Peer review of: Uniaxial-strain control of nematic superconductivity in SrxBi2Se3 (2020)

Reviewers' comments: 
Reviewer #1 (Remarks to the Author): 

Authors study Hc2 in doped BiSe, a putative odd parity superconductor with multi-component order 
parameter. There are very few superconductors of such kind; in addition to being fundamentally 
interesting, they may also be useful in the context of quantum information systems, since they can 
host Majorana fermions. 

Earlier experiments have shown rotational symmetry breaking in Hc2, down to two fold, which was 
taken as an indication of nematic superconductivity (since there is no visible structural or resistive 
anisotropy in the normal state). Unfortunately, earlier experiments failed to see reorientation of the 
anisotropy either by thermal cycling or strain. In the present experiment, authors give compelling 
evidence that this may be possible. 

The experiment shows that in the absence of externally applied strain (there is probably build-in strain, 
however), there is a signature of six-fold symmetry in the Hc2 pattern. In theory, Hc2 in only sensitive 
to the second order terms in Ginzburg-Landau functional. For one component superconductors in 
tetragonal, trigonal, or hexagonal systems there can therefore be no anisotropy in Hc2. The only way 
anisotropy can appear is if the crystal itself has only two-fold symmetry, or if the order parameter has 
multiple components. In the former case, Hc2 could have 2 fold anisotropy, but there is no reason to 
have any sign of six fold, as seen in the present experiment. This observation thus provides a 
compelling support for two-component odd-parity superconductivity. 

While I find the experimental results compelling, I do not agree with the interpretation in terms of Y 
domains. As stated above (see https://arxiv.org/pdf/1603.03406.pdf), the six-fold Hc2 anisotropy *is 
not* due to domain formation, since it is a result of *linear* analysis of GL equations. Indeed, 
experimentally, the six-star pattern only appear at large filed in Fig 2a, and at low fields (blue center) 
the pattern is only 2 fold -- indicating a single domain! 

Application of strain pushes the system apparently in the same direction as the built-in strain, making 
the two-fold anisotropy at small fields more pronounced, while removing the six-fold terms at high 
fields. This is consistent with an evolution between Fig 1 and 2 in 
https://arxiv.org/pdf/1603.03406.pdf. 

To summarize, I find the experiment excellent, worth publishing in Nature Communications; however, 
the modeling and interpretation have to be improved. 

Ivar Martin 


Reviewer #2 (Remarks to the Author): 

The manuscript by Kostylev et al. provides a detailed study on the effect of uniaxial strain on the 
magnetoresistance anisotropy in the mixed states of Sr-doped Bi2Se3. The approach is interesting. I 
am generally confident with their experimental approach and admire the detail they pursued the 
project. However, I have a few questions about the analysis. 

How does the normal state resistivity change under strain? The determination of the Hc2 depends on 
the normal state resistivity. Fig. 3 also depends on the resistivity normalized by the normal state 
resistivity. 

There is always a question about if the nematic order originates from the superconducting state or the 
electronic state anisotropy in the normal state. I do not quite get how the manuscript excludes this 
possibility. 
I am puzzled too by the model based on the G-L approach and multiple domains. The change of 
resistivity in the fluctuation regime is sensitive to the parameters. I am concerned that the model is 
too much overanalysis. 
Therefore, I would suggest a revision of the manuscript before the consideration for acceptance of the 
manuscript. 



Reviewer #3 (Remarks to the Author): 

In the paper by Kostylev et al. a detailed study of the strain dependence of the nematic 
superconductivity in SrxBi2Se3 is presented. The nematic superconductivity is monitored by the 
anisotropy in the upper critical field. This is a timely topic in the center of the field of topological 
superconductors and certainly of sufficiently broad scientific interest for Nature Communications. The 
paper is well written and the data and analysis are sound. The authors present the first control of 
superconductivity via uniaxial strain in doped Bi2Se3 superconductors, in that sense it is pioneering 
work. The experiments are properly described and the outcome is clear. The angular variation of the 
resistance is compared with a simulation to provide a further understanding of the data. 

The quality of the paper is high and I expect it to have a significant impact. I recommend publication 
in Nature Communications. Below are a few minor comments that should be addressed prior to 
publication. 

1. On page 1 line 6 "are" should read "is" and on page 2 line 25 "have"should read "has". 
2. On page 3, line 42 the sample to sample variation is discussed. Here a reference to the paper by 
Kuntsevich in New Journal of Physics (20-2018-103022) is missing. 
3. Kuntsevich et al. also published strain dependence in SrxBi2Se3 probed by the combination of XRD 
and transport. the authors should make a link and comparison to this work (Phys. Rev. B 100-
20190224509). 
4. I am puzzled by the change of Tc in Fig.S3 when the current is decreased from 250 to 50 microA. 
Normally such differences can be attributed to Joule heating of the sample for the larger current (the 
sample then has a higher temperature than the thermometer). The authors should explain whether 
local heating can be excluded. Overall the current in all experiments is large (250 microA) and 
absence of Joule heating should be demonstrated.