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Delocalized states in three-terminal superconductor-semiconductor nanowire devices
by P. Yu, B. D. Woods, J. Chen, G. Badawy, E. P. A. M. Bakkers, T. D. Stanescu, S. M. Frolov
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Submission summary
| Authors (as registered SciPost users): | Sergey Frolov |
| Submission information | |
|---|---|
| Preprint Link: | https://arxiv.org/abs/2108.07327v2 (pdf) |
| Date accepted: | 2023-04-18 |
| Date submitted: | 2023-01-31 08:10 |
| Submitted by: | Frolov, Sergey |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Theoretical, Experimental, Computational |
Abstract
We fabricate three-terminal hybrid devices with a nanowire segment proximitized by a superconductor, and with two tunnel probe contacts on either side of that segment. We perform simultaneous tunneling measurements on both sides. We identify some states as delocalized above-gap states observed on both ends, and some states as localized near one of the tunnel barriers. Delocalized states can be traced from zero to finite magnetic fields beyond 0.5 T. In the parameter regime of delocalized states, we search for correlated subgap resonances required by the Majorana zero mode hypothesis. While both sides exhibit ubiquitous low-energy features at high fields, no correlation is inferred. Simulations using a one-dimensional effective model suggest that delocalized states may belong to lower one-dimensional subbands, while the localized states originate from higher subbands. To avoid localization in higher subbands, disorder may need to be further reduced to realize Majorana zero modes.
Published as SciPost Phys. 15, 005 (2023)
Submission & Refereeing History
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Reports on this Submission
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The authors replied to my comments and questions in a satisfactory way. Fig. 6 strengthens the message of this work. However, the estimation of disorder is rather speculative and additional measurements and statistics are necessary to draw strong conclusions. Nonetheless, I believe that this work is a good starting point for characterizing disorder in these hybrid structures.
As a minor comment, the resolution of Fig. 5 and Fig. 6 is poor. I would appreciate if the authors could fix this technical issue.
Anonymous Report 1 on 2023-1-31 (Invited Report)
- Cite as: Anonymous, Report on arXiv:2108.07327v2, delivered 2023-01-31, doi: 10.21468/SciPost.Report.6646
Report
The revised manuscript has considerably improved. The Authors have removed some of the more speculative passages, and added a most useful Fig. 6 demonstrating that a quantitatice extraction of disorder strength is possible based on their measurements. Further systematic study is probably needed to assess the reliability and usefulness of this method, e.g. to assess the variance of the results when changing the input data, but that can be left to future work. As it stands, this addition satisfactorily answers one of my main criticisms from the previous report.
I do have one question on this new part. The Authors fix the correlation length of the disorder to 15 nm. They say this is based on the results of Ref. 35, though that reference reports values depending on charge impurity concentration. It would be useful if the Authors could say something about the sensitivity of the results on the correlation length, also because different sources of disorder than charge impurities (e.g. grains in the superconductor) may be characterized by different correlation lengths.
I found that most of the replies to my comments are also satisfactory. In some cases I stand corrected in my remarks, such as on the possible interpretation of the data in terms of Coulomb oscillations. In some other cases I am not fully convinced, but even in these cases I agree with the Authors that the methods and data have been shared to a degree such that readers can form their own judgement. This holds true, in particular, for my concerns related to the superconducting terminal being floating and the other technical details on three-terminal measurements.
Therefore, I don't find it helpful to prolong the debate on lingering disagreements, and based on the improvements I recommend publication of this work in SciPost (pending a possible minor revision on the correlation length based on my question above).
For completeness, let me just answer to two specific points.
First, the Authors state that:
"Bottom-line, identifying starters/nonstarters based on “hardness” of the gap at zero magnetic field is just another misconception among many affecting this field."
I never stated that a hard gap is an indicator of a disorder-free system. I maintain that my statement that a soft gap at B=0 is a "non-starter for a gapped topological phase to occur" is an accurate one. I wish the Authors did not rely on straw man arguments to accuse others of "misconceptions".
Second, the Authors write:
"References are not for giving credit, they are to help readers. We now add those references."
The Authors are free to choose their own deontology. I believe that references in scientific articles are appropriate for both purposes: to give proper credit and/or to help readers. This belief informed my suggested references. I am glad the Authors accepted the suggestions.