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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 |
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| Preprint Link: | https://arxiv.org/abs/2209.08766v1 (pdf) |
| Date submitted: | Sept. 20, 2022, 4:33 a.m. |
| 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
Report #1 by Anonymous (Referee 1) 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
- Interesting proposal to look at edge states between coexisting phases that arise from a first-order topological phase transition
- 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:
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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.
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Are the results sensitive to any underlying trapping potential, e.g., a harmonic trap?
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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.
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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
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Modify abstract to include concrete results, e.g., the conditions for the existence of a robust Majorana mode.
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Address comments above