arxiv: 2604.28105 · v2 · submitted 2026-04-30 · ❄️ cond-mat.mtrl-sci · cond-mat.other

Recognition: unknown

From Narrow-gap Semiconductor to Metallic Altermagnet: Optical Fingerprints of Co-Doped FeSb2

R. Mathew Roy , M. Povolotskiy , J. Kirschke , C. Prange , Y. Xia , V. Sundaramurthy , P. Puphal , M. Pinteric

show 6 more authors

M. van de Loo A. Kreyssig T. Zhang A. E. B\"ohmer M. Dressel M. Wenzel

Authors on Pith no claims yet

Pith reviewed 2026-05-08 02:54 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci cond-mat.other

keywords FeSb2altermagnetismcobalt dopingoptical conductivityinfrared spectroscopydensity functional theoryelectron-phonon couplingspin splitting

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The pith

Moderate cobalt doping transforms FeSb2 from a narrow-gap semiconductor into a metallic altermagnet stable to room temperature.

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

This paper establishes that substituting about 15 percent cobalt into FeSb2 changes the material from an insulator with a small energy gap into a metal that exhibits altermagnetic ordering, which holds at room temperature. The evidence comes from infrared light absorption showing new transitions at low energies around 0.1 electron volts that strengthen with more cobalt, which density functional theory traces directly to spin splitting caused by the altermagnetic arrangement. Doping also alters the vibration modes of the lattice, producing asymmetric Fano shapes that point to stronger interactions between electrons and phonons while the spin symmetry stays altermagnetic. A reader would care because finding such a tunable metallic altermagnet opens routes to study how spin, charge, and lattice degrees of freedom interact in a simple compound without needing extreme conditions.

Core claim

The authors demonstrate that moderate Co substitution in FeSb2 induces a metallic altermagnetic phase with spin-split bands of non-relativistic origin that produce observable interband transitions near 0.1 eV in optical conductivity, while SOC effects remain small near the Fermi level, and phonon spectra reflect enhanced electron-phonon coupling and local symmetry breaking without altering the altermagnetic spin symmetry, all persisting to room temperature.

What carries the argument

Altermagnetic spin ordering, which generates non-relativistic spin-split bands of about 0.2 eV responsible for the doping-induced low-energy optical transitions near 0.1 eV.

Load-bearing premise

The low-energy optical transitions arise solely from altermagnetic spin ordering as predicted by DFT calculations, rather than from disorder or other doping-induced electronic changes.

What would settle it

Detection of the 0.1 eV transitions in a Co-doped sample lacking altermagnetic order, or their absence in DFT models that remove spin splitting while keeping other doping effects, would disprove the claimed origin.

read the original abstract

The realization of bulk metallic altermagnetism has remained elusive despite the growing number of candidate materials. Here, we present evidence that moderate cobalt substitution ($\sim$15%) drives the correlated narrow-gap semiconductor FeSb$_2$ into a metallic altermagnetic state persisting up to room temperature. The infrared optical conductivity reveals low-energy interband transitions near 0.1 eV that emerge upon doping and grow with Co concentration. Density functional theory calculations show that these transitions originate exclusively from altermagnetic spin ordering, with spin split bands ($\sim$0.2 eV) of non-relativistic origin, together with spin-orbit coupling induced band splitting of the order of $\sim$5 meV near the Fermi level. Co substitution further leads to Fano lineshapes and mode mixing in the infrared-active phonons, reflecting enhanced electron-phonon coupling and local inversion symmetry breaking, while leaving the altermagnetic spin symmetry intact. Our results establish carrier-tuned FeSb$_2$ as a platform for exploring metallic $d$-wave altermagnetism and its coupling to lattice degrees of freedom.

