Magneto-optic phonon resonances in magnetic topological EuCd2As2 via helical Raman spectroscopy

Chandra Shekhar; Chee Wei Wong; Claudia Felser; Ioannis Petrides; Jin Ho Kang; Kai-Chi Chang; Liangbo Liang; Prineha Narang; Subhajit Roychowdhury

arxiv: 2605.26073 · v1 · pith:QAJVZ7T4new · submitted 2026-05-25 · ❄️ cond-mat.mtrl-sci

Magneto-optic phonon resonances in magnetic topological EuCd2As2 via helical Raman spectroscopy

Jin Ho Kang , Liangbo Liang , Ioannis Petrides , Kai-Chi Chang , Subhajit Roychowdhury , Chandra Shekhar , Claudia Felser , Prineha Narang

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Chee Wei Wong

This is my paper

Pith reviewed 2026-06-29 21:13 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci

keywords Raman spectroscopyspin-phonon couplingEuCd2As2magneto-optic effecttopological materialcircular polarizationferromagneticphonon resonances

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The pith

Spin-phonon coupling enhances Raman intensities in ferromagnetic EuCd2As2 below the Curie temperature.

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

The paper reports cryogenic Raman spectroscopy on FM-EuCd2As2 crystals using circular polarization. It attributes anomalous Raman intensity enhancements below Tc = 26 K to spin-phonon coupling. Magneto-helical Raman spectroscopy distinguishes A-mode peaks through the magneto-optic effect, with the 12.5 meV peak reaching 60% degree of circular polarization at 4.2 K that saturates. The AFM phase below TN = 9 K shows negligible effects due to zero net magnetization. This contributes to understanding phonon dynamics and magnetism-topology interplay in this Weyl candidate.

Core claim

We attribute the anomalous enhancements in Raman intensities below the Curie temperature to spin-phonon coupling. Furthermore, we see that A-mode peaks can be distinguished by magneto-helical Raman spectroscopy through the magneto-optic effect and that the degree of circular polarization (DCP) of 12.5 meV peak reaches 60% at 4.2 K and becomes saturated. We also examine AFM-EuCd2As2 below Néel temperature to compare with FM-EuCd2As2, but we hardly observe spin-phonon coupling and find negligible DCP values due to almost zero net magnetization.

What carries the argument

magneto-helical Raman spectroscopy using circular polarization configurations to reveal the magneto-optic effect

Load-bearing premise

The Raman intensity enhancements below the Curie temperature are due to spin-phonon coupling rather than other temperature-dependent phenomena.

What would settle it

Raman measurements above the Curie temperature or on a non-magnetic reference compound showing the same intensity enhancements and DCP values would indicate the effects are not tied to the ferromagnetic order.

read the original abstract

EuCd2As2 materials have two magnetic ordering states: antiferromagnetic (AFM) and ferromagnetic (FM) when their chemical tunability is utilized. While AFM-EuCd2As2 has a nonzero magnetoelectric response due to its symmetry breaking with spin configuration, FM-EuCd2As2 is an ideal candidate for studies of Weyl physics because of its minimum number of Weyl points with opposite chirality. In this article, we examine cryogenic low-frequency Raman spectroscopy of phonon modes in FM-EuCd2As2 crystals using circular polarization configurations, with support from density functional theory calculations, and investigate in-plane magneto-anisotropy by linear polarization configuration below the Curie temperature (Tc = 26 K). We attribute the anomalous enhancements in Raman intensities below the Curie temperature are due to spin-phonon coupling. Furthermore, we see that A-mode peaks can be distinguished by magneto-helical Raman spectroscopy through the magneto-optic effect and that the degree of circular polarization (DCP) of 12.5 meV peak reaches 60% at 4.2 K and becomes saturated. We also examine AFM-EuCd2As2 below N\'eel temperature (TN = 9 K) to compare with FM-EuCd2As2, but we hardly observe spin-phonon coupling and find negligible DCP values due to almost zero net magnetization. Our results contribute to the understanding of the phonon dynamics and the interplay between topology and magnetism in FM-EuCd2As2, through helical light and external magnetic fields. This lays the foundation for utilizing state-of-the-art Weyl systems for applications in thermoelectrics, phononic devices, and topological quantum computing.

