Band structure of a two-dimensional (2D) electron gas in the presence of 2D electric and magnetic modulations and of a perpendicular magnetic field

TL;DR

This paper investigates how 2D electric and magnetic modulations affect the energy spectrum of a 2D electron gas in a perpendicular magnetic field, revealing that the butterfly spectrum's structure remains robust under various conditions.

Contribution

It provides both numerical and analytical analysis of the energy spectrum, showing the invariance of the butterfly spectrum's structure and the effects of phase differences between modulations.

Findings

The butterfly spectrum remains qualitatively unchanged by modulation strengths.

At integer flux ratios, the spectrum collapses into a band.

Phase differences of π/2 suppress resistivity oscillations.

Abstract

Two-dimensional (2D) periodic electric modulations of a 2D electron gas split each Landau level into the well-known butterfly-type spectrum described by a Harper-type equation multiplied by an envelope function. This equation is slightly modified for 2D magnetic modulations but the spectrum remains qualitatively the same. The same holds if both types of modulations are present. The modulation strengths do not affect the structure of the butterfly-type spectrum, they only change its scale or its envelope. The latter is described by the ratio $\alpha$ of the flux quantum $h/e$ to the flux per unit cell. Exact numerical and approximate analytical results are presented for the energy spectrum as a function of the magnetic field. For integer $\alpha$ the internal structure collapses into a band for all cases. The bandwidth at the Fermi energy depends on the modulation strength, the electron density, and, when both modulations are present, on the phase difference between them. In the latter case if the modulations have a $\pi/2$ phase difference, the bandwidth at the Fermi energy is nearly independent of the magnetic field and the commensurability oscillations of the diffusive contribution to the resistivity disappear.