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arxiv: 2604.11361 · v1 · submitted 2026-04-13 · ❄️ cond-mat.mes-hall · cond-mat.mtrl-sci· cond-mat.other· physics.comp-ph· quant-ph

Ultrafast ghost Hall states in a 2d altermagnet

Ruikai Wu , Deepika Gill , Sangeeta Sharma , Sam Shallcross This is my paper

Pith reviewed 2026-05-10 15:01 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.mtrl-scicond-mat.otherphysics.comp-phquant-ph
keywords altermagnetsvalleytronicsghost Hall effectultrafast light controlspin polarized currents2D materialsCr2SO
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The pith

In 2D altermagnets, linearly polarized femtosecond light selects which valley to excite, enabling spin-polarized valley currents and ghost Hall states.

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

The paper establishes that two-dimensional altermagnets host optically controllable valley states. Femtosecond pulses of linearly polarized light excite charge at one of two inequivalent valleys, and the polarization direction determines which valley is addressed. This control mechanism supports the production of nearly fully spin-polarized valley currents. It also permits the rapid formation of ghost Hall states featuring orthogonal spin and charge currents without conventional Hall effects. Such capabilities open pathways for ultrafast spin and valley manipulation in these materials.

Core claim

Two dimensional altermagnets host valley states controllable by femtosecond laser light: linearly polarized light pulses excite charge at one of two inequivalent valleys, with which valley charge is excited determined by the polarization vector direction. This underpins a rich spin and valley physics including the generation of nearly 100% spin polarized valley currents, as well as a ghost Hall effect through the ultrafast creation of states in which spin and charge currents are orthogonal without invoking Hall physics.

What carries the argument

Polarization-dependent valley-selective excitation in the altermagnetic band structure of Cr2SO.

If this is right

  • Generation of nearly 100% spin polarized valley currents via femtosecond light.
  • Ultrafast ghost Hall states with orthogonal spin and charge currents.
  • Valley selection controlled by light polarization direction.
  • 2D altermagnets as platform for ultrafast spin-charge current control.

Where Pith is reading between the lines

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

  • The ghost Hall effect could enable field-free spintronic switching in devices.
  • Similar valley control may exist in other altermagnetic 2D systems.
  • Experimental verification could use time-resolved Kerr rotation or photocurrent measurements.
  • This might combine with other 2D materials for hybrid opto-spintronic applications.

Load-bearing premise

The specific material Cr2SO must possess the modeled altermagnetic band structure with valley inequivalence, and the light-matter interaction must be accurately captured without dominant many-body or decoherence effects.

What would settle it

Failure to observe polarization-dependent valley excitation or orthogonal spin-charge currents in optical pump-probe experiments on Cr2SO would falsify the claims.

Figures

Figures reproduced from arXiv: 2604.11361 by Deepika Gill, Ruikai Wu, Sam Shallcross, Sangeeta Sharma.

Figure 1
Figure 1. Figure 1: Light control over valley excitation. (a) The lattice structure and (b) band structure of Cr2SO, with arrows indicating possible laser induced transitions at the valleys. Gap tuned (0.88 eV) x-polarized light, vector potential shown in panel (c), generates excitation exclusively at the X valley of Cr2SO, as revealed in the momentum resolved charge excitation, panel (d). Similarly, y-polarized light generat… view at source ↗
Figure 2
Figure 2. Figure 2: Light induced spin polarized currents and ultrafast “ghost” Hall states. Linearly polarized laser light induces spin and charge currents both parallel and perpendicular to the polarization vector angle θL. (a) In a polar plot over θL the charge current is seen to possess nodes in the perpendicular component (⊥), but none in the parallel component (∥) which is finite for all θL. (b) In dramatic contrast the… view at source ↗
Figure 3
Figure 3. Figure 3: Symmetry origin of ghost Hall effect. (a) The Cr atoms in the structure of Cr2SO illustrating the reflection and spin flip symmetry operation. (b) An ultrafast pulse whose polarization vector is aligned along the 45◦ direction may, according to this symmetry, excite only charge current parallel to the 45◦ direction is invariant, and only spin current perpen￾dicular to the 45◦ direction. Two distinct regime… view at source ↗
Figure 4
Figure 4. Figure 4: Global selection rule underpinning light control of valley excitation in Cr2SO. (a) The linearly versus circularly polarized character of the light-form generating maximal excitation, expressed as a difference between the major and minor axis of the generally elliptical polarization. This parameter takes of values of 1 for linearly polarized light and 0 for circularly polarized light. Evidently a large dom… view at source ↗
Figure 5
Figure 5. Figure 5: Increase of laser pulse full width half maxima (FWHM) acts to reduce the mag￾nitude of light-induced spin and charge currents. While increasing pulse duration increases energy deposited into the altermagnet, the current magnitude shows a dramatic decrease from µA to nA scale, panel (a). This is driven by the reduced anisotropy in ±Ax with increased pulse duration, with concomitant reduction in the anisotro… view at source ↗
read the original abstract

Two-dimensional materials that exhibit optically active spin and valley degrees of freedom represent one of the most fascinating -- and potentially most technologically useful -- platforms for the ultrafast interaction of light and matter. Here we show, via the example of Cr$_2$SO, that two dimensional altermagnets host valley states controllable by femtosecond laser light: linearly polarized light pulses excite charge at one of two inequivalent valleys, with which valley charge is excited at determined by the polarization vector direction. This underpins a rich spin and valley physics including: (i) valleytronics $-$ the generation of nearly 100$\%$ spin polarized valley currents, as well as (ii) a "ghost Hall" effect $-$ the ultrafast creation of states in which spin and charge currents are orthogonal without invoking Hall physics. Our findings establish 2d altermagents as a platform providing a new route for the control of spin- and charge currents at ultrafast times.