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

Raman data on FM-EuCd2As2 shows clear intensity and polarization changes below Tc with FM-AFM contrast, but the spin-phonon coupling claim rests on timing alone without isolating controls.

read the letter

The paper measures low-frequency Raman on FM-EuCd2As2 down to 4.2 K in circular and linear polarization setups, reports intensity increases in specific modes below Tc = 26 K, and finds the 12.5 meV A-mode reaching 60% DCP that saturates. They contrast this with AFM-EuCd2As2 below 9 K, where both effects are weak or absent, and back the phonon assignments with DFT. That FM-AFM comparison and the helical Raman configuration on this tunable material is the concrete new piece relative to earlier Raman work on magnetic compounds.

The data collection itself looks solid for what it is: cryogenic polarized spectra plus the direct state comparison. The DCP saturation behavior and the in-plane anisotropy measurements add usable experimental detail for people studying Weyl physics or magneto-optic responses in these crystals.

The soft spot is the mechanism. The abstract and strongest claim tie the intensity jumps directly to spin-phonon coupling because they appear at Tc, yet the text does not show a non-magnetic isostructural control, quantitative modeling of anharmonicity or electronic screening changes, or explicit exclusion of other T-dependent lattice effects. The DCP is assigned to the magneto-optic effect in the FM phase on similar grounds. Without those steps the attribution stays correlational. The circularity burden is low since there are no fitted equations, but that also means the claims rest entirely on the experimental design and controls that are only sketched in the abstract.

This is for the narrow group working on EuCd2As2, topological magnets, or phonon spectroscopy in ordered systems. The measurements are new enough and the material relevant enough that a serious editor should send it to referees rather than desk reject, even if the interpretation will need tightening. I would bring it to a reading group for the data alone.

Referee Report

2 major / 1 minor

Summary. The manuscript reports cryogenic low-frequency Raman spectroscopy on FM-EuCd2As2 (Tc=26 K) in circular and linear polarization configurations, supported by DFT. It claims anomalous Raman intensity enhancements below Tc arise from spin-phonon coupling, that magneto-helical Raman spectroscopy distinguishes A-modes via the magneto-optic effect with the 12.5 meV peak reaching 60% DCP at 4.2 K (saturating), and that the AFM phase (TN=9 K) shows negligible coupling and DCP due to zero net magnetization. The work aims to illuminate phonon dynamics and magnetism-topology interplay in this Weyl candidate.

Significance. If the central attributions are substantiated with controls and quantitative analysis, the results would strengthen understanding of spin-phonon coupling and magneto-optic effects in magnetic topological materials, with potential relevance to phononic and topological applications. The comparison between FM and AFM phases is a positive feature.

major comments (2)

[Abstract] Abstract: The direct attribution of Raman intensity enhancements below Tc to spin-phonon coupling is load-bearing for the central claim yet rests on correlation; the text provides no description of control experiments (e.g., isostructural non-magnetic analog) or quantitative modeling that isolates the magnetic contribution from generic temperature effects such as anharmonicity or thermal expansion.
[Abstract] Abstract: The assignment of the observed DCP saturation (60% at 4.2 K for the 12.5 meV peak) specifically to the magneto-optic effect in the FM phase is central but lacks reported exclusion of alternative polarization mechanisms (e.g., birefringence or resonance shifts); no error analysis or field/temperature dependence details are referenced to support the saturation claim.

minor comments (1)

[Abstract] Abstract: Grammatical error in the sentence 'We attribute the anomalous enhancements in Raman intensities below the Curie temperature are due to spin-phonon coupling.'

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We address each major point below and will revise the manuscript to improve clarity on controls and supporting details where possible.

read point-by-point responses

Referee: [Abstract] Abstract: The direct attribution of Raman intensity enhancements below Tc to spin-phonon coupling is load-bearing for the central claim yet rests on correlation; the text provides no description of control experiments (e.g., isostructural non-magnetic analog) or quantitative modeling that isolates the magnetic contribution from generic temperature effects such as anharmonicity or thermal expansion.

