-metric theories. Together, Berry curvature and quantum metric impose a geometric lower bound on the pair size. Applying this framework to rhombohedral graphene, we find that the Berry-curvature-induced contribution can dominate and yields pair sizes comparable to experimentally inferred coherence lengths. These results identify Berry curvature as a central geometric ingredient controlling the microscopic length scale of superconductivity.

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cond-mat.supr-con 2026-05-05

Vortices govern transport in Ni/Bi bilayers

Vortex Transport in Ni/Bi Bilayer Superconductor with Strong Spin-Orbit and Exchange Interaction

Near Tc, out-of-plane fields produce antisymmetric resistance from Magnus-viscous competition on isolated vortices, explained by bulk s-wave

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Nickel/bismuth (Ni/Bi) bilayers are a promising platform for exploring unconventional superconductivity. Ferromagnetic Ni is coupled to Bi, a strong spin orbit metal that only becomes superconducting below approx 10 mK, forming a bilayer exhibits superconductivity at a much higher temperatures, a Tc of 3 to 4 K. Such a bilayer thus makes an ideal system to probe Cooper pairing in strong spin orbit coupled magnetic environments. Magneto transport studies near Tc reveal the behavior of vortex dynamics and exchange proximity effects. It is seen that isolated vortices of the bilayers respond sensitively to out of plane fields, producing antisymmetric transverse resistance peaks attributable to competing Magnus and viscous forces. Control experiments using a ferromagnetic insulator confirm that superconductivity extends throughout the bilayer, not just confined at the interface. Overall, the results provide a unified picture of transport dominated by vortex dynamics and show that a conventional s wave order parameter accounts for the observations, with any likely unconventional contributions being only subtle.

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cond-mat.supr-con 2026-05-05

YBCO electrons pair by exchanging oxygen adsorption modes at interfaces

Interfacial charge-induced adsorption mode for electron pairing in high-temperature superconductors

Valence-flexible ions create charge layers that yield a coupling constant of 43.4 and a gap near 17 meV matching STM data.

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The electron pairing mechanism by the interfacial charge-induced adsorption mode of high-temperature superconductors is revealed. For the YBCO superconductors, the coupling of electrons and valence-flexible state of oxygen ions forms a charge-regulated interfacial layer induced by the adsorption potential, and electrons are paired by sharing the optimized interfacial structure and exchanging the adsorption mode, generating strong attraction to form Cooper pairs. Then the effective interaction potential between electrons is exactly derived in details, as well as the electron-adsorption-mode coupling strength, in which the adsorption coupling constant is up to 43.4. Furthermore, we verify that d-wave come from the anisotropy of interfacial adsorption forces, and explain the pseudo-energy gap behavior. By using the one-dimensional Ginzburg-Landau equation in the absence of a magnetic field, we obtain the coherence length expression, and the coherence length calculated is very close to the literature results. By establishing the energy gap equation, we obtain the superconducting gap , which is very close to the measured result of 17meV by Scanning Tunneling Microscopy/Spectroscopy. These quantitative predictions close to the known results could verify our theoretical framework.

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cond-mat.supr-con 2026-05-05

D-wave midgap peaks broaden to V-shapes under roughness unlike p-wave

Revisiting the surface density of states of midgap Andreev edge states

Two scattering modes in the d-wave flat band spread the zero-energy peak while a single mode keeps the p-wave peak intact.

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We revisit the effect of surface roughness on midgap Andreev edge states (MAES) in p- and d- wave superconductors. For a perfectly specular surface, MAES form a flat band at the Fermi energy, which manifests as a sharp midgap peak in the surface density of states (SDOS). Previous theoretical studies have shown that MAES in p- and d-wave superconductors respond markedly differently to surface roughness. In the d-wave state, diffuse surface scattering significantly broadens the midgap peak in the SDOS, accompanied by the emergence of a V-shaped structure centered at the Fermi energy. In contrast, the midgap peak in the p-wave state remains robust against diffuse scattering. In this work, we clarify the physical origin of this contrasting behavior. A key aspect of our analysis is that the flat band in the d-wave state consists of two distinct types of MAES modes. We show that inter-mode diffuse scattering leads to substantial broadening of the midgap peak and to the formation of the V-shaped structure. By contrast, the robustness of MAES in the p-wave state arises from the presence of a single MAES mode in the flat band. These results provide new insight into the response of MAES to surface roughness.

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cond-mat.supr-con 2026-05-05 2 theorems

Altermagnetic junction produces spin-polarized supercurrent

Spin-polarized Josephson current induced by inhomogeneous altermagnetic interlayers

Equal layer thicknesses and π misorientation cancel pair oscillations to enhance current and generate net spin polarization without magnetic

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The pursuit of dissipationless spin supercurrents is a central theme in superconducting spintronics. We propose a field-free Josephson junction using an inhomogeneous altermagnetic interlayer with in-plane N\'{e}el vectors. We show that the current-phase relation and the critical Josephson current are highly sensitive to the misorientation angle between the altermagnetic layers' N\'{e}el vectors. Specifically, at a $\pi$ misorientation with equal layer thicknesses the spatial oscillations of the superconducting pair amplitude, governed by the center-of-mass momentum, undergo mutual cancellation. This compensation suppresses individual layer pair-breaking, significantly enhancing the critical current and eliminating $0$-$\pi$ transitions. Furthermore, the non-collinear alignment of the N\'{e}el vectors facilitates the emergence of a net spin-polarized Josephson current. This spin current serves as a distinct signature of spin-triplet pair correlations, generated by the spin-dependent momentum shifts inherent to the altermagnetic exchange field. Our results establish a highly tunable, field-free platform for the realization of dissipationless spintronic devices.

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cond-mat.supr-con 2026-05-05 2 theorems

π-misoriented altermagnets cancel oscillations to create spin supercurrent

Spin-polarized Josephson current induced by inhomogeneous altermagnetic interlayers

Equal layer thicknesses lead to mutual cancellation of pair amplitudes, boosting critical current and enabling net spin polarization without

abstract click to expand

The pursuit of dissipationless spin supercurrents is a central theme in superconducting spintronics. We propose a field-free Josephson junction using an inhomogeneous altermagnetic interlayer with in-plane N\'{e}el vectors. We show that the current-phase relation and the critical Josephson current are highly sensitive to the misorientation angle between the altermagnetic layers' N\'{e}el vectors. Specifically, at a $\pi$ misorientation with equal layer thicknesses the spatial oscillations of the superconducting pair amplitude, governed by the center-of-mass momentum, undergo mutual cancellation. This compensation suppresses individual layer pair-breaking, significantly enhancing the critical current and eliminating $0$-$\pi$ transitions. Furthermore, the non-collinear alignment of the N\'{e}el vectors facilitates the emergence of a net spin-polarized Josephson current. This spin current serves as a distinct signature of spin-triplet pair correlations, generated by the spin-dependent momentum shifts inherent to the altermagnetic exchange field. Our results establish a highly tunable, field-free platform for the realization of dissipationless spintronic devices.

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cond-mat.supr-con 2026-05-04 4 theorems

Ti4Co2O splits superconductivity into two pressure regimes

Two distinct superconducting regimes in Ti4Co2O under pressures

Low pressure brings Fermi-liquid transport and peak critical field; high pressure raises Tc with phonon scattering and no structural change.

