# Bulk boundary correspondence = difference in Chern numbers?

In topological insulators the bulk boundary correspondence is frequently stated as the principle that the number of edge modes equals the difference in Chern numbers at that edge. I found this e.g. in "Topological Band Theory and the Z2 Invariant" by C.L. Kane pp.18 . However in there is no "derivation" to this in the text by C.L. Kane.

Can anyone present to me an argument (maybe even a proof) that the number of edge states corresponds to the change in Chern numbers? A source where I can find such an argument would also be great!

• Adding comment from WangShaoyun: it seems it relates to the Atiyah–Singer index theorem, because n the Atiyah-Singer index theorem, topological index = analytical index and, in a topological insulator, bulk invariant = boundary invariant. This paper about the mathematical foundation of bulk boundary correspondence may be a good reference: Emil Prodan, Hermann Schulz-Baldes. Bulk and Boundary Invariants for Complex Topological Insulators: From K-Theory to Physics. arxiv.org/abs/1510.08744 Jun 22, 2021 at 12:35

## 4 Answers

There seems to be no general but simple derivation to this fact. However there are some papers concerning the issue that involve mathematical techniques too advanced for me still. The works that I found are

I also found some lecture notes on the topic also giving some more detailed background information at arXiv:1501.02874. Maybe it helps somebody.

• The first proof is due to Hatsugai for the Harper model. Then there are the proofs by Schulz-Baldes et al (K-theory) and Graf et al (functional analysis) which work for more general models. The proof you cite of Mong deals with very specific models but is perhaps the best place to start as one can really picture the geometric argument and one only needs to know complex analysis essentially.
– PPR
Jul 16, 2017 at 9:29
• I think the last arXiv link is strange. Feb 11, 2021 at 14:23

There is this paper of Fidkowski, Jackson, and Klich. They have the simplest description I've found so far. The idea is that no matter the model, you can form a spectrum-flattened version

$$H = 1 - 2P,$$

where $P$ projects onto the filled states. Then, an edge may be added by forming the Hamiltonian

$$H' = 1 - P - PVP,$$

where $V$ is a boundary profile function, which is $-1$ for $x < - \Delta$, $+1$ for $x > \Delta$, and $X/\Delta$ in some width $\Delta$. Thus, for $x \ll 0$ it is $H$ and for $x \gg 0$ it is the trivial Hamiltonian $1$.

The point that relates this all to the Chern number is that

$$PXP$$

is the Berry connection, so the Berry holonomy (in the direction perpendicular to the boundary) contributes a shift to the spectrum in the presence of a boundary.

In particular, in 2d, as we move around the Brillouin zone in the parallel direction, the holonomy winds $n$ times, where $n$ is the Chern number. It's easy to show the spectrum of this simple system is $m + nk_{||}/2\pi$ where $m \in \mathbb{Z}$ and $k_{||} \in [0,2\pi)$. Thus, at any value of the chemical potential, the bands cross zero energy $n$ times in the same direction, meaning there are $n$ modes with chiral $k_{||}$. One can identify these with the edge modes because the original $H$ has integer spectrum.

NOTE: I don't know if anyone showed that you can continuously connect the regimes where the domain wall looks like a step function (and the edge mode profile is like $e^{-x}$) to where the domain wall is a linear profile (and the edge mode is like $e^{-x^2}$). If somebody did this rigorously I would be very happy to look at it.

I would formulate the argument as follows:

1. Take a Thouless pump that adiabatically pumps charge, 1 negative energy state by 1 unit cell to the right per cycle. Thus the Chern number of the pump cycle is 1.

2. Because of unitarity, the net result of one cycle must be: 2A) 1 negative energy state must be converted to a positive energy state on the right end 2B) 1 positive energy state must be converted to a negative energy state on the left end 2C) 1 positive energy state must be pumped by 1 unit cell to the left.

3. Now promote time "t" to momentum "ky". You thus obtain a 2-dimensional system, which is infinite along y, but finite along x, with a left edge and a right edge. From 2A), you find that in the bulk gap, there is 1 edge state on the right edge moving up (more precisely, one more moving up than moving down) From 2B), you find an edge state on the left edge, moving down.

This is bulk--boundary correspondence (I just used Chern number 1 for simplicity, but it holds for any Chern number).

We try to spell this out in our lecture notes: https://arxiv.org/abs/1509.02295

This paper about the mathematical foundation of bulk boundary correspondence may be a good reference:

Emil Prodan, Hermann Schulz-Baldes. Bulk and Boundary Invariants for Complex Topological Insulators: From K-Theory to Physics. https://arxiv.org/abs/1510.08744

• Adding this connection is helpful - this would better serve as a comment on the original post, which you will be able to do eventually Jun 21, 2021 at 13:35
• I am sorry that I don't have enough reputation to add a comment, but I deleted my personal opinion which may make misunderstanding. Jun 21, 2021 at 16:04
• When you get the reputation, it will be worth adding the comment! Jun 21, 2021 at 17:49
• If you have the reputation, you can copy that into the comment and I will delete the answer soon. Jun 21, 2021 at 22:13