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Topological states between inversion symmetric atomic insulators
by Ana Silva and Jasper van Wezel
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Authors (as registered SciPost users):  Jasper van Wezel 
Submission information  

Preprint Link:  scipost_202101_00003v1 (pdf) 
Date submitted:  20210107 06:53 
Submitted by:  van Wezel, Jasper 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
One of the hallmarks of topological insulators is the correspondence between the value of its bulk topological invariant and the number of topologically protected edge modes observed in a ﬁnitesized sample. This bulkboundary correspondence has been welltested for strong topological invariants, and forms the basis for all proposed technological applications of topology. Here, we report that a group of weak topological invariants, which depend only by the symmetries of the atomic lattice, also induces a particular type of bulkboundary correspondence. It predicts the presence or absence of states localised at the interface between two inversionsymmetric band insulators with trivial values for their strong invariants, based on the space group representation of the bands on either side of the junction. We show that this corresponds with symmetrybased classiﬁcations of topological materials. The interface modes are protected by the combination of band topology and symmetry of the interface, and may be used for topological transport and signal manipulation in heterojunctionbased devices.
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Reports on this Submission
Anonymous Report 2 on 2021127 (Invited Report)
 Cite as: Anonymous, Report on arXiv:scipost_202101_00003v1, delivered 20210127, doi: 10.21468/SciPost.Report.2473
Report
The authors claim that a group of weak topological invariants, which depend only by the symmetries of the atomic lattice, induces a bulkboundary correspondence.
In particular, it is claimed that (i) these weak topological invariants predict the presence or absence of states localised at the interface between two inversionsymmetric band insulators with trivial values for their strong invariants, and (ii) the interface modes are protected by the combination of band topology and symmetry of the interface.
These statements are strong and counterintuitive, and I don’t find the supporting arguments in the manuscript convincing. The essential counterargument opposing these statements that it seems likely that it is possible to introduce a perturbation which obeys the symmetries but moves the interface states in energy to the conduction or valence band on either side of the interface. Thus, the interface states hybridise with the bulk states and are no longer localised at the interface. (Notice that the energy gap can be very small on one side of the interface, and therefore intuitively it seems that it would be very easy to hybridize the interface states with the bulk bands on that side of the interface.) This would mean that the interface states are not protected by the combination of the band topology and symmetry of the interface, and therefore also the weak topological invariants would not always predict the presence or absence of localised states at the interface.
It is probably possible to formulate a weaker statement, which is related to statement (i). Namely, I expect that quite generically these weak topological invariants are related to the presence or absence of localised states at the interface. The appearance of such interface states can be understood by first considering a smooth interface with slowly varying parameters connecting the two Hamiltonians, and then realising that one needs quite large perturbation to remove the resulting localised interface states appearing inside the bulk gap. Thus, the interface states will often survive in specific models even in the case of a sharp interface. Nevertheless, I want to emphasise that these localised interface states are not protected just by symmetry and topology; Their appearance requires additional assumptions about the model Hamiltonian.
I am willing to reconsider the paper for publication if the authors have additional arguments to support their statements that the interface states are indeed protected by the combination of band topology and symmetry of the interface. For this purpose I suggest that the authors construct an explicit example (as suggested also both by Daniel Varjas and the first referee), and study the robustness of the interface states with respect to introducing all possible symmetrypreserving perturbations which do not change the band topology.
Author: Jasper van Wezel on 20210503 [id 1402]
(in reply to Report 2 on 20210127)Please see attached for our replies to both referees.
Attachment:
Anonymous Report 1 on 2021122 (Invited Report)
 Cite as: Anonymous, Report on arXiv:scipost_202101_00003v1, delivered 20210122, doi: 10.21468/SciPost.Report.2455
Strengths
1) this is a very wellwritten paper
2) the splitting into main text/appendices is appropriate, and the latter provide many useful details on the authors' results
3) the results are general and apply in a modelindependent way
4) I believe this work will lead to multiple followup theoretical and experimental investigations
Weaknesses
1) it would really help the readers to see one simple example Hamiltonian showing the behavior discussed in the paper
2) I'm confused about the use of the term "topological protection." It might be helpful to better clarify this term.
