Energy spectrum of valence band in HgTe quantum wells on the way from a two to the three dimensional topological insulator

TL;DR

This study investigates the valence band energy spectrum in HgTe quantum wells, revealing discrepancies between experimental results and theoretical models, and highlighting the need for improved theoretical descriptions.

Contribution

It provides experimental measurements of the valence band spectrum in HgTe quantum wells and challenges existing four-band kp theoretical predictions.

Findings

Double degeneracy of states at the valence band top in certain wells

Cyclotron mass increases with hole density from 0.2 to 0.3 times electron mass

Experimental data strongly contradicts four-band kp model predictions

Abstract

The magnetic field, temperature dependence and the Hall effect have been measured in order to determine the energy spectrum of the valence band in HgTe quantum wells with the width (20-200)nm. The comparison of hole densities determined from the period Shubnikov-de Haas oscillations and the Hall effect shows that states at the top of valence band are double degenerate in teh entry quantum wells width the width range. The cyclotron mass determined from temperature dependence of SdH oscillations increases monotonically from (0.2-0.3) mass of the free electron, with increasing hole density from 2e11 to 6e11 cm^-2. The determined dependence has been compared to theoretical one calculate within the four band kp model. The experimental dependence was found to be strongly inconsistent with this predictions. It has been shown that the inclusion of additional factors (electric field, strain) does not remove the contradiction between experiment and theory. Consequently it is doubtful that the mentioned kp calculations adequately describe the valence band for any width of quantum well.