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Giant spatial anisotropy of magnon lifetime in altermagnets
by António T. Costa, João C. G. Henriques, Joaquín Fernández-Rossier
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Submission summary
| Authors (as registered SciPost users): | António Costa |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202411_00053v1 (pdf) |
| Date submitted: | 2024-11-26 16:17 |
| Submitted by: | Costa, António |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Theoretical, Computational |
Abstract
Altermagnets are a new class of magnetic materials with zero net magnetization (like antiferromagnets) but spin-split electronic bands (like ferromagnets) over a fraction of reciprocal space. As in antiferromagnets, magnons in altermagnets come in two flavours, that either add one or remove one unit of spin to the S=0 ground state. However, in altermagnets these two magnon modes are non-degenerate along some directions in reciprocal space. Here we show that the lifetime of altermagnetic magnons has a very strong dependence on both flavour and direction. Strikingly, coupling to Stoner modes leads to a complete suppression of magnon propagation along selected spatial directions. This giant anisotropy will impact electronic, spin, and energy transport properties and may be exploited in spintronic applications.
Author indications on fulfilling journal expectations
- Provide a novel and synergetic link between different research areas.
- Open a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work
- Detail a groundbreaking theoretical/experimental/computational discovery
- Present a breakthrough on a previously-identified and long-standing research stumbling block
Current status:
Reports on this Submission
Report #3 by Anonymous (Referee 2) on 2025-1-7 (Invited Report)
- Cite as: Anonymous, Report on arXiv:scipost_202411_00053v1, delivered 2025-01-07, doi: 10.21468/SciPost.Report.10451
Strengths
The manuscript is timely as there is a lot of interest in electronic and spin properties of altermagnets in general, with recently opened (by Ref. [9]) exploration of magnon excitation in this novel class of magnetic materials. The findings of the paper are novel and interesting, as the theory presented in Ref. [9] is simplistic one assuming noninteracting quasiparticles, which seems to be insufficient according to present papers and some other studies completed after Ref. [9].
Weaknesses
No major weakness, but some improvement in presentation is needed.
Report
The paper presents perhaps one of the first exploration of broadening of magnon bands from simplistic linear spin wave theory (used in Ref. [9]) due to interaction with other excitations in magnetic solids, such as Stoner excitations in the case of the present manuscript. This topic has been explored for conventional metallic ferromagnets [PHYSICAL REVIEW B 84, 174418 (2011)], as well as in Ref. [9], but conclusions of Ref. [9] are quite different from the present manuscript. Due to impact of spatial anisotropic magnon lifetime on future experimental studies and technological applications of altermagnetic magnons.
Requested changes
1. The authors label the effect they study "Stoner damping", but it would be better aligned with the rest of the literature to call it "Landau damping" (which is due to Stoner excitations). This is terminology used in Ref. [9], prior papers exploring it for conventional magnets, see, e.g., PHYSICAL REVIEW B 84, 174418 (2011) on "Different dimensionality trends in the Landau damping of magnons in iron, cobalt, and nickel: Time-dependent density functional study".
2. The title of the paper does not really highlight the main effect studied as there are many potential reasons for finite magnon lifetime. Akin to reference mentioned in 1., it would be better to add "magnon lifetime due to Landau damping" into the title.
3. The authors of Ref. [9] have apparently done extensive study of Landau damping effect on altermagnetic magnons, concluding that: "We also
show that, overall, the Landau damping of this metallic altermagnet is suppressed due to the spin-split electronic structure, as compared to an artificial antiferromagnetic phase of the same RuO2 crystal with
spin-degenerate electronic bands and chirality-degenerate magnon bands." As this is strikingly different from the conclusion of the present manuscript, so some discussion the origin of this discrepancy is necessary.
4. Eq. 1 would be far easier to understand if all of its labels were also placed in Fig. 1, meaning add label to each site, add labels on dashed lines which represent hopping etc.
5. Another Hubbard model for altermagnets was proposed in https://doi.org/10.1103/PhysRevB.108.L100402. It would be good to mention it and/or compare to Eq. 1.