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We report on the pressure dependence of superconducting transition temperature Tc and upper critical field Bc2(0) through electrical transport of the Ti4Co2O superconductor (eg.,the superconducting transition temperature Tc = 2.5 K and the Bc2(0)=7.2T=2.9Tc). We find that the Tc exhibits non-monotonic pressure dependence:it rises monotonically at first with a pressure coefficient of dTc/dP=0.034 K/GPa, but rapidly decreases around 10-20 GPa, and then increases with the dTc/dP = 0.023 K/GPa, up to= 4.31 K at 69.7 GPa. Concurrently, the Bc2(0)exhibits a dome shaped pressure dependence, with its maximum at 5 GPa of almost twice the value at ambient pressure, exceeding the weak-coupling Pauli paramagnetic limit Bp throughout the whole pressure range. By comparing the normal-state and superconducting properties, we identify two distinct superconducting regimes, with a low-pressure superconducting phase characterized by an enhanced Bc2(0)values and Fermi-liquid normal-state electrical transport (the exponent n = 2), and a high-pressure superconducting phase with a monotonically increased Tc and an enhancement in phonon scatterings (the exponent n = 4). Room-temperature synchrotron X-ray diffraction indicates that there is no structural transition up to 55.8 GPa, which gives a relatively large bulk modulus of 192 GPa in comparison with other alloy superconductors. First-principles calculations suggest that the nonmonotonic Tc maybe closely related to the evolution of the density of states of Ti4Co2O upon compression, which is different from those of isostructural superconductors Ti4Ir2O and Nb4Rh2C. Our results show that even in the Ti4Co2O with weak spin-orbit coupling, superconductivity remains highly sensitive to the external stimuli such as pressure.

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cond-mat.supr-con 2026-05-04

Nickelate crosses to good metal at tilt-free transition

Metallic crossover through the tilt-free transition in La₃Ni₂O₇ at high pressure and temperature

Infrared shows La3Ni2O7 carrier density rising 100-fold as pressure removes crystal tilt, coinciding with 80 K superconductivity.

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La$_3$Ni$_2$O$_7$, a bilayer nickelate with Ruddlesden-Popper structure, undergoes a pressure-induced structural transition from a tilted Amam phase to an untilted Fmmm (or I4/mmm) phase near 10-15 GPa, concomitant with the emergence of high-T$_c$ superconductivity (T$_c$ $\sim$ 80 K). Despite intense interest, the phase boundaries and the impact of structural changes on the electronic properties remain unclear. Here, we combine high-pressure and high-temperature Raman and synchrotron-based infrared spectroscopies to map the structural and electronic evolutions. Raman measurements confirm the pressure-driven structural transition and reveal the emergence of Fano line shapes, indicating enhanced electron-phonon coupling. High-temperature data show analogous spectral signatures above 544 K, suggesting an unreported upper temperature limit of the Amam phase within the T-P phase diagram of this system. Infrared reflectivity measurements evidence a concomitant metallization, with a tremendous two-order-of-magnitude increase in carrier density, marking a crossover from a bad metal to a good metal. These results establish a unified picture of the structural transition and its strong coupling to the electronic properties.

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cond-mat.supr-con 2026-05-04

Impurity scattering turns cuprate resistivity quadratic below pseudogap scale

Impurity-Scattering Assisted Umklapp Scattering as the Origin of Low-Temperature Resistivity in the Normal-State of Cuprate Superconductors

Assisted umklapp processes from spin excitations stay strong in overdoped samples but weaken when the antinodal spin pseudogap opens, giving

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The transport experiments reveal that the low-temperature resistivity in the normal-state of cuprate superconductors is quadratic in temperature (T-quadratic) in the underdoped pseudogap phase, while it is linear in temperature (T-linear) in the overdoped strange-metal phase, however, the full understanding of these different behaviours is still a challenging issue. Here starting from the microscopic electronic structure of cuprate superconductors, the low-temperature resistivity in the normal-state is investigated from the underdoped pseudogap phase to the overdoped strange-metal phase. It is shown that the mechanism requires both the impurity scattering and the umklapp scattering: the impurity scattering is needed to restrict the modification of the distribution function to at around the antinodal region,while the impurity-scattering assisted umklapp scattering from a spin excitation is at the heart of the behaviour in the low-temperature resistivity, where the doping dependence of the temperature scale exists, and presents a similar behavior of the antinodal spin pseudogap crossover temperature. In the low-temperature region above the temperature scale in the overdoped strange-metal phase, the resistivity is T-linear, however, in the low-temperature region below the temperature scale in the underdoped pseudogap phase, the opening of the spin pseudogap lowers the spin excitation density of states at around the antinodal region, which reduces the strength of the electron umklapp scattering from a spin excitation associated with the antinode, and thus leads to a T-quadratic behaviour of the resistivity.

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cond-mat.supr-con 2026-05-04

Supercurrent induces magnetism for perfect superconducting diode

Superconducting diode effect in correlated electron systems by nonreciprocal magnetism

Nonreciprocal antiferromagnetic order sets critical currents in a correlated model near quantum criticality.

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The superconducting diode effect (SDE), characterized by a nonreciprocal critical current in superconductors, has recently been observed in strongly correlated electron systems and near quantum criticality, pointing to unconventional mechanisms beyond weak-coupling theories. Here we investigate the SDE in the Rashba-Zeeman-Hubbard model, which captures $d$-wave superconductivity in an antiferromagnetic quantum critical regime, using the Dyson-Gor'kov equation with the fluctuation exchange approximation. We show that electron correlations suppress the conventional intrinsic SDE arising from depairing currents. More importantly, a supercurrent nonreciprocally induces antiferromagnetic order, which fundamentally governs the critical current and enables perfect diode efficiency. Our results reveal a previously unrecognized correlation-driven mechanism of the SDE and establish strongly correlated superconductors as a platform for superconducting diode physics.

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cond-mat.supr-con 2026-05-04

Zero resistance reached at 2 micrometers in LSMO junctions

Long range proximity effects in planar structures involving the halfmetal ferromagnet La0.7Sr0.3MnO3 and Pt interlayers

Pt interlayers allow triplet supercurrents to cross half-metallic ferromagnets over distances previously considered difficult, hinting at a

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Over the last decade, there has been steady research on superconducting junctions with a ferromagnet as the weak link, and where triplet correlations can transport supercurrents over a substantial distances. Of particular interest are halfmetallic ferromagnets, in which only one spin band is present, so that, presumably, the induced supercurrent is fully spin-polarized. We have earlier reported on a study of triplet transport in planar La0.7Sr0.3MnO3(LSMO) nanostrip Josephson junctions with NbTi superconducting contacts, where we found high values for the supercurrents, and large junction lengths (up to 1.3 {\mu}m). Here, we extend that work by studying the dependence of the critical current Ic on the length of the nanostrip between the contacts and the width of the strip. All junctions show strong supercurrents, but we do not observe simple systematics. Apparently, the fabrication process does not allow sufficient control over some of its parameters. To gain more insight in the mechanism for triplet generation at the LSMO/NbTi interface, we also studied the effect of Pt as an interlayer between the LSMO and the NbTi. For this, we etched a NbTi/Pt electrode structure on a full film of LSMO. The results are highly promising, showing sharp superconducting transitions and zero-resistance states being reached at an electrode distance of 2 {\mu}m, with indications that larger distances should be feasible.

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cond-mat.supr-con 2026-05-04

Zero-field Josephson diode signals TRS breaking in TaS2

Signatures of time-reversal-symmetry breaking in multiband 2H-TaS2 revealed by zero-field Josephson nonreciprocity

Nonreciprocity without magnetic field plus nonlinear Hall response together indicate multiband phase structure in the superconductor.