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

Co doping makes FeSb2 metallic with new ~0.1 eV optical transitions that DFT attributes to altermagnetic splitting, but the claim rests on that attribution without direct magnetic confirmation.

read the letter

The core result is that ~15% Co substitution turns the narrow-gap semiconductor FeSb2 metallic while producing low-energy interband transitions around 0.1 eV that grow with doping. DFT calculations tie those transitions to non-relativistic spin splitting of ~0.2 eV from altermagnetic order, plus smaller SOC effects. They also report Fano lineshapes and phonon mode mixing that point to stronger electron-phonon coupling and local symmetry breaking, all while keeping the altermagnetic spin symmetry intact. That combination of doping-tuned optics and phonon changes is the concrete new piece; it gives an experimental handle on a metallic d-wave altermagnet candidate that was not shown before in this system. The optical spectra and the DFT band structures look internally consistent on their own terms, and the temperature persistence up to room temperature is stated clearly from the data. The soft spot is exactly where the stress-test flagged it: the paper asserts the 0.1 eV feature originates exclusively from the altermagnetic order, yet the only support is the DFT match. No neutron scattering, spin-resolved ARPES, or symmetry-selective transport is presented to rule out impurity bands, disorder-induced absorption, or simple band-filling effects from the extra carriers. Without those checks the optical fingerprint remains plausible but not unique. The phonon analysis is more straightforward and less dependent on the magnetic interpretation. This paper is aimed at the altermagnetism community and at people who use infrared spectroscopy on correlated materials. A reader looking for a new platform to study metallic altermagnetism coupled to the lattice will find usable data and a clear starting point, even if the magnetic assignment needs more work. It is worth sending to peer review because the material system and the optical approach are fresh enough that referees can tighten the interpretation without starting from zero.

Referee Report

2 major / 2 minor

Summary. The manuscript claims that moderate (~15%) cobalt substitution transforms the correlated narrow-gap semiconductor FeSb2 into a metallic altermagnet with room-temperature persistence. Infrared optical conductivity shows doping-induced low-energy interband transitions near 0.1 eV that DFT calculations attribute exclusively to non-relativistic altermagnetic spin splitting (~0.2 eV) plus small SOC-induced splittings (~5 meV) near the Fermi level. Co doping also induces Fano lineshapes and mode mixing in infrared-active phonons, interpreted as enhanced electron-phonon coupling with preserved altermagnetic symmetry.

Significance. If the central attribution holds, the work would identify a tunable platform for metallic d-wave altermagnetism and its coupling to phonons. The optical-DFT comparison offers a potential spectroscopic fingerprint, and the absence of free parameters in the DFT analysis is a positive feature. However, the significance is tempered by the lack of independent experimental confirmation of the alternating spin texture.

major comments (2)

[Abstract and DFT calculations] Abstract and the DFT comparison section: the statement that the ~0.1 eV interband transitions 'originate exclusively from altermagnetic spin ordering' is load-bearing for the central claim but rests on visual comparison of DFT bands to the measured conductivity without quantitative fitting, error bars on transition energies, or explicit exclusion of alternative origins such as impurity bands or disorder-induced mid-gap absorption.
[Magnetic characterization] Magnetic characterization section: no direct experimental signature of altermagnetic order (neutron scattering, spin-resolved ARPES, or symmetry-selective transport) is reported. The optical fingerprint therefore remains the sole evidence, leaving the metallic altermagnet interpretation vulnerable to non-magnetic doping effects.

minor comments (2)

[Figure 3] Figure 3 (optical conductivity spectra): the low-energy feature near 0.1 eV should include a quantitative decomposition (e.g., Drude-Lorentz fit parameters with uncertainties) to allow direct comparison with the DFT-predicted ~0.2 eV splitting.
[Phonon section] Phonon analysis: the claim that altermagnetic spin symmetry remains intact despite local inversion-symmetry breaking requires an explicit symmetry table or calculation showing which phonon modes are allowed under the altermagnetic point group.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the careful and constructive review. We address the two major comments below, providing additional quantitative analysis where feasible and acknowledging the limitations of indirect evidence for altermagnetic order.

read point-by-point responses

Referee: [Abstract and DFT calculations] Abstract and the DFT comparison section: the statement that the ~0.1 eV interband transitions 'originate exclusively from altermagnetic spin ordering' is load-bearing for the central claim but rests on visual comparison of DFT bands to the measured conductivity without quantitative fitting, error bars on transition energies, or explicit exclusion of alternative origins such as impurity bands or disorder-induced mid-gap absorption.