Recommendation
Ask for minor revision
Report #1 by Anonymous (Referee 1) on 2024-12-27 (Invited Report)
- Cite as: Anonymous, Report on arXiv:scipost_202411_00053v1, delivered 2024-12-27, doi: 10.21468/SciPost.Report.10392
Report
The authors of the study “Giant spatial anisotropy of magnon lifetime in altermagnets”, have investigated magnons in altermagnets and many-body effects on them with special focus on stoner excitations. For this purpose, they have adopted a Hubbard model of altermagnetism that maintains the parametric continuous transition from the metallic phase to the insulating phase altermagnets and includes the important features of altermagnets such as non-relativistic spin splitting band structure as well as the correlation effects. According to their findings, altermagnetism which comes as a result of reducing the symmetry of the lattice gives rise to interesting spatial anisotropy of magnons with unique directionality of the magnon propagation and lifetime in metallic altermagnets. They relate their finding in directionality dependence of magnon excitation and propagation to broken chirality of the two magnon dispersion which is a unique feature of altermagnets. In addition, they show that even when magnons could be excited in a certain direction with well-defined quasiparticle character, they might be highly suppressed at higher energies and wavevectors due to the Stoner excitations. Nonetheless, their findings reveal that magnons in insulating altermagnets agrees very well with linear spin wave theory and therefore the correlation effects are negligible in this type of altermagnets.
I found the study very intriguing and interesting in many aspects such as its novelty, well-written manuscript, and advanced theory. I highly recommend this manuscript for the publication in SciPost Physics. Nonetheless, there are minor issues that I would like to ask the authors to address.
1. After the Hamiltonian in equation (1) is introduced, the parameters are not well described which makes it difficult for the reader to understand. I suggest a complete description of the model and all its parameters to be given after equation (1).
2. It is mentioned that spin dynamics in the intermediate regime (3τ<U<10τ) the spin dynamics cannot be properly described by a spin-only model. The authors should provide some more evidence and if possible or provide some references for the readers.
3. In the case of the insulating altermagnets, it is said that magnon energy of one channel is 40% different from the magnon energy of the other channel. However, in Figure 2, the difference does not look like to be that large. Would you please clarify this confusion?
4. Appendix E is very disorganized. There is no logical connection between paragraphs. I suggest revisiting it.
Recommendation
Ask for minor revision
Author: António Costa on 2025-02-11 [id 5210]
(in reply to Report 1 on 2024-12-27)
1.
We thank the referee for the suggestion. An improved description of the structure of the hopping matrix has been added to the text just after Eq. 1. We also added legends to fig, 1 to make the meaning of the various elements clearer.
2.
We have provided two pieces of evidence that we believe should convince the readers of the limitations of the spin-only model in the intermediate coupling regime . The first one is the poor fitting between the spin-only dispersion relation and the magnon energies extracted from the fermionic model (Fig. 9). The second, more conclusive one is the clear departure from the simple pole structure in the magnon spectral density, expected for magnons well described by spin-only models. This is shown in Fig. 10. We have added a reference in which spin models fail to describe magnons in a different context (new ref. 19).
3.
We thank the referee for pointing that out. We have mixed up the discussion of the strong coupling limit (which appears in the main text) and the intermediate coupling limit (which is presented in the appendix). The referee is absolutely right that the energy differences in the strong coupling limit are considerably smaller than 40%. The larger differences appear in the intermediate coupling limit. We have amended the text around Fig. 2 accordingly, and transferred the statement about the intermediate coupling regime to appendix D.
4.
We thank the referee for pointing that out. Appendix E was indeed very disorganized. We have organized it in the revised version.
Author: António Costa on 2025-02-11 [id 5211]
(in reply to Report 3 on 2025-01-07)2. We have changed the title to address the referee's concern. We have also included a sentence in the abstract to stress the fact that the lifetimes we talk about in this manuscript are caused by Landau damping by Stoner excitations.
3. We have indeed discussed the results of ref. 9 in contrast to ours. The discussion appears in lines 190 to 199 of the originally submitted manuscript.
4. We thank the referee for the suggestion. We have added appropriate labels to fig. 1 to make it easier to interpret and more informative.
5. The model proposed in the work mentioned by the referee is essentially the same as the one we employed, with slightly different notation. We have cited this work in our manuscript (Ref. 5).