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Superconductors that spontaneously break time-reversal symmetry host complex order parameters and are widely regarded as a hallmark of unconventional superconductivity. Whether such symmetry breaking can also arise in superconductors with nominally isotropic spin-singlet pairing remains an open question. Here we report a zero-field Josephson diode effect in noncentrosymmetric 2H-TaS2/2H-NbSe2 van der Waals junctions. The diode efficiency shows no systematic correlation with supercurrent amplitude, TaS2 thickness, or normal-state resistance, arguing against simple extrinsic, purely interfacial, or transparency-driven mechanisms. Time-reversal-symmetric scenarios are further tested using symmetry-controlled and molecule-intercalated control devices, in which the nonreciprocal response is absent or strongly reduced. Normal-state Hall transport in TaS2 exhibits a nonlinear response consistent with multiband correlated electronic states. Within a Josephson framework, our modelling shows that interband scattering acts as a phase-locking mechanism generating an intrinsic anomalous phase difference and a nonsinusoidal asymmetric current-phase relation, leading to finite zero-field rectification. Together, zero-field Josephson nonreciprocity and nonlinear Hall transport provide complementary evidence for a multiband superconducting phase structure in 2H-TaS2, consistent with intrinsic time-reversal-symmetry breaking.

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cond-mat.supr-con 2026-05-01

Monte Carlo simulation shows fermion quadruplets condensing

Demonstration of a fermion Quadrupling Condensate via Quantum Monte Carlo Simulation

Correlated hopping model allows unbiased sampling and reveals transition at the hopping energy scale

abstract click to expand

Fermionic condensation typically occurs via pairing. In recent decades, however, a fundamental question has emerged: whether alternative forms of order exist, such as condensates of fermion quadruplets. These states--including ``charge-4e" superconductors and ``charge-0" counterflow condensates--lie beyond the standard Bardeen-Cooper-Schrieffer framework, and require strong fluctuations and correlation effects that invalidate the BCS mean-field description. This makes the problem notoriously difficult to study numerically at a microscopic level, as it involves both strong interactions and the fermionic sign problem. Here, we present a microscopic fermionic model featuring correlated hopping that significantly mitigates the sign problem, enabling rigorous Monte-Carlo-based analysis. Using large-scale simulations, we demonstrate the existence of a fermion-quadrupling condensate with a transition temperature comparable to the hopping energy scale. These results provide direct numerical evidence for quartic fermionic order in a microscopic system and suggest that these exotic states are also experimentally accessible in ultracold atomic gases.

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cond-mat.supr-con 2026-05-01

Doping lets superconductivity win in hybrid bilayers

Enhancement of superconducting stiffness in hybrid superconducting-metallic bilayers

Partially polarized Kondo lattices show superconducting correlations dominating density waves, surviving small gaps unlike half-filling.

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Boosting superconductivity by metallic reservoirs is the essence of Kivelson's bilayer proposal. One layer provides pairing to the electrons, while the weakly coupled metal provides additional phase coherence to those pairs by mediating extended-range pair-pair coupling. Demonstrating significant and unambiguous performance gains with strong-coupling methods for such set-ups had been difficult. In the present work, we study these systems doped away from half-filling, corresponding to a partially spin-polarized 1D Anderson- or Kondo-lattice. We show that this breaks the coexistence of dominant superconducting and density-density correlations decisively in favour or the former. Consequently, we provide evidence that in this doped regime, superconducting near-long-range order is not precluded by a small charge-gap in the thermodynamic limit, as we have recently shown to be the case at half-filling [JE Ebot $et$ $al.$, arXiv:2602.11153 [cond-mat.supr-con]]. We study the complex manner in which the enhancement of superconductivity in the pairing layer depends on the parameters of the metal, and especially that both pairing-limited and stiffness-limited regimes may appear in these systems. In addition to superconducting bilayers, our results are relevant, via a particle-hole transformation, for heavy-fermion Kondo-lattice materials in magnetic fields, as we provide previously lacking insight on the competition between antiferromagnetic and easy-plane magnetism, as well as a route for comprehensive indirect tests of Kivelson's bilayer proposal well beyond previous capabilities.

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cond-mat.supr-con 2026-05-01

AFM dissipation spectroscopy detects superconducting edge channels in oxide interfaces

Local probing of superconductivity at oxide interfaces with atomic force microscopy

Nonlinear bias dependence in energy loss is confined to 200 nm channels and matches transport data up to the critical field.

abstract click to expand

Superconductivity in strontium titanate has remained enigmatic for more than 50 years. The LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterointerface enables systematic dimensional confinement, from a two-dimensional electron gas to quasi-one-dimensional nanostructures, providing access to this quantum state. Transport measurements in patterned devices reveal puzzling phenomena, including width-independent critical currents and anomalous pairing suggestive of one-dimensional behavior, but direct local probes of the patterned interface and its superconducting response have been lacking. Here we use ultralow-temperature non-contact atomic force microscopy, dissipation spectroscopy, and Kelvin probe force microscopy to locally probe signatures of superconductivity in patterned LAO/STO devices. Spatially resolved energy-dissipation measurements reveal superconducting signatures, with features confined in some devices to edge channels approximately 200 nm wide. Dissipation spectra exhibit a characteristic nonlinear bias dependence that provides a local diagnostic of superconductivity, consistent with the intermediate carrier-density regime near the superconducting dome, and persisting up to the critical field. These results establish atomic force microscopy as a local probe of superconductivity in patterned LAO/STO structures and provide a route to addressing longstanding questions about quantum confinement and transport anomalies in correlated oxide nanostructures.

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cond-mat.supr-con 2026-05-01

Nonzero switching currents seen above critical field in Al two-width wires

Unusual critical currents in quasi-one-dimensional superconducting aluminum two-width structures in a magnetic field

Experiments near Tc reveal nonlocal dependence on junction transport and disagreement with Ginzburg-Landau theory.

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We measured unusual critical currents as functions of temperature in the zero field and as functions of a magnetic field perpendicular to the substrate surface at a given temperature close to the critical temperature in thin-film long quasi-one-dimensional superconducting aluminum two-width structures consisting of narrow and wide wires with different critical temperatures. It is found that the experimental critical switching current as a function of the field at a given temperature, determined by the appearance of a dc voltage on a short section of the structure, is nonlocal (dependent on electron transport in the area containing the junction line between the narrow and wide wires). When current flows through the narrow and wide wires of the structure, the switching currents, experimental and calculated within the framework of the Ginzburg-Landau theory, differ radically from each other. A nonzero switching current exists in high fields greater than the maximum critical magnetic field in a quasi-one-dimensional superconducting wire. In the aluminum two-width structures studied here, the unusual measured switching current challenges description by known theories.

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cond-mat.supr-con 2026-05-01

Kinetic inductance difference shifts critical currents in asymmetric Al rings

Shift of the maxima of the critical currents of different polarity relative to the zero magnetic flux along the flux axis in a superconducting asymmetric aluminum ring

The temperature-dependent phase shift equals the inductance difference between wide and narrow halves only when their critical temperaturesd

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We measured the rectification of an ac voltage in a structure of superconducting circularly-asymmetric aluminum rings in series, permeated with a magnetic flux and biased with a low-frequency alternating current (without a dc component). This rectification is due to the shift of the maxima of the critical currents of different polarity relative to the zero flux in opposite directions along the flux axis in the asymmetric ring. For the first time, we propose a model for a temperature-dependent phase shift equal to difference between dimensionless kinetic inductances of wide and narrow semirings having the same length and thickness. The shift is not zero in the case of different critical currents densities in both semirings. This is possible only in a situation of different critical temperatures of both semirings. The model describes well the temperature-dependent shift of the maxima of the critical currents, answers the long-standing mysterious challenge of the shift and removes extremely strange contradiction between the results of different measurements, previously found in circularly-asymmetric aluminum structures.

