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arxiv: 2605.06150 · v1 · submitted 2026-05-07 · ❄️ cond-mat.supr-con

Recognition: unknown

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

Guowei Liu, Hailang Qin, Hanbin Deng, Jia-Xin Yin, Mu-Wei Gao, Xiao-Yu Yan, Yuanyuan Zhao

Authors on Pith no claims yet

Pith reviewed 2026-05-08 04:21 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con
keywords Josephson scanning tunneling microscopykagome superconductorspoint-contact regimezero-bias conductanceseries resistancepair density wave
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The pith

Zero-bias conductance saturates in deep Josephson point contacts of kagome superconductors

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates Josephson scanning tunneling microscopy on kagome superconductors by driving the junction into the deep point-contact regime, where the normal-state resistance is only 0.15 times h over 2e squared, about 2 kiloohms. It finds that the zero-bias conductance, which signals the Josephson current, stops following the usual quadratic increase with normal-state conductance. The conductance levels off instead, and this leveling is traced to series resistance present in the measurement circuit. The observation warns that zero-bias saturations or apparent quantizations seen in experiments on Majorana zero modes or similar phenomena could be artifacts when the tip-sample resistance is extremely low. The authors also locate a sweet spot in coupling strength where the method can still serve as an atomic-scale tool for pair-density wave studies in low-temperature superconductors like the AV3Sb5 family.

Core claim

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.

What carries the argument

Deep point-contact Josephson scanning tunneling microscopy on kagome superconductors, where zero-bias conductance saturation is produced by external series resistance rather than intrinsic junction behavior.

If this is right

  • The expected quadratic scaling of zero-bias conductance with normal-state conductance fails at very low resistances.
  • Saturation of zero-bias conductance is explained by circuit series resistance.
  • Interpretations of zero-bias saturation as evidence for exotic states require caution at small junction resistances.
  • An optimum regime exists for JSTM to probe pair-density wave states in low-Tc kagome superconductors.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Independent calibration of circuit resistance in JSTM setups could prevent misattribution of saturation effects.
  • Similar saturation phenomena may occur in point-contact spectroscopy of other superconductors when pushed to strong coupling.
  • Future experiments on Majorana candidates should verify that observed quantizations are not limited by external resistances.

Load-bearing premise

The saturation of zero-bias conductance is caused exclusively by series resistance in the external circuit rather than by additional junction or material effects that appear only in the deep point-contact regime.

What would settle it

Directly measuring or subtracting the series resistance and checking whether the zero-bias conductance saturation disappears or the quadratic dependence is restored at low junction resistances.

read the original abstract

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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript reports Josephson scanning tunneling microscopy experiments on kagome superconductors pushed into the deep point-contact regime with normal-state junction resistances down to ~2 kΩ. It claims that the zero-bias conductance deviates strongly from the expected quadratic dependence on normal-state conductance and exhibits saturation, which the authors attribute to series resistance in the measurement circuit. The work cautions against misinterpreting such saturation in studies of exotic states such as Majorana zero modes and identifies an optimum regime for JSTM probing of pair-density wave states in low-Tc AV3Sb5 materials.

Significance. If the series-resistance interpretation is independently validated, the results provide a practical cautionary framework for low-resistance Josephson junctions that could affect interpretations in topological superconductivity and related fields. The identification of an optimal JSTM regime for low-Tc kagome systems may also enable more reliable atomic-scale studies of pair-density waves.

major comments (1)
  1. [Discussion of saturation (results section)] The attribution of zero-bias conductance saturation exclusively to external series resistance R_s is load-bearing for the central claim. The manuscript must demonstrate that R_s is obtained independently (e.g., from the high-bias I-V slope above the gap or a separate calibration) and then quantitatively predicts the observed saturation level. If R_s is instead inferred by fitting the saturation itself, the argument is circular and cannot exclude intrinsic junction or material effects that appear only at small R_n, such as local heating, gap suppression, or enhanced multiple Andreev processes.
minor comments (1)
  1. [Abstract] The abstract notation '0.15 h/2e^2 ≃ 2 kΩ' would benefit from a brief parenthetical reminder of the quantum resistance value for broader accessibility.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for identifying the need to strengthen the evidence for the series-resistance interpretation. We have revised the results section to explicitly demonstrate the independent determination of R_s and its quantitative predictive power.

read point-by-point responses

  1. Referee: [Discussion of saturation (results section)] The attribution of zero-bias conductance saturation exclusively to external series resistance R_s is load-bearing for the central claim. The manuscript must demonstrate that R_s is obtained independently (e.g., from the high-bias I-V slope above the gap or a separate calibration) and then quantitatively predicts the observed saturation level. If R_s is instead inferred by fitting the saturation itself, the argument is circular and cannot exclude intrinsic junction or material effects that appear only at small R_n, such as local heating, gap suppression, or enhanced multiple Andreev processes.

    Authors: We agree that an independent determination of R_s is essential to avoid circularity. In the revised manuscript we have added a new paragraph and supplementary figure that extract R_s directly from the linear slope of the I-V curve at high bias (well above the gap voltage), where the junction is in the normal state and the current is limited only by the external circuit resistance. Using this independently measured R_s, we then compute the expected zero-bias conductance saturation level via the standard voltage-divider relation G_ZB = G_n / (1 + G_n R_s) and show that it reproduces the observed plateau without any additional fitting parameters. We also include a brief discussion ruling out local heating (currents remain below 100 nA) and gap suppression (the gap edge remains visible in dI/dV spectra at the same low R_n). revision: yes

Circularity Check

0 steps flagged

No circularity in experimental observations and attribution

full rationale

The paper reports direct experimental measurements of zero-bias conductance versus normal-state conductance in Josephson point contacts on kagome superconductors, documenting a deviation from quadratic scaling and a saturation effect. These quantities are measured outputs, not quantities derived from a mathematical chain or fitted parameters that reduce to the inputs by construction. The attribution of saturation to series resistance is presented as an explanatory model based on circuit elements, without evidence in the provided text of R_s being extracted solely from the saturation data in a self-referential loop. No self-citations, ansatzes, or uniqueness theorems are invoked as load-bearing steps. The work is self-contained as an observational study.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper rests on standard Josephson tunneling theory for the expected quadratic dependence and on basic circuit analysis for the series-resistance explanation; no new free parameters, ad-hoc axioms, or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption Josephson current in the tunneling regime exhibits quadratic dependence of zero-bias conductance on normal-state conductance
    This is the baseline behavior from which the paper reports deviation upon entering the deep point-contact regime.

pith-pipeline@v0.9.0 · 5538 in / 1270 out tokens · 68398 ms · 2026-05-08T04:21:46.799920+00:00 · methodology

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Reference graph

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    W. Chen, W. Ren, N. Kennedy, M. H. Hamidian, S. Uchida, H. Eisaki , P. D. Johnson, S. O'Mahony, and J. C. Seamus Davis, Identification of a nematic pair density wave state in Bi 2Sr2CaCu2O8+x, Proc. Natl. Acad. Sci. U.S.A. 119, e2206481119 (2022). 9 Figures FIG. 1 dI/dV spectra acquired on the Cs(V 0.86Ta0.14)3Sb5 sample using a non-superconducting and a ...

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