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Robust semiDirac points and unconventional topological phase transitions in doped superconducting Sr2IrO4 tunnel coupled to t2g electron systems
by Mats Horsdal, Timo Hyart
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Submission summary
Authors (as registered SciPost users):  Mats Horsdal · Timo Hyart 
Submission information  

Preprint Link:  https://arxiv.org/abs/1705.02987v3 (pdf) 
Date accepted:  20171113 
Date submitted:  20171018 02:00 
Submitted by:  Horsdal, Mats 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
SemiDirac fermions are known to exist at the critical points of topological phase transitions requiring finetuning of the parameters. We show that robust semiDirac points can appear in a heterostructure consisting of superconducting Sr2IrO4 and a t2g electron system (t2gES) without finetuning. They are topologically stable in the presence of the symmetries of the model, metallic t2gES and a single active band in Sr2IrO4. If the t2g metal is coupled to two different layers of Sr2IrO4 (effectively a multiband superconductor) in a threelayerstructure the semiDirac points can split into two stable Dirac points with opposite chiralities. A similar transition can be achieved if the t2gES supports intrinsic triplet superconductivity. By considering Sr2RuO4 as an example of a t2gES we predict a rich topological phase diagram as a function of various parameters.
Author comments upon resubmission
Dear Editor,
we would like to thank you for communicating us the referee report. We thank the referee for her/his positive report and careful reading of the manuscript.
The referee has requested two changes on the manuscript.
 Referee: It might help to have a Table listing all the different models (bilayer or trilayer, simplified or microscopically accurate,...) used in the paper and the conclusion reached for each of them. The paper is quite long and contains a lot of information, so it might be worth trying to summarize the main points even more.
Response: We agree with the referee, and we have included a Table listing the different models and the conclusions reached for each of them in the last paragraph of the introduction.
 Referee: The Fermi surfaces of bulk Sr2RuO4 actually have some nontrivial dependence on the outofplane momentum k_z if one looks at their spinorbital content [arXiv:1303.5444]. It might therefore be an oversimplification to take the kz=0 Fermi surface to model a few layer system. Furthermore, the only monolayers of Sr2RuO4 that were made [ 1605.05669] showed some reconstruction and therefore have Fermi surfaces that are different from the bulk ones. In particular, the gamma pocket seemed to have gone through a Lifshitz transition and to be on the other side of the van Hove singularity. It might be good to check the results for a tightbinding model specifically fitted to the fewlayer physics and not the bulk one.
Response: The use of the bulk tightbinding parameters for Sr2RuO4 is a simplification. As mentioned by the referee the Fermi surfaces have a weak dependence on the outofplane momentum and in thin layers the gamma band can be closer to a Lifshitz transition than in the bulk system. Moreover, when heterostructures are created the band structure parameters will again be modified because of the interface reconstructions (as already pointed out in the manuscript). Predicting the correct band structure parameters for the heterostructures requires density functional theory calculations and even then there are large uncertainties in these predictions. The important point in our paper is however that the band structure parameters are not important for our qualitative conclusions. Similar kind of transitions will exist in the phase diagram as long as the symmetries are present and the tunneling matrices are topologically nontrivial. The effect of the band structure parameters is only a renormalization of the critical points of phase transitions in the phase diagrams shown in the paper. We also point out that based on the reference mentioned by the referee the gamma band can be on either side of the Lifshitz transition depending on whether the t2g material is chosen to be Sr2RuO4 or Ba2RuO4 and the choice of the substrate material. Therefore, our current choice of using the bulk Sr2RuO4 tight binding parameters is well justified. Moreover, the possibility of changing the tight binding parameters for example by strain engineering opens new interesting possibilities because it may allow to drive the system through topological phase transitions in a controlled way. We have discussed this in the revised version of the manuscript on p11.
We have addressed all the comments of the referee and we believe that the paper is now suitable for publication in SciPost.
Yours sincerely, Mats Horsdal and Timo Hyart
List of changes
 Added a short summary of the models studied in the paper at the end of Section I
 Added Table I
 Added discussion about bulk vs singlelayer Sr2RuO4 on p11.
Added references [18], [86] and [87].
 Added sentence below eq. (52) in supplementary material:
Since $\kappa^{n\cal{T}}_{m,m'}$ and $\kappa^{n\cal{N}}_{m,m'}$ are only nonvanishing for $m=m'$, we will in the following simplify notation by renaming $\kappa^{n\cal{T}}_{m,m'}$ and $\kappa^{n\cal{N}}_{m,m'}$ to $\kappa^{n\cal{T}}_{m}$ and $\kappa^{n\cal{N}}_{m}$, respectively.
 Changed $\kappa^{n\cal{N}}_{m,m'}$ to $\kappa^{n\cal{N}}_{m}$ in (55) in the Supplementary material.
Published as SciPost Phys. 3, 041 (2017)