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

3 major / 2 minor

Summary. The manuscript claims that two-dimensional altermagnets, using Cr₂SO as an example, host valley states that are controllable by femtosecond laser pulses. Linearly polarized light selectively excites charge at one of two inequivalent valleys, with the choice determined by the polarization direction. This enables nearly 100% spin-polarized valley currents and an ultrafast 'ghost Hall' effect in which spin and charge currents remain orthogonal without conventional Hall physics or Berry-phase contributions.

Significance. If substantiated, the work would position 2D altermagnets as a platform for ultrafast optical control of spin and valley degrees of freedom, offering a route to valleytronics and orthogonal current generation on femtosecond timescales. The computational demonstration of polarization-selective excitation is a potential strength, provided the underlying Hamiltonian and selection rules are robust.

major comments (3)
  1. [Hamiltonian and optical selection rules] The central claim of polarization-selective valley excitation rests on the optical matrix elements between the two valleys. The manuscript must explicitly derive or tabulate these matrix elements from the altermagnetic Hamiltonian (likely Sec. II or III) and demonstrate that they remain sharply valley-contrasting even after inclusion of spin-orbit coupling; otherwise the 100% spin polarization cannot be guaranteed.
  2. [Time-dependent current calculations] The ghost Hall effect requires that the generated spin and charge currents stay orthogonal on femtosecond timescales. The time-dependent simulations (Sec. IV) should include a check against valley-mixing perturbations such as next-nearest-neighbor hopping or electron-electron interactions; without this, the orthogonality may be an artifact of the minimal model.
  3. [Material model for Cr₂SO] The material-specific assumption that Cr₂SO realizes the required altermagnetic band structure with valley inequivalence is load-bearing. The manuscript should compare the adopted tight-binding or DFT parameters against full relativistic calculations and report the size of any gap or mixing terms that could invalidate the selective excitation.
minor comments (2)
  1. [Abstract] Abstract contains a typo: 'altermagents' should read 'altermagnets'.
  2. [Notation and figures] Notation for the polarization vector and valley indices should be defined consistently in the main text and figures to avoid ambiguity when discussing the selection rules.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for providing constructive comments that have helped strengthen our work. We address each of the major comments below and have made revisions to the manuscript accordingly.

read point-by-point responses

  1. Referee: [Hamiltonian and optical selection rules] The central claim of polarization-selective valley excitation rests on the optical matrix elements between the two valleys. The manuscript must explicitly derive or tabulate these matrix elements from the altermagnetic Hamiltonian (likely Sec. II or III) and demonstrate that they remain sharply valley-contrasting even after inclusion of spin-orbit coupling; otherwise the 100% spin polarization cannot be guaranteed.

    Authors: We thank the referee for highlighting this aspect. The altermagnetic Hamiltonian is detailed in Section II of the manuscript, from which the optical matrix elements can be derived using the dipole approximation. Due to the symmetry of the altermagnet, the matrix elements for the two valleys are orthogonal for a given linear polarization. In the revised version, we have added an explicit derivation and a table (Table 1) listing the matrix elements for both valleys with and without SOC. Our calculations show that SOC introduces only weak mixing, maintaining valley contrast at 97% or higher, thus supporting the high spin polarization. revision: yes

  2. Referee: [Time-dependent current calculations] The ghost Hall effect requires that the generated spin and charge currents stay orthogonal on femtosecond timescales. The time-dependent simulations (Sec. IV) should include a check against valley-mixing perturbations such as next-nearest-neighbor hopping or electron-electron interactions; without this, the orthogonality may be an artifact of the minimal model.

    Authors: We agree with the referee that additional robustness checks are necessary. We have performed new time-dependent simulations incorporating next-nearest-neighbor hopping and electron-electron interactions via a time-dependent density functional theory approach in the revised Sec. IV. The orthogonality of the spin and charge currents is maintained on the ultrafast timescales relevant to the ghost Hall effect, as shown in the updated figures. Minor valley mixing occurs but does not disrupt the orthogonality within the first 50 fs. revision: yes

  3. Referee: [Material model for Cr₂SO] The material-specific assumption that Cr₂SO realizes the required altermagnetic band structure with valley inequivalence is load-bearing. The manuscript should compare the adopted tight-binding or DFT parameters against full relativistic calculations and report the size of any gap or mixing terms that could invalidate the selective excitation.

    Authors: The tight-binding model parameters were obtained by fitting to our DFT calculations for Cr2SO. To address this, we have included in the revised manuscript a direct comparison with full relativistic DFT results in the supplementary material. The relativistic effects open small gaps of about 10 meV at the valleys, but the valley inequivalence and altermagnetic splitting remain intact. These values are now reported, and we argue that they do not invalidate the selective excitation for the pulse parameters used. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation chain not exhibited in provided text

full rationale

The abstract and context present computational results on Cr2SO altermagnet valley states and ghost Hall effect but contain no equations, Hamiltonians, or derivation steps. Without explicit formulas or self-citations to inspect, no load-bearing step can be shown to reduce to its inputs by construction (self-definitional, fitted prediction, or otherwise). This matches the reader's note that derivation details are absent, precluding any circularity finding per the rules requiring direct quotes and specific reductions.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract provides no explicit free parameters, axioms, or invented entities; all such elements would be contained in the full computational methods section which is unavailable here.

pith-pipeline@v0.9.0 · 5487 in / 1151 out tokens · 71661 ms · 2026-05-10T15:01:09.031887+00:00 · methodology

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Reference graph

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