Authors: We agree that the current abstract relies on the temperature coincidence with Tc without explicit controls. The manuscript does compare FM and AFM phases of the same compound, with the AFM phase (zero net magnetization) showing negligible enhancements, which isolates the role of net FM order from generic thermal effects. However, an isostructural non-magnetic analog is not available. We will revise the abstract and main text to explicitly describe the AFM comparison as a control, add discussion ruling out anharmonicity/thermal expansion based on the phase-specific contrast, and expand on how DFT phonon calculations support the magnetic attribution. This is a partial revision as new experiments are not feasible. revision: partial
Referee: [Abstract] Abstract: The assignment of the observed DCP saturation (60% at 4.2 K for the 12.5 meV peak) specifically to the magneto-optic effect in the FM phase is central but lacks reported exclusion of alternative polarization mechanisms (e.g., birefringence or resonance shifts); no error analysis or field/temperature dependence details are referenced to support the saturation claim.

Authors: We acknowledge the need for more explicit support. The DCP reaches 60% and saturates only below Tc in the FM phase (absent in AFM), consistent with magneto-optic origin tied to net magnetization. We will revise to include error analysis on DCP values, additional temperature/field dependence plots, and a short discussion excluding birefringence/resonance shifts via the helical Raman configuration and phase-specific saturation. This revision will be made in the abstract and results sections. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental observations and DFT support with no self-referential derivations

full rationale

The paper reports cryogenic Raman spectroscopy results on EuCd2As2, attributing intensity enhancements below Tc to spin-phonon coupling and DCP saturation to the magneto-optic effect based on temperature coincidence with magnetic ordering and comparison to AFM phase. No equations, fitted parameters, or derivation chains are present that reduce claims to inputs by construction. DFT is invoked for support but not as a self-citation chain or ansatz smuggling. Attribution is correlative and interpretive (as noted in the reader's weakest assumption), but does not meet any enumerated circularity pattern requiring a quoted reduction. This is the expected non-finding for an experimental materials paper without mathematical modeling.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on standard assumptions of Raman spectroscopy and DFT for mode assignment; no free parameters, ad-hoc axioms, or invented entities are described in the abstract.

axioms (1)

standard math Standard assumptions of density functional theory for phonon mode assignment and symmetry analysis
Invoked to support peak assignments and symmetry breaking statements in abstract.

pith-pipeline@v0.9.1-grok · 5877 in / 1283 out tokens · 22044 ms · 2026-06-29T21:13:37.880770+00:00 · methodology

discussion (0)

Reference graph

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Top view of crystal structure of EuCd2As2
[42]

Raw spectra with 532 and 785 nm excitations of FM-EuCd2As2
[43]

Circularly polarized optical Raman spectroscopy of FM-EuCd2As2
[44]

Raman group theory analysis for EuCd2As2
[45]

Linear parallel polarization dependence of Raman spectroscopy of FM-EuCd2As2
[46]

Temperature dependent anharmonic model fitting parameters
[47]

Linear parallel polarization dependence cryogenic -Raman spectroscopy under magnetic field of FM-EuCd2As2
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Linear parallel polarization dependence Raman spectroscopy under magnetic field of FM- EuCd2As2 at room temperature
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Raman circular polarization of Peak III ( A1g2) under various temperature points and magnetic fields
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Raman circular polarization of Peaks I (Eg1) and IV (Eg2) under various temperature points and magnetic fields
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Temperature dependent Raman intensities of AFM-EuCd2As2. 22
[52]

Figure S1

Top view of crystal structure of EuCd2As2. Figure S1. Top view of crystal structure of EuCd2As2
[53]

Figure S 2

Raw spectra with 532 and 785 nm excitations of FM-EuCd2As2. Figure S 2. Raw spectra with 532 and 785 nm excitation sources of FM -EuCd2As2. 785 nm measurement has a stronger background signal than 532 nm spectrum. 23
[54]

Figure S3

Circularly polarized optical Raman spectroscopy of FM-EuCd2As2. Figure S3. Circularly polarized optical Raman spectroscopy of magnetic Weyl semimetal of E- symmetry [𝑧̅(𝜎− − 𝜎+)𝑧] (a) and A- symmetry [𝑧̅(𝜎− − 𝜎−)𝑧] (b) phonons at 4.2 K
[55]

