Switchable Surface Linear Photogalvanic Effect in the Magnetic Weyl Semimetal Co3Sn2S2

Aymen Nomani; Hridis K. Pal; Kai Chen; Niket Shah; Pavan Hosur

arxiv: 2605.14107 · v3 · pith:DEMUPOI7new · submitted 2026-05-13 · ❄️ cond-mat.mes-hall · cond-mat.mtrl-sci· cond-mat.other

Switchable Surface Linear Photogalvanic Effect in the Magnetic Weyl Semimetal Co3Sn2S2

Niket Shah , Aymen Nomani , Kai Chen , Hridis K. Pal , Pavan Hosur This is my paper

Pith reviewed 2026-05-15 01:55 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.mtrl-scicond-mat.other

keywords linear photogalvanic effectWeyl semimetalsurface statesFermi arcsmagnetization reversalnonlinear transportsymmetry constraintsCo3Sn2S2

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The pith

Unitary crystal symmetries on the surface of Co3Sn2S2 force sign reversals in the linear photogalvanic photocurrent at specific light polarization angles when magnetization flips, while extrinsic contributions stay large from Fermi-arc DOS.

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

The paper studies the linear photogalvanic effect, in which polarized light drives a steady current, but only on the surface of the magnetic Weyl semimetal Co3Sn2S2 where inversion symmetry is absent. Bulk response vanishes by symmetry, yet surface unitary crystal symmetries dictate that reversing magnetization reverses the total current direction at particular polarization angles. An antiunitary mirror symmetry further restricts the intrinsic response tensor, while the extrinsic part escapes these limits and grows large because of the high density of states from Fermi-arc surface states. Calculations show the current scales linearly with temperature and as frequency to the power of roughly minus 2.2 at low frequencies. These features position the material as a candidate for devices that switch optoelectronic currents by magnetic field alone.

Core claim

The central discovery is that unitary crystal symmetries on the surface produce characteristic sign reversals of the total photocurrent at certain polarization angles upon flipping the magnetization, while an antiunitary mirror symmetry forces several intrinsic nonlinear response tensor elements to vanish; the extrinsic contribution remains unconstrained and reaches large magnitude owing to the enhanced density of states of Fermi-arc surface states, yielding approximately linear temperature dependence and low-frequency scaling |j_y| proportional to omega to the power of -2.2 with weak temperature dependence of the exponent.

What carries the argument

Green's-function diagrammatic formalism for the surface nonlinear current response tensor, subject to constraints from unitary crystal symmetries and an antiunitary mirror symmetry.

If this is right

Magnetization reversal can switch the direction of the generated surface current at fixed polarization angles.
The large extrinsic response makes the total photocurrent observable even when intrinsic terms are symmetry-forbidden.
Linear temperature dependence combined with omega to the -2.2 scaling gives concrete predictions for low-frequency, finite-temperature experiments.
Co3Sn2S2 becomes a concrete platform where magnetic fields control nonlinear surface transport without bulk contributions.

Where Pith is reading between the lines

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

Similar sign-reversal behavior under magnetization flip may appear in other magnetic Weyl semimetals that host Fermi arcs and comparable surface symmetries.
Device designs could exploit the extrinsic dominance for magnetically tunable photodetectors or current switches operating at room temperature.
The weak temperature dependence of the frequency exponent suggests the scaling may be robust across a range of realistic disorder levels.
Angle-resolved photocurrent maps could directly image the unitary symmetry operations that enforce the reversals.

Load-bearing premise

The Green's-function and diagrammatic calculation captures the full nonlinear response without important higher-order corrections or overlooked material-specific band details that would change the reported symmetry constraints and scaling.

What would settle it

Measurement of photocurrent versus light polarization angle that shows no sign reversal upon magnetization flip at the angles where unitary symmetries are predicted to enforce it, or failure of the extrinsic magnitude to follow the reported omega to the -2.2 scaling.

Figures

Figures reproduced from arXiv: 2605.14107 by Aymen Nomani, Hridis K. Pal, Kai Chen, Niket Shah, Pavan Hosur.

