Mexican Hat and Rashba Bands in Few-Layer van der Waals Materials

TL;DR

This paper investigates the Mexican hat and Rashba band structures in few-layer van der Waals materials, revealing their impact on electronic and thermoelectric properties through ab initio calculations and analytical models.

Contribution

It provides a comprehensive analysis of the band structures and thermoelectric effects in various few-layer materials, highlighting the role of interlayer coupling and vertical bias.

Findings

Large band-edge density of modes enhances thermoelectric performance.

Interlayer coupling significantly affects electronic properties.

Vertical bias in bilayer graphene offers a test-bed for these effects.

Abstract

The valence band of a variety of few-layer, two-dimensional materials consists of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a `Mexican hat' or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III-VI materials GaS, GaSe, InS, InSe, for Bi$_{2}$Se$_{3}$, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III-VI materials and Bi$_{2}$Se$_{3}$ is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.