164), which in turn belongs to the points group D 3d

Raman group theory analysis for EuCd2As2 The crystal structure of EuCd2As2 is in the space group 𝑃3̅𝑚1 (no. 164), which in turn belongs to the points group D 3d. This points group has doubly degenerate Eg symmetric Raman tensor and one non-degenerate A1g Raman tensor [1-3]. R(𝐸𝑔)=( 𝑐 0 0 0 −𝑐 𝑑 0 𝑑 0 ) or ( 0 𝑐 𝑑 𝑐 0 0 𝑑 0 0 ) (S1) R(𝐴1𝑔)= ( 𝑎 0 0 0 𝑎 0 0...
[56]

Linear parallel polarization dependence of Raman signal below Curie-temperature at 8 K (a) and 20 K (b) and above Curie-temperature at 30 K (c) and 40 K (d)

Linear parallel polarization dependence of Raman spectroscopy of FM-EuCd2As2 Figure S4. Linear parallel polarization dependence of Raman signal below Curie-temperature at 8 K (a) and 20 K (b) and above Curie-temperature at 30 K (c) and 40 K (d). 26
[57]

Temperature dependent anharmonic model fitting parameters. Peak Position Peak I (Eg1) Peak III (A1g1) Peak IV (A1g2) Peak V (Eg2) ω0 0.11 ± 0.01 0.247 ± 0.006 0.368 ± 0.004 0.45 ± 0.08 C -0.02 ± 0.01 -0.021 ± 0.008 -0.034 ± 0.005 -0.10 ± 0.09 x 0.14 ± 0.07 0.08 ± 0.03 0.08 ± 0.01 0.2 ± 0.1 Table S1. Fitting parameters for the peak positions by anharmonici...
[58]

Linear parallel polarization dependence of Raman signal under 500 mT magnetic field

Linear parallel polarization dependence cryogenic-Raman spectroscopy under magnetic field of FM-EuCd2As2 Figure S5. Linear parallel polarization dependence of Raman signal under 500 mT magnetic field. The magnetic field is in an upward direction. 27
[59]

Figure S6

Linear parallel polarization dependence Raman spectroscopy under magnetic field of FM-EuCd2As2 at room temperature. Figure S6. Linear parallel polarization dependence of Raman signal at room temperature (a) and under 500 mT magnetic field (b). The magnetic field is in an upward direction
[60]

Figure S7

Raman circular polarization of Peak III (A1g2) under various temperature points and magnetic fields. Figure S7. Raman circular polarization of Peak III (A1g2) between 4.2 K to 100 K temperature points under magnetic fields. At 4.2 and 10 K above 400 mT, the DCP values have flat response. 28
[61]

Figure S8

Raman circular polarization of Peaks I ( Eg1) and IV (Eg2) under various temperature points and magnetic fields. Figure S8. Raman circular polarization of Peaks I (Eg1) and IV (Eg2) between 4.2 K to 100 K temperature points under magnetic fields
[62]

Figure S 9

Temperature dependent Raman intensities of AFM-EuCd2As2. Figure S 9. Raman intensities of temperature dependent measurement of AFM -EuCd2As2 in parallel polarization configuration. The overall intensities are divided by room temperature intensities I (296K). The vertical dashed line represents the Néel temperature of AFM-EuCd2As2 sample. 29 Supporting References
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Top view of crystal structure of EuCd2As2

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Raw spectra with 532 and 785 nm excitations of FM-EuCd2As2

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Circularly polarized optical Raman spectroscopy of FM-EuCd2As2

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Raman group theory analysis for EuCd2As2

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Linear parallel polarization dependence of Raman spectroscopy of FM-EuCd2As2

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Temperature dependent anharmonic model fitting parameters

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Linear parallel polarization dependence cryogenic -Raman spectroscopy under magnetic field of FM-EuCd2As2

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Linear parallel polarization dependence Raman spectroscopy under magnetic field of FM- EuCd2As2 at room temperature

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Raman circular polarization of Peak III ( A1g2) under various temperature points and magnetic fields

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Raman circular polarization of Peaks I (Eg1) and IV (Eg2) under various temperature points and magnetic fields

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Temperature dependent Raman intensities of AFM-EuCd2As2. 22