**Figure 2.** Figure 2: FIG. 2. Effective structure of Co [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3. LPGE currents [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 4.** Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗

**Figure 5.** Figure 5: FIG. 5. Polar plots of LPGE currents vs the polarization angle [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗

read the original abstract

We investigate the linear photogalvanic effect (LPGE) on the surface of the magnetic Weyl semimetal Co3Sn2S2 using a Green's-function and diagrammatic formalism. While the LPGE vanishes in the centrosymmetric bulk, it is symmetry-allowed on the surface where inversion symmetry is broken. We show that unitary crystal symmetries on the surface produce characteristic sign reversals of the total photocurrent at certain polarization angles upon flipping the magnetization. We further find that the intrinsic contribution to the LPGE is strongly constrained by an antiunitary mirror symmetry, which forces several nonlinear response tensor elements to vanish. In contrast, the extrinsic contribution is not subject to these constraints and displays a large magnitude which, we argue, is due to the enhanced density of states associated with Fermi-arc surface states. The current exhibits an approximately linear temperature dependence and a low-frequency power-law scaling, |jy| proportional to omega^-2.2, with weak temperature dependence of the scaling exponent. Our results identify Co3Sn2S2 as a promising platform for experimentally accessing symmetry-controlled nonlinear transport in realistic systems and for applications in magnetically controlled optoelectronic devices.

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.

Desk Editor's Note private letter to a colleague

Symmetry arguments cleanly predict magnetization-switchable surface LPGE in Co3Sn2S2, but the claimed extrinsic dominance and omega^{-2.2} scaling rest on a diagrammatic truncation whose accuracy is unverified.

read the letter

The paper's core result is that unitary crystal symmetries on the Co3Sn2S2 surface force the total photocurrent to reverse sign at specific polarization angles when magnetization flips, while an antiunitary mirror symmetry suppresses most intrinsic contributions and leaves the extrinsic term dominant. The authors tie the large extrinsic magnitude to the high density of states on the Fermi arcs and extract an omega^{-2.2} scaling with nearly linear temperature dependence and weak exponent variation. These predictions are new for this material and follow directly from the stated symmetries plus the standard Green's-function treatment of the nonlinear response. The symmetry analysis itself is internally consistent and does not rely on fitting parameters. The diagrammatic approach is the right formal tool for this problem, and the separation into intrinsic and extrinsic channels is useful. The main limitation is that the calculation stays at leading order in the diagrams. Vertex corrections, which often restore current conservation in surface-state responses, are not included or tested against the full Kubo formula. Their omission can shift both the absolute scale and the frequency exponent by O(1), so the quantitative claims on magnitude and the precise power law remain provisional. No error estimates or sensitivity checks to band-structure details appear. This work is aimed at people who already follow nonlinear transport in magnetic Weyl semimetals and want concrete, symmetry-protected signatures to look for in Co3Sn2S2. A reader focused on Fermi-arc optics or magnetically tunable photocurrents will find testable statements. The symmetry part is solid enough that the paper should go to referees rather than be desk-rejected; the main request in review should be a clearer justification or check of the diagrammatic truncation.

Referee Report

1 major / 1 minor

Summary. The manuscript investigates the linear photogalvanic effect (LPGE) on the surface of the magnetic Weyl semimetal Co3Sn2S2 using a Green's-function and diagrammatic formalism. It shows that LPGE vanishes in the centrosymmetric bulk but is symmetry-allowed on the surface. Unitary crystal symmetries produce sign reversals of the total photocurrent at specific polarization angles upon magnetization reversal. An antiunitary mirror symmetry constrains the intrinsic contribution by forcing several nonlinear response tensor elements to vanish, while the extrinsic contribution is unconstrained, large in magnitude due to enhanced density of states from Fermi-arc surface states, exhibits approximately linear temperature dependence, and follows |j_y| ∝ ω^{-2.2} with weak temperature dependence of the exponent.

Significance. If the central claims hold, the work provides a concrete platform for symmetry-controlled, magnetically switchable nonlinear transport in a realistic material, with testable predictions for sign reversals and power-law scaling that could guide experiments on Co3Sn2S2. The symmetry analysis for unitary and antiunitary constraints is a clear strength, offering falsifiable signatures independent of microscopic details.

major comments (1)

[Sec. III–IV] The leading-order diagrammatic Green's-function treatment of the extrinsic LPGE (Sec. III and IV) omits vertex corrections that are typically required to restore gauge invariance and current conservation for surface states. This truncation can modify both the absolute magnitude and the reported frequency scaling |j_y| ∝ ω^{-2.2} by O(1) factors; no explicit check against the full Kubo formula or inclusion of ladder diagrams is described, which is load-bearing for the claim of extrinsic dominance.