Authors: We agree that the original presentation relied primarily on visual comparison and that a more quantitative treatment strengthens the attribution. In the revised manuscript we have added a multi-Lorentzian decomposition of the measured conductivity, reporting fitted transition energies with uncertainties that are now tabulated against the DFT-predicted interband energies (including both the ~0.2 eV altermagnetic splitting and the smaller SOC contributions). We have also inserted a dedicated paragraph that explicitly considers impurity-band and disorder-induced absorption scenarios. These alternatives are inconsistent with (i) the systematic blue-shift and intensity growth of the feature with Co concentration, (ii) its persistence to room temperature, and (iii) the absence of analogous absorption in non-magnetic reference calculations. To reflect the strength of the evidence we have changed the abstract wording from “originate exclusively” to “originate primarily from altermagnetic spin ordering, as supported by the DFT comparison.” revision: yes
Referee: [Magnetic characterization] Magnetic characterization section: no direct experimental signature of altermagnetic order (neutron scattering, spin-resolved ARPES, or symmetry-selective transport) is reported. The optical fingerprint therefore remains the sole evidence, leaving the metallic altermagnet interpretation vulnerable to non-magnetic doping effects.

Authors: We recognize that neutron scattering, spin-resolved ARPES, or symmetry-selective transport would constitute more direct confirmation of the alternating spin texture. Such measurements lie outside the present experimental scope, primarily because of the small magnetic moments and the difficulty of obtaining sufficiently large single crystals. Nevertheless, the optical data provide a spectroscopic fingerprint whose energy scale, doping dependence, and temperature robustness are reproduced only when the non-relativistic altermagnetic splitting is included in the DFT. We have expanded the discussion section to compare the measured conductivity against both altermagnetic and non-magnetic metallic reference calculations, demonstrating that simple carrier doping without spin splitting fails to generate the observed ~0.1 eV feature. While we agree the evidence remains indirect, the optical-DFT agreement constitutes a non-trivial consistency check that is difficult to reconcile with purely non-magnetic scenarios. The revised text now explicitly flags the desirability of future direct probes. revision: partial

standing simulated objections not resolved

Direct experimental confirmation of the alternating spin texture via neutron scattering or spin-resolved ARPES is not available in the current study.

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper reports experimental infrared conductivity data on Co-doped FeSb2 showing emergent ~0.1 eV interband transitions, then compares these to standard DFT band-structure calculations performed in an assumed altermagnetic configuration. The attribution of the transitions to non-relativistic spin splitting is a direct output of those calculations rather than a quantity defined in terms of itself or a fitted parameter that is subsequently relabeled as a prediction. No load-bearing self-citations, ansatz smuggling, or uniqueness theorems imported from prior author work appear in the provided text; the chain from doping to metallic state to optical fingerprints rests on independent experimental spectra and first-principles modeling.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The interpretation relies on the assumption that DFT correctly captures non-relativistic altermagnetic band splitting and that observed phonon changes do not alter the underlying spin symmetry; no new entities are postulated.

axioms (1)

domain assumption Density functional theory calculations accurately reproduce the altermagnetic spin splitting and associated optical transitions in the doped compound.
Invoked to assign the 0.1 eV features exclusively to altermagnetic order.

pith-pipeline@v0.9.0 · 5570 in / 1214 out tokens · 29316 ms · 2026-05-08T02:54:19.786300+00:00 · methodology

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Works this paper leans on

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From Narrow-gap Semicon- ductor to Metallic Altermagnet: Optical Fingerprints of Co-doped FeSb2

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