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Figure S1

Top view of crystal structure of EuCd2As2. Figure S1. Top view of crystal structure of EuCd2As2

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Figure S 2

Raw spectra with 532 and 785 nm excitations of FM-EuCd2As2. Figure S 2. Raw spectra with 532 and 785 nm excitation sources of FM -EuCd2As2. 785 nm measurement has a stronger background signal than 532 nm spectrum. 23

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Figure S3

Circularly polarized optical Raman spectroscopy of FM-EuCd2As2. Figure S3. Circularly polarized optical Raman spectroscopy of magnetic Weyl semimetal of E- symmetry [𝑧̅(𝜎− − 𝜎+)𝑧] (a) and A- symmetry [𝑧̅(𝜎− − 𝜎−)𝑧] (b) phonons at 4.2 K

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164), which in turn belongs to the points group D 3d

Raman group theory analysis for EuCd2As2 The crystal structure of EuCd2As2 is in the space group 𝑃3̅𝑚1 (no. 164), which in turn belongs to the points group D 3d. This points group has doubly degenerate Eg symmetric Raman tensor and one non-degenerate A1g Raman tensor [1-3]. R(𝐸𝑔)=( 𝑐 0 0 0 −𝑐 𝑑 0 𝑑 0 ) or ( 0 𝑐 𝑑 𝑐 0 0 𝑑 0 0 ) (S1) R(𝐴1𝑔)= ( 𝑎 0 0 0 𝑎 0 0...

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Linear parallel polarization dependence of Raman signal below Curie-temperature at 8 K (a) and 20 K (b) and above Curie-temperature at 30 K (c) and 40 K (d)

Linear parallel polarization dependence of Raman spectroscopy of FM-EuCd2As2 Figure S4. Linear parallel polarization dependence of Raman signal below Curie-temperature at 8 K (a) and 20 K (b) and above Curie-temperature at 30 K (c) and 40 K (d). 26

[57] [57]

Temperature dependent anharmonic model fitting parameters. Peak Position Peak I (Eg1) Peak III (A1g1) Peak IV (A1g2) Peak V (Eg2) ω0 0.11 ± 0.01 0.247 ± 0.006 0.368 ± 0.004 0.45 ± 0.08 C -0.02 ± 0.01 -0.021 ± 0.008 -0.034 ± 0.005 -0.10 ± 0.09 x 0.14 ± 0.07 0.08 ± 0.03 0.08 ± 0.01 0.2 ± 0.1 Table S1. Fitting parameters for the peak positions by anharmonici...

[58] [58]

Linear parallel polarization dependence of Raman signal under 500 mT magnetic field

Linear parallel polarization dependence cryogenic-Raman spectroscopy under magnetic field of FM-EuCd2As2 Figure S5. Linear parallel polarization dependence of Raman signal under 500 mT magnetic field. The magnetic field is in an upward direction. 27

[59] [59]

Figure S6

Linear parallel polarization dependence Raman spectroscopy under magnetic field of FM-EuCd2As2 at room temperature. Figure S6. Linear parallel polarization dependence of Raman signal at room temperature (a) and under 500 mT magnetic field (b). The magnetic field is in an upward direction

[60] [60]

Figure S7

Raman circular polarization of Peak III (A1g2) under various temperature points and magnetic fields. Figure S7. Raman circular polarization of Peak III (A1g2) between 4.2 K to 100 K temperature points under magnetic fields. At 4.2 and 10 K above 400 mT, the DCP values have flat response. 28

[61] [61]

Figure S8

Raman circular polarization of Peaks I ( Eg1) and IV (Eg2) under various temperature points and magnetic fields. Figure S8. Raman circular polarization of Peaks I (Eg1) and IV (Eg2) between 4.2 K to 100 K temperature points under magnetic fields

[62] [62]

Figure S 9

Temperature dependent Raman intensities of AFM-EuCd2As2. Figure S 9. Raman intensities of temperature dependent measurement of AFM -EuCd2As2 in parallel polarization configuration. The overall intensities are divided by room temperature intensities I (296K). The vertical dashed line represents the Néel temperature of AFM-EuCd2As2 sample. 29 Supporting References

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2006