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arxiv: 2509.05620 · v2 · submitted 2025-09-06 · ❄️ cond-mat.mes-hall · cond-mat.mtrl-sci· cond-mat.other

Quantization of spin circular photogalvanic effect in altermagnetic Weyl semimetals

Pith reviewed 2026-05-18 18:22 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.mtrl-scicond-mat.other
keywords altermagnetsWeyl semimetalsphotogalvanic effectspin currentssymmetry groupsoptical responsemagnetic materials
0
0 comments X

The pith

Altermagnetic Weyl semimetals generate a quantized spin current under circularly polarized light, an effect forbidden by symmetry in antiferromagnets.

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

The paper establishes that a spin-current version of the circular photogalvanic effect can be quantized in certain Weyl semimetals. This response arises only when the magnetic order belongs to the altermagnetic class. Antiferromagnets lack the symmetry that permits the necessary second-order spin response. By using spin point groups to classify allowed responses, the authors show which crystal symmetries support the effect. They then build a model Hamiltonian and perform first-principles calculations to find a concrete material where the Weyl points and the quantized response coexist.

Core claim

In altermagnetic Weyl semimetals the spin circular photogalvanic effect is quantized because the spin space group symmetries both enforce Weyl points and allow a second-order spin current that is linear in the light intensity and circular polarization. This quantization is unique to altermagnets; the corresponding spin point group of antiferromagnets forbids the response altogether.

What carries the argument

Spin point group classification of second-order spin-current responses, which reveals that the quantized circular photogalvanic effect for spin is permitted solely in altermagnets.

If this is right

  • The classification identifies all symmetry-allowed second-order spin responses across magnetic groups.
  • A symmetry-guided tight-binding model explicitly realizes the quantized spin photocurrent.
  • First-principles calculations locate Weyl crossings in a specific altermagnetic material candidate.

Where Pith is reading between the lines

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

  • Detection of this spin current could serve as a direct optical readout of altermagnetic order.
  • Similar quantized responses might appear in other frequency regimes or under different light polarizations.
  • The approach could generalize to predict new spintronic phenomena in other symmetry-broken phases.

Load-bearing premise

That the spin point group analysis exhaustively lists every possible second-order term in the spin current without omissions from higher-order or extrinsic contributions.

What would settle it

A direct measurement showing that the spin photocurrent in an altermagnetic Weyl semimetal is either unquantized or absent when the light is circularly polarized.

Figures

Figures reproduced from arXiv: 2509.05620 by Hiroki Yoshida, Jan Priessnitz, Libor \v{S}mejkal, Shuichi Murakami.

Figure 1
Figure 1. Figure 1: FIG. 1. Schematic illustration of the quantized spin circular [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Calculation of the conductivity tensor for a tight [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: (c) shows the band structure along the same path as [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
read the original abstract

We theoretically predict a spin-current analog of the quantized circular photogalvanic effect in Weyl semimetals. This phenomenon is forbidden in antiferromagnets by symmetry but uniquely allowed in altermagnets, highlighting a novel and intrinsic characteristic of altermagnetism. To systematically explore second-order spin current responses, we classify all symmetry-allowed responses based on spin point groups. Furthermore, we provide a comprehensive classification of altermagnetic Weyl semimetals by identifying spin space groups that host symmetry-enforced Weyl points. Utilizing this classification, we construct a symmetry-guided tight-binding model and confirm our predictions. Finally, we identify Weyl crossings in a material candidate via first-principle calculations. Our work unveils a distinctive optical response of altermagnets, paving the way for a new frontier in altermagnetism.

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 paper predicts a quantized spin-current analog of the circular photogalvanic effect (CPGE) in altermagnetic Weyl semimetals. This response is claimed to be forbidden by symmetry in antiferromagnets but uniquely allowed in altermagnets. The authors classify all symmetry-allowed second-order spin current responses using spin point groups, provide a classification of altermagnetic Weyl semimetals via spin space groups, construct a symmetry-guided tight-binding model to demonstrate the quantization, and identify Weyl crossings in a material candidate using first-principles calculations.

