Zero Flux Localization: Magic Revealed

TL;DR

This paper reveals how introducing a non-Abelian spin field component can lead to perfect localization and flat bands in inhomogeneous magnetic fields with zero total flux, with implications for moiré materials and quantum Hall states.

Contribution

It demonstrates that non-Abelian fields enable flat bands in zero-flux magnetic systems and provides exactly solvable models illustrating this phenomenon.

Findings

Perfect localization achieved with non-Abelian spin fields.

Flat bands occur at specific quantized flux values.

Exact solutions involve elliptic functions on a torus.

Abstract

Flat bands correspond to the spatial localization of a quantum particle moving in a field with discrete or continuous translational invariance. The canonical example is the flat Landau levels in a homogeneous magnetic field. Several significant problems -- including flat bands in moir\'e structures -- are related to the problem of a particle moving in an inhomogeneous magnetic field with zero total flux. We demonstrate that while perfectly flat bands in such cases are impossible, the introduction of a "non-Abelian component" -- a spin field with zero total curvature -- can lead to perfect localization. Several exactly solvable models are constructed: (i) a half-space up/down field with a sharp 1D boundary; (ii) an alternating up/down field periodic in one direction on a cylinder; and (iii) a doubly periodic alternating field on a torus. The exact solution on the torus is expressed in terms of elliptic functions. It is shown that flat bands are only possible for certain magic values of the field corresponding to a quantized flux through an individual tile. These exact solutions clarify the simple structure underlying flat bands in moir\'e materials and provide a springboard for constructing a novel class of fractional quantum Hall states.