On occurrence of spectral edges for periodic operators inside the Brillouin zone

TL;DR

This paper investigates whether spectral edges of periodic operators always occur at high symmetry points of the Brillouin zone, providing examples where they do not and analyzing the stability of such edges under perturbations.

Contribution

It constructs examples showing spectral edges can occur inside the Brillouin zone and demonstrates the stability of high symmetry point edges in generic cases.

Findings

Spectral edges can occur inside the Brillouin zone.

Edges at high symmetry points are stable under small perturbations.

Practical cases often exhibit spectral edges at high symmetry points.

Abstract

The article discusses the following frequently arising question on the spectral structure of periodic operators of mathematical physics (e.g., Schroedinger, Maxwell, waveguide operators, etc.). Is it true that one can obtain the correct spectrum by using the values of the quasimomentum running over the boundary of the (reduced) Brillouin zone only, rather than the whole zone? Or, do the edges of the spectrum occur necessarily at the set of ``corner'' high symmetry points? This is known to be true in 1D, while no apparent reasons exist for this to be happening in higher dimensions. In many practical cases, though, this appears to be correct, which sometimes leads to the claims that this is always true. There seems to be no definite answer in the literature, and one encounters different opinions about this problem in the community. In this paper, starting with simple discrete graph operators, we construct a variety of convincing multiply-periodic examples showing that the spectral edges might occur deeply inside the Brillouin zone. On the other hand, it is also shown that in a ``generic'' case, the situation of spectral edges appearing at high symmetry points is stable under small perturbations. This explains to some degree why in many (maybe even most) practical cases the statement still holds.