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cond-mat.supr-con 2026-05-01

Narrower aluminum wires show reduced critical temperatures

Critical temperatures and critical currents of wide and narrow quasi-one-dimensional superconducting aluminum structures in zero magnetic field

New measurements find lower Tc and Ic density in narrow quasi-1D structures, with SNS Josephson junctions appearing near the transition.

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We measured the critical temperatures and critical switching and retrapping currents of wide and narrow thin-film quasi-one-dimensional superconducting aluminum structures of the same thickness in zero magnetic field. For the first time, we found that the narrower the structure, the lower the critical temperature and critical current density in the structure. Probably, the influence of depairing centers that are on dirty longitudinal boundaries of the structure, is the stronger than the narrower the structure. It is found for the first time that, in most cases, the temperature-dependent switching critical current in both structures is approximated by two functions. At temperatures below the temperature corresponding to the bottom of the resistive N-S transition of structures, the switching critical current is described by the Kupriyanov-Lukichev theory. At temperatures close to the top of the N-S transition, the switching current is linear with temperature and coincides with the critical Josephson current. At these temperatures, Josephson SNS junctions are formed in structures.

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cond-mat.supr-con 2026-05-01

Local Tc scaling uncovers magnetic QCP inside CeCoIn5 superconductor

Magnetic Quantum Criticality inside the Superconducting State Revealed by Penetration Depth Scaling with Local T_{mathrm c}

Zero-temperature penetration depth peaks when plotted against local transition temperature, showing suppressed superfluid stiffness at the Q

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We demonstrate a magnetic quantum critical point embedded within the superconducting state of Zn-doped CeCoIn$_5$, revealed by a pronounced peak in the magnetic penetration depth at zero temperature $\lambda(0)$. Using scanning SQUID microscopy, we determine the local superconducting transition temperature $T_{\mathrm c}$ and $\lambda(0)$. By parameterizing $\lambda(0)$ in terms of the local $T_{\mathrm c}$ rather than nominal Zn substitution, we circumvent the ambiguity caused by doping inhomogeneity and enable a more precise extraction of the critical exponent. The extracted exponent exceeds the clean spin-density-wave value, indicating a disorder-modified quantum critical regime. The enhancement of $\lambda(0)$ reflects the suppression of the superfluid stiffness and is consistent with critical scaling. Our approach provides a route to uncover intrinsic quantum critical behavior hidden by inhomogeneity in unconventional superconductors.

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cond-mat.supr-con 2026-04-30

Superfluid 3He reorients its axes at field-independent Tx

Phase Stability of Superfluid ³He in Anisotropic Aerogel

Uniform aerogel strain orients the chiral and spin-orbit vectors, producing a spontaneous change explained by temperature-dependent Ginzburg

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The A and B phases of superfluid 3 He have vector degrees of freedom that reflect their characteristic broken symmetries, respectively chiral and spin-orbit rotation axes. Anisotropic disorder in the superfluid, imbibed in uniformly strained silica aerogel, orients these degrees of freedom, thereby affecting phase stability. These degrees of freedom have been found to spontaneously reorient at a field-independent transition temperature Tx , that can be accounted for with a temperature dependent anisotropic Ginzburg-Landau model.

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cond-mat.supr-con 2026-04-30

Superconductor converts FMR into standing spin waves

Superconductivity-Enabled Conversion of Ferromagnetic Resonance into Standing Spin Waves

An extra resonance appears in garnet-niobium bilayers only below the transition due to triplet pairs and vortices.

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Superconductors can transport spin without Joule dissipation, yet their coherent coupling to short-wavelength magnons in insulating magnets remains largely unexplored. Here we demonstrate experimentally and theoretically that a conventional diffusive superconductor can enable the conversion of the uniform ferromagnetic-resonance (FMR) mode into perpendicular standing spin waves (PSSWs) in an adjacent ferrimagnetic insulator. In Bi-substituted iron-garnet/Nb bilayers, the microwave transmission develops an additional resonance feature that appears only below the Nb transition temperature and lies close to the uniform FMR peak. A microscopic theory that self-consistently couples the quasiclassical Keldysh--Usadel description of the superconducting condensate to the Landau--Lifshitz--Gilbert dynamics shows that the conversion requires two ingredients: (i) an interfacial spin-transfer torque mediated by spin-polarized triplet Cooper pairs and (ii) a depth-dependent effective field produced by Abrikosov vortices (electromagnetic proximity). The resulting susceptibility reproduces the measured lineshapes and establishes superconductivity as an active control knob for exchange standing-wave modes in magnetic insulators.

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cond-mat.supr-con 2026-04-30

Non-local conductance reveals inelastic scattering at 3Δ in superconducting wires

Non-local Tunneling Spectroscopy of Inelastic Quasiparticle Relaxation in Superconducting 1-D Wires

Dual-bias tunneling on NIS devices extracts energy-dependent quasiparticle relaxation times from anti-symmetric signals.

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Non-local conductance experiments using tunnel junctions can provide valuable spectroscopic information on both the transport and relaxation of quasiparticles in superconductors, as these techniques directly probe the quasiparticle charge and energy imbalance even at mK temperatures. In this work, we employ mesoscopic three terminal Cu and Al NIS devices to study non-local quasiparticle transport over length-scales on the order of the superconducting coherence length in this regime. Via a dual-bias scheme, which utilizes detector biases both above and below the superconducting gap, we are able to extract the effect of quasiparticle energy imbalance via its impact on the self consistent pair potential by symmetry considerations. We observe non-local conductance features due to pair-breaking which are anti-symmetric with respect to the polarity of the voltage bias, with a sharp onset during single electron tunneling at energies around $3\Delta$. We compare these findings with quasiclassical simulations including inelastic effects to obtain estimates of the energy dependent inelastic scattering time. In addition, we demonstrate kinetic effects due to a large applied supercurrent which can also be captured in this formalism and decomposed with respect to the particle-hole symmetry and supercurrent direction, and discuss further opportunities for the advancement of this method.

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cond-mat.supr-con 2026-04-30

Researchers show that adding a layer to Weyl semimetals like PtBi2 can create…

Engineering superconductivity on the surface of Weyl semimetals

Depositing a suitable layer on Weyl semimetals induces surface van Hove singularities that significantly enhance superconducting critical…

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Ten years after the experimental discovery of Weyl semimetals, theoretical and experimental work has pointed to the possibility of realizing surface-only superconductivity at relatively high temperatures in these materials. A consensus is developing that this unusual form of superconductivity is mediated by surface electronic states unique to Weyl semimetals, known as Fermi arcs. In this work, we show that the topological protection of these exotic states can be exploited to engineer high critical temperatures. Motivated by a real-material example (PtBi$_2$), we demonstrate that surface van Hove singularities can be induced by depositing a suitable additional layer on top of the Weyl surface. We also investigate the role of these singularities in raising the critical temperature, showing that it is significantly enhanced when the chemical potential lies in their vicinity. More generally, our results demonstrate how topological protection can be exploited to manipulate surface electronic states, thereby opening experimentally accessible routes toward engineering high-temperature two-dimensional superconductivity and other exotic phases.