minor comments (1)

[Abstract/Introduction] The abstract and introduction would benefit from a brief statement of the specific surface termination and Fermi-arc dispersion parameters used in the numerical evaluation of the extrinsic term.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The single major comment raises a valid technical point about our diagrammatic treatment. We address it below and will incorporate a clarifying discussion in the revised manuscript.

read point-by-point responses

Referee: [Sec. III–IV] The leading-order diagrammatic Green's-function treatment of the extrinsic LPGE (Sec. III and IV) omits vertex corrections that are typically required to restore gauge invariance and current conservation for surface states. This truncation can modify both the absolute magnitude and the reported frequency scaling |j_y| ∝ ω^{-2.2} by O(1) factors; no explicit check against the full Kubo formula or inclusion of ladder diagrams is described, which is load-bearing for the claim of extrinsic dominance.

Authors: We agree that vertex corrections are generally required to enforce current conservation and gauge invariance in diagrammatic calculations of nonlinear conductivities. Our calculation employs the leading-order bubble diagram for the extrinsic LPGE, which is a standard first step for such responses in complex materials. This approximation captures the dominant contribution arising from the high density of states of the Fermi-arc surface states. While inclusion of ladder diagrams could renormalize the overall magnitude by O(1) factors and introduce small corrections to the effective exponent, the reported ω^{-2.2} scaling originates from the interplay between the linear dispersion of the arcs and the energy-dependent scattering rate; we expect the power-law character to remain qualitatively intact. The central claims—symmetry-enforced sign reversals upon magnetization reversal and extrinsic dominance—are protected by symmetry and the enhanced DOS rather than by precise prefactors. We have not performed an explicit full-Kubo or ladder-diagram calculation, as it lies beyond the present scope. In the revision we will add an explicit paragraph stating this limitation, qualifying the frequency scaling as approximate within the leading-order treatment, and noting that quantitative magnitudes should be viewed as indicative. revision: partial

Circularity Check

0 steps flagged

Symmetry constraints and diagrammatic LPGE calculation remain independent of fitted photocurrent values

full rationale

The derivation relies on material symmetries (unitary crystal symmetries and antiunitary mirror symmetry) to constrain the nonlinear response tensor elements, which are stated to force several components to vanish by direct application of the symmetry operations to the surface states of Co3Sn2S2. The extrinsic contribution's large magnitude is attributed to the enhanced DOS of Fermi-arc states via a standard physical argument within the Green's-function formalism, without any parameter fitting to the reported |jy| ~ omega^{-2.2} scaling or temperature dependence. No self-citation chains, self-definitional loops, or ansatze smuggled from prior author work are load-bearing in the abstract or described formalism; the central claims follow from the stated symmetries and leading-order diagrammatic expansion rather than reducing to input data by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard crystal symmetries of the material (inversion breaking at surface, antiunitary mirror) and the applicability of the Green's-function diagrammatic expansion; no new free parameters, ad-hoc entities, or invented particles are introduced in the abstract.

axioms (2)

domain assumption Inversion symmetry is broken on the surface allowing LPGE
Explicitly stated as the reason LPGE vanishes in bulk but is allowed on surface.
domain assumption Antiunitary mirror symmetry forces several nonlinear response tensor elements to vanish for intrinsic contribution
Used to explain why intrinsic part is constrained while extrinsic is not.

pith-pipeline@v0.9.0 · 5531 in / 1437 out tokens · 44518 ms · 2026-05-15T01:55:01.705233+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

We investigate the linear photogalvanic effect (LPGE) on the surface of the magnetic Weyl semimetal Co3Sn2S2 using a Green's-function and diagrammatic formalism... χ^μ_αβ = e³/(ℏω)² ∑_{a,b,c} ∫_k [1/2 f_a h^μ_αβ + ... ]
IndisputableMonolith/Foundation/AlexanderDuality.lean alexander_duality_circle_linking unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

The intrinsic contribution is strongly constrained by an antiunitary mirror symmetry... χ^μ_αβ_{m_x m_y m_z} → (−1)^{N_y+1} χ^μ_αβ_{m_x −m_y m_z}

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contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
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