SciPost Submission Page
Diffusion and relaxation of topological excitations in layered spin liquids
by Aprem P. Joy, Roman Lange, Achim Rosch
Submission summary
| Authors (as registered SciPost users): | Aprem Joy |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202512_00033v1 (pdf) |
| Code repository: | https://zenodo.org/records/17880707 |
| Data repository: | https://zenodo.org/records/17880707 |
| Date submitted: | Dec. 15, 2025, 11 a.m. |
| Submitted by: | Aprem Joy |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approach: | Theoretical |
The author(s) disclose that the following generative AI tools have been used in the preparation of this submission:
Chat-GPT used for spelling and grammar check.
Abstract
Relaxation processes in topological phases such as quantum spin liquids are controlled by the dynamics and interaction of fractionalized excitations. In layered materials hosting two-dimensional topological phases, elementary quasiparticles can diffuse freely within the layer, whereas only pairs (or more) can hop between layers - a fundamental consequence of topological order. Using exact solutions of emergent nonlinear diffusion equations and particle-based stochastic simulations, we explore how pump-probe experiments can provide unique signatures of the presence of $2d$ topological excitations in a $3d$ material. Here we show that the characteristic time scale of such experiments is inversely proportional to the initial excitation density, set by the pump intensity. A uniform excitation density created on the surface of a sample spreads subdiffusively into the bulk with a mean depth $\bar z$ scaling as $\sim t^{1/3}$ when annihilation processes are absent. The propagation becomes logarithmic, $\bar z \sim \log t$, when pair-annihilation is allowed. Furthermore, pair-diffusion between layers leads to a new decay law for the total density, $n(t) \sim (\log^2 t)/t$ - slower than in a purely $2d$ system. We discuss possible experimental implications for pump-probe experiments in finite-size system.
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
Strengths
The motivation for studying this model is twofold: Firstly, it is designed to capture the propagation and pairwise annihilation of topological excitations in multilayer systems. The authors suggest that such physics could arise in materials that host layers of 2D quantum spin liquids, and potentially be used as a means of detecting these phases. Secondly, the model itself has intrinsic theoretical interest in light of ongoing work on the dynamics of fractons, which exhibit constrained hopping processes such as the one described here.
Overall, I find this to be an interesting and broadly well-motivated study. The model itself is rather simple, but to my knowledge has not appeared in the literature before, and exhibits interesting phenomena such as subdiffusion that are markedly different from the usual unconstrained diffusion-annihilation processes often studied. The authors provide a very clear and succinct exposition of their analysis, which is fairly straightforward but convincing, and is backed up by numerical simulations.
Weaknesses
Report
This paper will be of interest to a number of different communities, spanning those interested in topological phases of matter and their detection in the solid state, as well as exotic many-body dynamics such as fracton models. The multilayer nature of 1D and 2D is often overlooked in theoretical studies such as these, and the ideas in this paper could spark further work which addresses these effects. Once the points below are addressed, I will be happy to recommend publication.
- Line 108. It is stated that in the dilute limit, the quasiparticles of a quantum spin liquid can be modelled as classical diffusive particles. But why should the quasiparticles propagate classically and diffusively rather than quantum-coherently and ballistically? Indeed, in previous analyses of the dynamics of spinons, e.g. [Sachdev and Young, Phys Rev Lett 78 2220 (1997)], the dynamics of spinons is described by a semiclassical theory with ballistic dynamics. Moreover, since quasiparticles are necessarily created in pairs, one would expect a pump pulse to create many spinon pairs at momentum +/-k with k nonzero, which have a nonzero group velocity.
The model of diffusing particles is in itself still interesting, so I do not see this as a major flaw in the paper, but I do think the connection between the model (1) and the quasiparticles of a QSL should be given a little more explanation. E.g. are the authors envisaging some environmental coupling that destroys quantum-coherent propagation? 2. On a similar note, a connection is made to possible experimental probes of QSLs in Section 3.5. The idea of using a pump pulse of light to generate excitations in the first place certainly seems sensible, but the “experimental signature” chosen is the time taken for excitations to move from one side of the slab to the other. How do the authors imagine that this would be detected? Time-resolved spectroscopies are almost exclusively not spatially resolved, since the frequency range required to couple to magnetic excitations is THz, for which the wavelength is very long. Is there a signature that could be seen even without any spatial resolution? What specific observable would one look at, and how would this show up in THz spectroscopy?
Recommendation
Ask for minor revision