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cond-mat.supr-con 2026-04-30

Quasiparticles generate negative voltages near Tc in aluminum structures

Negative nonlocal and local voltages (resistances) in a quasi-one-dimensional superconducting aluminum structure

Measurements show negative nonlocal and local voltages arise from current across the normal-superconducting boundary in a magnetic field.

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To study a nonlocal electron transport in an aluminum superconducting quasi-one-dimensional structure, we measured negative nonlocal (local) direct current voltages in the structure in a magnetic field near the critical temperature. The structure is a normal-superconducting at $T_{cn}<T<T_{cw}$ ($T_{cn}$ and $T_{cw}$ are the critical temperatures for narrow and wide wires, respectively, making up this structure). Negative voltage arises due to a quasiparticle current flowing through the N-S interface. We plotted the experimental and theoretical temperature and magnetic-field dependences of current, resistance and voltage corresponding to the peak of negative voltage, taking into account either equilibrium or nonequilibrium superconducting fluctuations.

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cond-mat.supr-con 2026-04-30

Nematic domains program a 75% efficient superconducting diode in FeSe

Programmable superconducting diode from nematic domain control in FeSe

Microsecond pulses reconfigure twin boundaries to set diode polarity after the device is made.

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The superconducting diode effect (SDE) allows polarity-dependent critical currents when time-reversal and current-inverting spatial symmetries are broken. Superconducting diodes show promise for applications, but inversion asymmetry is usually encoded in sample geometry or non-centrosymmetric crystals, rendering them static circuit elements. Here we demonstrate a programmable superconducting diode whose functionality is encoded in correlated electronic domains. We use the nematic superconductor FeSe as a platform and report a large intrinsic SDE with efficiencies up to $\eta \sim 75\%$ due to vortices interacting with nematic twin boundaries. The domain wall configuration thus encodes the SDE of the device. Through intense microsecond current pulses to quench the nematic order at rates exceeding $10^7$ K/s, we modify the domain pattern and control the polarity and strength of the SDE. These results establish a new paradigm in which superconducting circuit elements can be programmed through patterns imprinted into correlated electronic states.

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cond-mat.supr-con 2026-04-30 Recognition

Quantization alone accounts for Meissner effect

The Meissner effect does not require radial charge flow

The persistent current is an experimental fact from angular momentum rules that Lorentz forces on radial flows cannot explain.

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The Meissner effect is the expulsion of magnetic flux from the interior of a bulk superconductor in the presence of the constant critical magnetic field by the persistent current circulating near the surface of the superconductor. The conventional theory of superconductivity explains the appearance of the persistent current in the Meissner effect and other macroscopic quantum phenomena observed in superconductors as a consequence of the quantization of angular momentum of Cooper pairs. According to the alternative theory of hole superconductivity the persistent current appears due to the Lorentz force acting on a radial charge flow rather than due to quantization. Therefore, the author of this theory, Jorge Hirsch, argues in his numerous publications that a radial charge flow is required to explain the Meissner effect. This article draws attention to the fact that the appearance of the persistent current because of quantization is not only the statement of the conventional theory of superconductivity, but first of all the experimental fact that cannot be explained using the Lorentz force. Therefore, the explanation of the Meissner effect does not require radial charge flow.

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cond-mat.supr-con 2026-04-30

Metallic crossover precedes structural change in GaNb4Se8

From Wavefunction Collapse to Superconductivity: Evolution of the Electronic State in Compressed GaNb4Se8

Transport turns metallic near 5 GPa while the cubic lattice persists until 20 GPa, leading to superconductivity in the correlated metal.

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Understanding how electronic transport evolves from localized to itinerant regimes in correlated cluster solids remains an important challenge in condensed-matter physics. Here we investigate the pressure-dependent transport properties of the lacunar spinel GaNb4Se8, a cluster Mott insulator at ambient conditions. At low pressures, the resistivity follows Efros-Shklovskii variable-range hopping, indicating Coulomb-gap-controlled carrier localization (x ~ 6.1 Angstrom). A crossover toward metallic transport begins near ~ 5 GPa, whereas a crystallographic transition from the cubic phase to a monoclinic C2 phase occurs at significantly higher pressure (~ 20 GPa), establishing a hierarchy characterized by the decoupling of electronic delocalization from structural symmetry change. At higher pressures, superconductivity (xi(0) ~ 80-90 Angstrom) emerges from the correlated metallic regime. These results identify GaNb4Se8 as a platform for studying correlation-controlled transport evolution in cluster-based solids.

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cond-mat.supr-con 2026-04-29

Shot noise unmasks trivial zero-bias peaks in Fe(Se,Te)

Exposing impostor Majorana zero modes through atomic-scale shot-noise

Zero-bias conductance peaks look robust but noise reveals ordinary particle and hole character, exposing them as impostors rather than Major

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A robust zero-bias conductance peak in putative $p$-wave superconductors is often regarded as the primary signature of a Majorana zero mode. Yet similar features can also arise from trivial bound states. This ambiguity has limited the reliability of conventional spectroscopy as a diagnostic tool, raising a long-standing problem of how to detect such impostors. Here, we address this issue with an alternative approach, atomic-scale shot-noise spectroscopy, that goes beyond conductance measurements. Through a detailed investigation of multiple defect-bound zero-bias states in the widely studied superconductor Fe(Se,Te), we observe that differential conductance can exhibit an apparently `robust' zero-bias peak. However, shot-noise measurements consistently reveal the fingerprint of the individual particle- and hole character hidden in the tunnelling conductance, unambiguously exposing the trivial nature of the zero-bias peak. Our results establish shot-noise spectroscopy as a decisive diagnostic for ruling out false Majorana signatures in atomic-scale experiments.

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cond-mat.supr-con 2026-04-29

Ultrasound detects fourth phase boundary in UTe2

Thermodynamic Identification of the Internal Superconducting Phase Boundary in UTe₂ for H parallel b

An anomaly in the C33 mode marks a bulk boundary near 14 T that ends at a tetracritical point and constrains the high-field order parameter.

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The $H$--$T$ phase diagram of UTe$_2$ for magnetic field along the hard $b$ axis contains an unresolved internal boundary near $\mu_0H \sim 14$--15~T, previously inferred from ac susceptibility and transport experiments but lacking thermodynamic evidence. We report ultrasound results for several elastic modes in an ultraclean UTe$_2$ single crystal with $T_c>2$~K for $H \parallel b$ down to 0.33~K and up to 18~T. A pronounced anomaly in the longitudinal $C_{33}$ mode, with a weaker response in $C_{44}$ and no resolvable anomaly in $C_{55}$, establishes this feature as a bulk thermodynamic phase boundary and reveals a symmetry-selective coupling to lattice strain. The phase line remains nearly constant in field near 14~T and terminates near 13.5~T and 1.25~K at a tetracritical point, providing the thermodynamic evidence for the fourth phase boundary in the $H$--$T$ phase diagram. The results constrain the order-parameter structure of the high-field phase and support field-induced multicomponent superconductivity in UTe$_2$.

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cond-mat.supr-con 2026-04-29

Disordered vacancies raise Tc from 7 K to 11 K

Bragg-Williams order competes with superconductivity

Reversible Bragg-Williams ordering of indium vacancies tunes superconductivity through electron-phonon coupling without doping changes.

