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Andreev bound states at boundaries of polarized 2D Fermi superfluids with s-wave pairing and spin-orbit coupling
by Kadin Thompson, Joachim Brand, Ulrich Zuelicke
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Submission summary
| Authors (as registered SciPost users): | Joachim Brand · Kadin Thompson · Ulrich Zuelicke |
| Submission information | |
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| Preprint Link: | https://arxiv.org/abs/2209.08766v1 (pdf) |
| Date submitted: | 2022-09-20 04:33 |
| Submitted by: | Zuelicke, Ulrich |
| Submitted to: | SciPost Physics |
| Ontological classification | |
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| Academic field: | Physics |
| Specialties: |
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| Approach: | Theoretical |
Abstract
Two-dimensional (2D) Fermi gases subject to s-wave pairing, spin-orbit coupling and large-enough Zeeman spin splitting are expected to form a topological superfluid. While the general argument of bulk-boundary correspondence assures the existence of topologically protected zero-energy quasiparticle excitation at such a system's boundaries, it does not fully determine the physical properties of the low-energy edge states. Here we develop a versatile theoretical method for elucidating microscopic characteristics of interface-localized subgap excitations within the spin-resolved Bogoliubov-deGennes formalism. Our analytical results extend current knowledge about edge excitations existing at the boundary between vacuum and a 2D superfluid that is in its topological or nontopological regime. We also consider the Andreev bound states that emerge at an interface between coexisting time-reversal-symmetry-breaking topological and nontopological superfluids and juxtapose their unusual features with those of vacuum-boundary-induced edge excitations. Our theory provides a more complete understanding of how the spin-orbit-coupled polarized 2D Fermi gas can be tailored as a platform for realizing unconventional Majorana excitations.
Current status:
Reports on this Submission
Report
Within the spin-resolved Bogoliubov–de Gennes formalism, the authors studied the interface properties of superfluids consisting of 2D Fermi gases subject to s-wave pairings, very small spin-orbit couplings and large Zeeman fields. They considered the subgap states in two systems: the edge sates of topological or nontopological superfluid and the bound states at the interface between topological and nontopological superfluids. The paper is well written with detailed derivations, simple analytical expressions and careful illustrations. I have only one concern that the abstract is unclear in delivering the message. The authors may provide an introduction to the theoretical formalism and the implications of the subgap states. I would like to recommend its publication after improving the abstract.
Report #1 by Anonymous (Referee 2) on 2022-11-28 (Invited Report)
- Cite as: Anonymous, Report on arXiv:2209.08766v1, delivered 2022-11-28, doi: 10.21468/SciPost.Report.6213
Strengths
1. Interesting proposal to look at edge states between coexisting phases that arise from a first-order topological phase transition
2. Relatively simple analytic results for the Andreev bound states
Weaknesses
- The predicted edge states might be difficult to realize and probe in cold-atom experiments
Report
This paper considers the spin-orbit-coupled polarized 2D Fermi gas and investigates the low-energy excitations that can exist at the interface between two coexisting phases with differing topology. In particular, the authors consider the boundary between a topological superfluid and a nontopological one, which could be realized at a first-order topological transition where there is spatial coexistence between different superfluids. Using the Bogoliubov–de Gennes formalism, they derive analytic expressions for the case where the spin-orbit coupling strength is smaller than the Zeeman splitting and they show that it is important to include the coupling between opposite-spin sectors (which is naturally present in this setup). They show that there can be a robust Majorana mode at the interface between topological and nontopological superfluids, provided that the Andreev bound states are well separated in energy.
Overall, this is a well-written paper and it provides a solid contribution to the field. It is quite technical in parts, which is appropriate for the aim of the paper, but it would have helped to have a clear summary of the results in the abstract, i.e., beyond vague statements like “the results extend current knowledge….”. I would have also liked to see more discussion of how the predicted states might be realized and probed in cold atom experiments. My specific comments are below:
1. What is the effect of temperature on the results? In particular, how robust are the edge states to thermal fluctuations? This is pertinent to cold-atom experiments where it is often challenging to access low temperatures, especially when applying lasers to simulate spin-orbit coupling.
2. Are the results sensitive to any underlying trapping potential, e.g., a harmonic trap?
3. It was not clear to me how these states might be probed in practice. The authors refer to a proposal in Ref [69] which is based on a tunnelling measurement, but it would be good see some more details of how this might be adapted to the cold-atom case.
4. I would be curious to know how the results might generalize to other types of interactions, e.g., dipolar interactions. Could this be used to further enhance p-wave superfluidity?
Requested changes
- Modify abstract to include concrete results, e.g., the conditions for the existence of a robust Majorana mode.
- Address comments above