Significance. If the quantization and the symmetry-based distinction hold, the result would establish a distinctive optical signature of altermagnetism that is absent in conventional antiferromagnets, with potential implications for spintronic detection of altermagnetic order. The symmetry-guided model construction and first-principles support are positive elements that strengthen the work when the central symmetry argument is independently verified.

major comments (1)
  1. The central claim that the quantized spin CPGE is forbidden in antiferromagnets but permitted only in altermagnets rests on the spin point group classification of second-order spin current responses. The manuscript does not report an independent cross-check against full magnetic space group tables or the Bilbao Crystallographic Server to confirm that the relevant tensor components identically vanish under AFM symmetries. If any operation retained in the complete magnetic group is missed by the spin-group reduction, the prohibition fails and the uniqueness to altermagnets does not hold.
minor comments (1)
  1. The distinction between spin point groups and spin space groups is introduced in the abstract and classification sections but could be explained more explicitly in the main text for readers outside the immediate subfield.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading and constructive feedback on our manuscript. We address the major comment regarding the symmetry classification below and outline the revisions we will make to strengthen the presentation.

read point-by-point responses
  1. Referee: The central claim that the quantized spin CPGE is forbidden in antiferromagnets but permitted only in altermagnets rests on the spin point group classification of second-order spin current responses. The manuscript does not report an independent cross-check against full magnetic space group tables or the Bilbao Crystallographic Server to confirm that the relevant tensor components identically vanish under AFM symmetries. If any operation retained in the complete magnetic group is missed by the spin-group reduction, the prohibition fails and the uniqueness to altermagnets does not hold.

    Authors: We thank the referee for raising this important point on verification. Spin point groups provide the natural and minimal symmetry framework for classifying spin-dependent responses in altermagnets, as they encode the coupled spin-lattice symmetries that distinguish altermagnetism from conventional antiferromagnetism. In the latter, the presence of effective time-reversal symmetry (arising from combined operations in the full magnetic space group) forces the relevant second-order spin-current tensor components to vanish, which is already captured by the spin-point-group reduction used in our classification. Nevertheless, to directly address the concern and eliminate any ambiguity, we will add an explicit cross-check in the revised manuscript. This will include representative examples drawn from magnetic space group tables and the Bilbao Crystallographic Server, confirming that the quantized spin CPGE tensor is identically zero under standard AFM symmetries while remaining allowed under the spin space groups of altermagnets. These additions will be placed in a new subsection or supplementary note and will not alter the central conclusions or the tight-binding and first-principles results. revision: yes

Circularity Check

0 steps flagged

Symmetry classification and explicit model construction yield independent verification of the quantized spin CPGE

full rationale

The derivation begins with a symmetry classification of second-order spin-current responses under spin point groups, followed by identification of symmetry-enforced Weyl points in altermagnetic spin space groups. A tight-binding model is then constructed to respect these symmetries, and the spin CPGE is computed directly from it, producing the quantized value as a consequence of the allowed tensor components and topological properties. First-principles calculations independently locate candidate Weyl crossings. No equation or result reduces by construction to a fitted parameter, self-citation chain, or renamed input; the central claim follows from standard group-theoretic enumeration and explicit Hamiltonian diagonalization rather than tautological re-expression of the premises.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper rests on spin point group classifications and symmetry-enforced Weyl points whose validity is assumed from domain knowledge; no explicit free parameters or invented entities are stated in the abstract.

axioms (1)
  • domain assumption Spin point groups classify all symmetry-allowed second-order spin current responses.
    Invoked to systematically explore responses and identify those unique to altermagnets.

pith-pipeline@v0.9.0 · 5692 in / 1282 out tokens · 55614 ms · 2026-05-18T18:22:49.714913+00:00 · methodology

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