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Orderings in charge and spin have been extensively studied to unravel their correlation to emergent superconductivity over the past decades. Bragg-Williams order (BWO), a classical structural order parameter describing site occupancy in alloys, has long been speculated to influence superconducting behavior. Yet, its role still remains ambiguous, largely due to the difficulty of isolating BWO from concomitant charge doping or competing electronic instabilities. Here, we establish In2/3PSe3 as a platform wherein indium vacancies are reversibly configurable between ordered and disordered states via thermal treatment. We show that the disordered phase undergoes a pressure-induced superconducting transition with a Tc of 11 K, significantly higher than the 7 K observed in its ordered counterpart. This constitutes a rare instance in which pure BWO variation drives a substantial shift in Tc. By combining a Ginzburg-Landau phenomenological analysis with a BCS-McMillan microscopic description, we demonstrate that BWO naturally suppresses superconductivity through electron-phonon interactions, a mechanism supported by ultra-low-wavenumber Raman measurements. Our findings support BWO as an independent order parameter that competes directly with superconductivity, extending the concept of competing orders beyond conventional electronic and magnetic degrees of freedom.

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cond-mat.supr-con 2026-04-29

Rashba geometry sets quasi-linear Tc(θ) at LaAlO3/KTaO3 interfaces

Geometric Rashba Control of Polar Pairing at LaAlO₃/KTaO₃ Interfaces

A sin(θ) vertex in a minimal Eliashberg model with PNR fluctuations reproduces the observed angular dependence and higher Tc scale than SrTi

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At LaAlO$_3$/KTaO$_3$ interfaces, the superconducting $T_c$ exhibits a striking quasi-linear dependence on crystallographic orientation, coexisting with switchable polar nanoregions (PNRs). We propose an effective minimal Eliashberg framework in which overdamped PNR fluctuations provide the pairing glue, while geometric Rashba coupling controls its angular dependence. Within a reduced isotropic helicity-band description, the dynamic Rashba vertex scales as $\sin(\theta)$, yielding a pairing strength $\lambda(\theta)=\lambda_0+C\sin^2(\theta)$. Exact Matsubara-Eliashberg numerical solutions show that this non-linear mapping naturally yields the same qualitative quasi-linear $T_c(\theta)$ dependence within the reduced model. Because the Rashba-activated polar channel is amplified by the large atomic spin-orbit coupling of Ta $5d$ orbitals, the same framework also rationalizes why KTaO$_3$ interfaces exhibit both a much stronger orientational dependence and a substantially higher $T_c$ scale than their SrTiO$_3$ counterparts.

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cond-mat.supr-con 2026-04-29

Hydrogenation lifts FeSe superconductivity past 40 K

Critical Role of Hydrogen in Unconventional Superconductors: The Case of Hydrogenated FeSe Layers

Orbital shifts from correlations strengthen electron-phonon pairing and create a two-gap state in stable FeSeH layers.

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Hydrogenation is known to tune superconductivity in a wide range of materials. While its microscopic role has been clarified in phonon-mediated superconductors such as hydrogenated MgB2, LaH10, and H3S, much less is known for hydrogenated cuprates and iron-based superconductors, where even the underlying structural motifs remain elusive. Using hydrogenated FeSe as a prototypical example, we reveal how hydrogen affects superconductivity in the presence of strong electronic correlations: correlation-induced orbital renormalization shifts hydrogen-derived spectral weight from the high-energy region toward the Fermi surface (FS), remarkably enhancing the electron-phonon coupling (EPC). We predict a structurally stable FeSeH phase where, compared to bare FeSe, hydrogen incorporation reshapes the FS topology and increases the number of channels for electron-phonon scattering, while simultaneously introducing high-frequency phonons that strengthen pairing. First-principles EPC calculations combined with dynamical mean field theory (DMFT) yield a superconducting transition temperature (Tc) exceeding 40 K. Fully anisotropic Eliashberg theory reveals a two-gap superconducting state, consistent with the gap structure experimentally observed in doped FeSe. Our findings identify correlation-enhanced EPC as a plausible microscopic mechanism for iron-based superconductivity and offer a new perspective on pairing in strongly correlated systems. In addition, this work establishes hydrogenated FeSe as a promising platform for engineering two-dimensional superconductors and superconducting quantum devices.

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cond-mat.supr-con 2026-04-29

Pseudogap tied to oxygen impurities

Differentiation of electron doping and oxygen reduction in electron-doped cuprates

Surface alkali dosing adds electrons without changing oxygen, eliminating antiferromagnetic reconstruction but leaving the pseudogap intact.

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Electron-doped cuprates require not only electron doping by chemical substitution but also post-growth reduction annealing for realizing superconductivity. However, electron concentration can also be varied by reduction annealing, making it challenging to disentangle the respective influences of electron concentration and oxygen non-stoichiometry. Here, by combining alkali-metal dosing and angle-resolved photoemission spectroscopy, we monitored changes in the electronic structure of an electron-doped cuprate while supplying additional electrons to its surface without modifying oxygen content. Whereas a Fermi surface reconstruction due to long-range antiferromagnetic order was suppressed by alkali-metal deposition, the pseudogap -- which is associated with short-range spin/charge correlations and can be suppressed by efficient reduction annealing -- was found to persist. The results highlight significant contribution of impurity oxygen atoms to pseudogap formation in electron-doped cuprates.

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cond-mat.supr-con 2026-04-29

Nd-garnet sensor maps superconductor Jc up to 13 T

High-field magneto-optical imaging of superconducting critical states beyond 10 T using a paramagnetic garnet sensor

Magneto-optical imaging reconstructs local critical current density and current vectors in iron-based crystals at 12 K and 20 K.

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Spatially resolved characterization of the critical current density Jc in superconductors under high magnetic fields is crucial for both fundamental understanding and practical applications. However, conventional techniques primarily provide bulk-averaged values, making it difficult to resolve local variations of Jc, especially in high magnetic fields. In this work, we develop a magneto-optical imaging (MOI) technique that enables visualization of superconducting critical states in steady magnetic fields up to 13 T. This is achieved by employing a paramagnetic Nd-garnet indicator combined with a polarizing microscope system. Using this method, we directly image the magnetic flux distribution in a bulk single crystal of an iron-based superconductor Ba(Fe1-xCox)2As2 (x = 0.075) at 12 K and 20 K across the entire sample area (approximately 1 mm). From the measured magnetic field distributions, we quantitatively reconstruct the spatial distribution of the critical current density. The extracted field dependence of Jc is in good agreement with that obtained from conventional magnetization measurements. Furthermore, we demonstrate vector mapping of current flow within the sample by converting the magnetic field distribution into local current-density distributions. Our results establish high-field MOI as a powerful approach for spatially resolved evaluation of superconducting critical states and provide a new pathway for investigating inhomogeneous current transport in superconductors under high magnetic fields.

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cond-mat.supr-con 2026-04-29

Nonlocal Cooper pairs form in finite topological superconductors

Nonlocal Cooper pairs in finite topological superconductors and their relation to Majorana nonlocality

Low-frequency Green's functions show end-to-end correlations growing with length while local ones vanish, tied to hybridized Majorana modes.

