# Temperature-driven single-valley Dirac fermions in HgTe quantum wells

## TL;DR

This study demonstrates the emergence of single-valley Dirac fermions in HgTe quantum wells at a critical temperature, showing a temperature-induced topological phase transition and validating results with theoretical calculations.

## Contribution

It provides experimental evidence of temperature-driven topological phase transition and Dirac fermion formation in HgTe quantum wells, supported by theoretical modeling.

## Key findings

- Observation of a temperature-driven transition from topological insulator to semiconductor.
- Identification of gapless state with massless Dirac fermions at 90 K.
- Experimental energies agree with 8-band Kane Hamiltonian calculations.

## Abstract

We report on temperature-dependent magnetospectroscopy of two HgTe/CdHgTe quantum wells below and above the critical well thickness $d_c$. Our results, obtained in magnetic fields up to 16 T and temperature range from 2 K to 150 K, clearly indicate a change of the band-gap energy with temperature. The quantum well wider than $d_c$ evidences a temperature-driven transition from topological insulator to semiconductor phases. At the critical temperature of 90 K, the merging of inter- and intra-band transitions in weak magnetic fields clearly specifies the formation of gapless state, revealing the appearance of single-valley massless Dirac fermions with velocity of $5.6\times10^5$ m$\times$s$^{-1}$. For both quantum wells, the energies extracted from experimental data are in good agreement with calculations on the basis of the 8-band Kane Hamiltonian with temperature-dependent parameters.