Report
The authors consider interface states between 2D atomic insulators in class A, which the authors define as systems with vanishing Chern number. Focusing on space groups p2, p3, and pmm, the authors show a simple and generic criterion predicting localized interface states. This is based on the logarithmic derivative of the wavefunction, which is constrained by bulk topology, namely by the symmetry labels of the bands.
I really enjoyed reading this paper, it is very well written. Results are presented in a stepbystep pedagogical way, making the authors' discussion easy to follow also for nonspecialists. I think this work is timely and of interest to the community working on topological systems, and it will motivate experimental studies on these kinds of interface states, possibly in various metamaterial platforms.
I only have two concerns with regard to this work, and I would like to ask the authors to please address them before I can recommend publication. I recognize that I am biased in this regard, since I have already read the question asked by Daniel Varjas on the SciPost submission page, and I fully agree with him.
The two points are:
1) Please add an example. I understand that the work is general, and that the results apply in a modelindependent way: this is one of the strengths of the paper. However, it would really help the readers to have something concrete to point to while going through these general results. With even a simple example in an appendix somewhere, I believe the quality of this paper would be greatly improved.
2) Several times throughout the paper, but especially at the beginning of Section 4, the authors refer to interface states which should be present at the Fermi level. At the end of Section 5, the authors say that these interface states may sensitively depend on crystal terminations. This makes me confused with respect to the way in which the authors define "topological protection."
My confusion is as follows:
As far as I can understand (again, here an example would go a long way), the symmetry labels force interfacebound states to exist, but they don't force these states to continuously interpolate in energy between the valence and the conduction band. My naive guess (which could be wrong) is that these states exist at some particular energy, $E_p$, which is in general different from 0 since there is no chiral or particlehole symmetry in class A. The interface between two inversionsymmetric bulks is in general not inversionsymmetric, so it might be possible to change the microscopic details of the interface without worrying about inversion breaking. In this case, isn't it possible for me to freely move these interface states in energy, simply by changing the microscopic details of the interface itself? For instance, could I add a chemical potential just to the interface, and push the interface states up in energy until they overlap with the bulk bands?
Note that this would be very different from the "topological protection" of strong TIs (e.g. quantum spinHall effect) and what people used to call weak TIs (e.g. 3D stack of 2D QSHE). In those systems, the boundaries are invariant under the symmetries (time reversal, translation) and the boundary states exist at the Fermi level no matter how one changes the microscopic details of the interface, provided its symmetries are left intact.
Are the states discussed in this paper and their amount of topological protection different from Shockley states? Does this paper discuss the conditions for which Shockley states exist in p2, p3, and pmm?
The authors should clarify their definition of topological protection when it comes to the constraints on the energy of interface states. Also they should clarify the connection/difference between the interface states they consider and conventional Shockley states. For instance, with a toymodel example it could be possible to show the states and see if/how they move in energy away from the Fermi level when a chemical potential is added only to the interface region.
Requested changes
See report.
Author: Jasper van Wezel on 20210503 [id 1401]
(in reply to Report 1 on 20210122)Please see attached for our replies to both referees.
Daniel Varjas on 20210108 [id 1129]
It would be very useful if the authors supported their findings with at least one concrete example. In its current form the manuscript only presents very general but somewhat counterintuitive claims. Could the authors show a specific system (given by its Hamiltonian and the action of the symmetries) demonstrating topological interface modes at a heterojunction of two atomic insulators? It would also be useful to elucidate whether the authors mean the term "atomic insulator" as a system with trivial strong invariant, or as a trivial atomic system with respect to the space group symmetries as well.
Author: Jasper van Wezel on 20210113 [id 1148]
(in reply to Daniel Varjas on 20210108 [id 1129])Dear dr. Varjas,
thank you for the helpful comments, which we greatly appreciate.
We tried to explain in the document that 'atomic insulator' is intended here to refer to a material with simple bands that have Chern number zero, without further restrictions. We will try to formulate this more clearly.
We also appreciate your suggestion of including a simulation for an example heterojunction. In fact, we purposely chose not to do that in the current manuscript, because we would like to present a general symmetrybased argument that can be broadly applied. In that light, we feel that a detailed analysis of what must necessarily be an arbitrarily chosen example, is beyond the scope of the current article.
However, if you are interested, we would be very happy to collaborate with you, and look for potentially interesting implementations of the ideas presented here.