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We identify two fundamental properties of the Gor'kov Green's function of finite one-dimensional topological superconductors. In the low-frequency (low-energy) regime, the normal and anomalous Green's functions, which describe single-particle and Cooper-pair correlations, respectively, become identical up to a phase factor. Moreover, they exhibit pronounced nonlocality: correlations between the two ends of the system grow exponentially with system length, whereas local correlations at either end vanish in the zero-frequency limit. These striking features signify the emergence of unconventional nonlocal Cooper pairs associated with a nonlocal fermionic mode composed of hybridized Majorana end modes. The nonlocal Cooper pairs are directly linked to fermion parity and to the nonlocal transport properties of finite topological superconductors. By focusing on pair correlations, our analysis advances the understanding of Majorana nonlocality, a key concept in topological quantum computation.

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cond-mat.supr-con 2026-04-28

Triplet networks trap noninteger flux through d-vector frustration

Nonintegral Flux Trapping in Frustrated Josephson Networks of Triplet Superconductors

Anisotropic couplings generate emergent phases that spontaneously pin fractional flux quanta above a critical strength.

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In a Josephson junction network, anisotropic coupling between spin triplet pairing correlations can lead to frustrated $d$ vector textures that support spontaneous Josephson currents and nonintegral flux trapping. Such networks can appear in superconducting polycrystals, as well as single-crystal superconductors. In analogy to classical spin systems, in which the presence of geometric frustration and anisotropic superexchange can lead to nontrivial spin textures, Josephson networks with anisotropic Josephson couplings cannot simultaneously optimize their $\mathrm{U}(1)$ superconducting phase difference and relative $d$ vector orientations. The internal pairing structure of Cooper pairs twists as they tunnel across the Josephson junction, and the $d$ vector texture enters as an emergent geometric phase which can spontaneously trap fractional flux. For unitary triplet pairing order, this mechanism can support $\pi$-flux trapping above a critical value of antisymmetric Josephson coupling, and is distinct from usual half-quantum vortices. The results of this work reveal new routes to engineer frustrated Josephson networks from the interplay of magnetic textures and spin triplet superconducting pairing order.

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cond-mat.supr-con 2026-04-28

Flat bands from lone pairs boost superconductivity in FeSe to 23 GPa

Cosine bands, flat bands and superconductivity in orthorhombic iron selenide

Calculations link lone pair electrons to flat bands that enhance electron-hole pairing under pressure in beta FeSe1-x.

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Electronic band structures (EBSs) for orthorhombic beta FeSe1-x at less than 16 K and up to 23 GPa using experimentally determined cell dimensions are evaluated for cosine-shaped bands near, or crossing, EF. Cosine shaped bands are present in reciprocal directions parallel to the c axis at all pressures. Calculations using a P1 cell derived from Cmma symmetry with a 2c superlattice moderates the effect of intersecting bands to 9.0 GPa. This approach enables determination of a superconducting gap consistent with experimentally determined values. Key influences on charge distribution and transfer in the interplanar region of beta FeSe1-x are lone pair electrons which feature as flat bands (FBs) near EF along GZ in an EBS. FBs also influence the topology of Fermi surfaces as pressure increases and in directions parallel to the c* direction (i.e. offset along ky) within the Brillouin zone. At the Fermi surface along b*, cosine bands split and align favorably for electron-hole pairing with nodal inflection points located at EF. For P greater than 12.0 GPa, FBs interact with folded cosine bands invoking additional band dispersions. These calculations suggest that FBs participate in, and with increased pressure, enhance and sustain the superconducting properties of beta FeSe1-x to 23 GPa.

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cond-mat.supr-con 2026-04-27

CeCoIn5 Campbell depth deviates from square-root field law

Campbell penetration depth in a single crystal of heavy fermion superconductor CeCoIn₅

Abrupt slope changes point to vortex lattice symmetry shifts while critical current stays linear with temperature

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The temperature and magnetic field dependent magnetic penetration depth, $\lambda_m(T,H)$, was measured in a single crystal of a heavy fermion superconductor CeCoIn$_5$ using a frequency-domain tunnel diode resonator. In addition to the London penetration depth, which yields the superfluid density, measurements in a finite DC magnetic field provide Campbell penetration depth, $\lambda_C(T,H)$, which is directly linked to the true (unrelaxed) critical current density, $J_c$. The measured $\lambda_C(H)$ in CeCoIn$_5$ deviates significantly from the conventional $\sim \sqrt{H}$ behavior, and its slope changes abruptly at the characteristic magnetic field values. Considering that our sample is in the clean limit, we interpret this deviation as a fingerprint of the vortex lattice symmetry change. The temperature dependence $J_c(T)$ of CeCoIn$_5$ calculated from $\lambda_C(T)$ is nearly $T$-linear over the entire temperature range, also in stark contrast to expectations in a conventional type-II superconductor. Our results provide new evidence for unconventional superconductivity in CeCoIn$_5$ from the never-before-measured Campbell penetration depth.

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cond-mat.supr-con 2026-04-27

LiFeAs vortex lattice is stripes of half-quantum pairs

Unconventional mixed state in the nematic superconductor LiFeAs

Spectroscopy shows magnetic penetration via bound half-flux vortices instead of standard single-quantum Abrikosov ones.

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In the mixed state of type-II bulk superconductors, the magnetic field penetrates in the form of vortices enclosing one magnetic flux quantum: this is the conventional Abrikosov vortex lattice. Here, by using transverse muon-spin spectroscopy, we demonstrate the presence of an unconventional vortex lattice in LiFeAs single crystals. We also show evidence that the new mixed phase consists of stripes of "coreless" vortices, which are bound states of two spatially separated half-quantum vortices.

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cond-mat.supr-con 2026-04-27

WTe2 with reduced disorder shows record magnetoresistance and 1.8 K superconductivity

Record magnetoresistance, enhanced superconductivity, and fermiology in WTe2

Cleaner crystals from refined growth enable first quantum oscillations in doped monolayers and a tunable superconducting dome.

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The diverse electronic properties of transition metal chalcogenides can be very sensitive to crystal imperfections. A new crystal growth technique, known as horizontal flux transport, offers a route to improved crystal quality. By refining this technique and applying it to the topological semimetal WTe2, we achieved crystals with an order of magnitude less disorder as determined by electrical transport and scanning tunneling microscopy measurements. At low temperatures these crystals exhibit the largest magnetoresistance reported in a metal. Exfoliated monolayers show quantum oscillations for the first time in the electrostatically doped metallic states, enabling determination of band degeneracies and the valley splitting induced by an electric field. Moreover, they exhibit a gated superconducting dome with a greatly enhanced critical temperature approaching 1.8 K. This advance opens up new avenues for employing WTe2 in topological electronics and gated superconducting devices, and promises comparable breakthroughs with other chalcogenides.

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cond-mat.supr-con 2026-04-27

Fermi pockets form at doping 0.007 in cuprate inner planes

Persistent Fermi Pockets and Robust Electron Pairing in Lightly Doped CuO₂ Planes of Cuprate Superconductors

Shielded planes reveal abrupt metallization and 33 meV gaps coexisting with antiferromagnetism.

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High temperature superconductivity in cuprate superconductors is generally considered to be generated from doping the Mott insulators. The fundamental nature of the doped parent compounds as well as the microscopic origin of electron pairing remain critical issues in understanding the emergence of superconductivity. Here, using high-resolution spatially-resolved laser angle-resolved photoemission spectroscopy, we investigate the intrinsic electronic structures of the CuO$_2$ planes in multilayer cuprates Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+\delta}$ (n=5$\sim$8). The inner CuO$_2$ planes are well shielded from the disorders and provide a rare and ideal platform to probe the intrinsic electronic phase diagram. We observe well-defined Fermi pockets with hole doping levels as low as 0.007, demonstrating an abrupt transition from the parent Mott insulator to a metallic state upon the introduction of an infinitesimal amount of doping. The innermost CuO$_2$ planes (IP$_0$) display gapless Fermi pockets, while the second innermost planes (IP$_1$) exhibit anisotropic superconducting gaps up to $\sim$33$\,$meV, indicative of robust electron pairing coexisting with strong antiferromagnetic order. Our findings provide a revised framework for understanding the doping-driven transitions and pairing mechanisms in cuprate superconductors.

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cond-mat.supr-con 2026-04-27

CaSb2 vortex clusters show stronger edges and quieter cores

Visualizing Vortex Cluster Dynamics in the Weak Type-II Superconductor CaSb₂

Local imaging finds boundary-enhanced magnetism and suppressed motion inside clusters, hinting at non-monotonic forces in a single-gap super

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Scanning SQUID imaging of CaSb$_2$ reveals dense vortex clusters with enhanced boundary susceptibility and suppressed internal vortex motion, which features inconsistent with both isolated vortex and flux tube behaviors. These measurements provide the first local visualization of magnetic dynamics within vortex clusters in a weakly pinned superconductor, offering a new route to probe non-monotonic vortex-vortex interactions that are typically expected in single-band type-II/1 or multiband type-1.5 superconductors. Although the superfluid density follows a single-gap BCS model and the Ginzburg-Landau parameter of CaSb$_2$ lies slightly outside the type-II/1 regime, vortex clustering and spatially inhomogeneous dynamics are clearly observed, indicating physics beyond existing microscopic theories for single-band superconductors.

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cond-mat.supr-con 2026-04-27

Frequency shifts determine electron mean free path in SRF cavities

Resonance Frequency Shift Measurements of SRF Cavities at DESY

New setup tracks changes during the superconducting-to-normal transition and studies anomalous dips near critical temperature.

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The variation of the resonance frequency and intrinsic quality factor of superconducting radio-frequency cavities during the transition from the superconducting to the normal-conducting state provides essential insight into the fundamental superconducting properties of the cavity material. Investigating these transition dynamics is crucial for the continued advancement of niobium cavities whose near-surface regions are intentionally modified through the controlled introduction of interstitial atoms, such as oxygen and nitrogen, leading to the emergence of several novel behaviors whose underlying mechanisms are not yet fully understood. This work reports on the development and commissioning of a dedicated frequency-shift measurement setup. In its initial implementation, the system establishes a precise framework for determining the electron mean free path within both the superconducting penetration depth and the normal-conducting skin depth. It further enables investigation of an anomalous dip in the temperature dependence of the frequency shift near the critical temperature in cavities containing interstitial atoms in the near-surface lattice, a novel phenomenon previously reported in the literature. A recent upgrade, currently in the final stage of validation, significantly improves measurement accuracy and reproducibility. The improved setup enables comprehensive studies of the frequency shift and quality factor over the full temperature range above 7 K, contributing to a deeper understanding of the superconducting properties.

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cond-mat.supr-con 2026-04-27

Hypothetical NaAlH3 phase predicted to superconduct at 74 K

Theoretical prediction of strong-coupling superconductivity in a hypothetical NaAlH3 phase at ambient pressure

Strong electron-phonon coupling in the cubic structure yields high critical temperature at ambient pressure in density-functional theory.

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We present a comprehensive first-principles investigation of a hypothetical cubic Pm-3m phase of the ternary hydride NaAlH3, focusing on its lattice dynamics, electronic structure, and electron-phonon-mediated superconducting properties at ambient pressure. Using density functional theory and the Migdal-Eliashberg formalism, we find an exceptionally strong electron-phonon coupling ($\lambda=2.23$), resulting in a superconducting critical temperature of up to 73.7 K for a Coulomb pseudopotential $\mu^* = 0.1$. Phonon dispersion calculations, complemented by ab initio molecular dynamics simulations, indicate dynamic and thermal stability within the adopted theoretical framework. The electronic structure exhibits a metallic character with substantial contributions from Al- and Na-derived states at the Fermi level. The resulting superconducting gap ratio ($2\Delta(0)/k_B T_c \approx 4.8$) and specific heat jump ($\Delta C/\gamma T_c \approx 2.2$) significantly exceed BCS weak-coupling predictions, highlighting the strong-coupling nature of superconductivity in this hypothetical phase.

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cond-mat.supr-con 2026-04-27

Magnetic switching of superconducting gap creates non-volatile memory

Non-volatile superconducting tunnelling magnetoresistance memory enabled by exchange-field gap engineering

The device achieves nanowatt read power and zero standby dissipation down to 0.25 K with full compatibility for superconducting circuits.

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Scalable, low-dissipation memory operating below 4 K is a critical requirement for superconducting and quantum computing systems. Existing cryogenic memory technologies rely on CMOS derivatives or hybrid architectures that incur leakage, refresh overhead or limited compatibility with superconducting logic. Here we demonstrate a superconducting tunnelling magnetoresistance device that functions as a non-volatile cryogenic memory element across the full superconducting temperature range. By integrating a de Gennes spin valve with a superconducting tunnel junction in a current perpendicular-to-plane geometry, we realise exchange-field control of the superconducting energy gap. This produces two magnetically switchable gap voltages and robust quasiparticle tunnelling magnetoresistance down to 0.25 K.The device operates at millivolt bias with nanowatt-level read power and zero standby dissipation. Its vertical junction architecture and Nb-based materials platform enable compatibility with superconducting logic and scalable cryogenic memory arrays.

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cond-mat.supr-con 2026-04-24

Orbital delocalization suppresses long-range magnetism in nickelate films

3d_{z²} orbital delocalization and magnetic collapse in superconducting (La,Pr)₃Ni₂O_{7-δ} films

Strain and oxygenation shift spectral weight and broaden dd excitations, melting SDW coherence while short-range magnons survive to enable a

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The recent discovery of Ruddlesden--Popper (RP) nickelate thin-film superconductors has opened a new frontier in unconventional superconductivity. Its realization requires both compressive epitaxial strain and highly oxidative growth conditions, yet the microscopic pathway from the parent phase to the superconducting phase remains elusive. Here, X-ray absorption spectra and resonant inelastic X-ray scattering are employed to track this evolution by independently tuning strain and oxygen content in (La,Pr)$_3$Ni$_2$O$_{7-\delta}$ thin films. We uncover a remarkable two-step narrative. First, signatures of delocalization emerge in the same way upon two independent tunings: Spectral weight transfers from a ''Upper Hubbard''-like peak to the hole-like peak associated with O $2p_z$ state, and in parallel, the initially localized Ni $3d_{z^2}$ orbital becomes more itinerant followed by the broadening and weakening of $dd$ orbital excitations. Second, as itinerancy increases, long-range spin-density-wave (SDW) order is suppressed in both intensity and correlation length, indicating direct competition with superconductivity. Yet, short-range magnons persist: they become damped but their bandwidth stays unchanged. Our results paint a coherent picture that both strain and oxygenation drive the RP bilayer nickelates towards the superconducting instability, where the O $2p_z$ and Ni $3d_{z^2}$ orbitals become delocalized. Concomitantly, the long-range magnetic order loses coherence and gets suppressed. These findings establish an orbital-selective route to RP nickelate superconductivity, in which the delocalization of the $2p_z$ and $3d_{z^2}$ orbitals and the robust short-range magnons upon the melting of SDW order are prerequisites, providing strong constraints for theory and the roadmap for designing nickelate superconductors.

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