Importance of the semimetallic state for the quantum Hall effect in HfTe$_{5}$

TL;DR

This study explores how the semimetallic state in HfTe$_{5}$ is crucial for the emergence of the 3D quantum Hall effect, showing pressure-induced phase transitions and the importance of low carrier density for QHE.

Contribution

It demonstrates the pressure-dependent transition from semimetallic to insulating phases in HfTe$_{5}$ and highlights the role of the semimetallic groundstate in enabling the quantum Hall effect.

Findings

Pressure induces a transition from semimetallic to insulating phase around 1.5 GPa.

Quasi-3D quantum Hall effect occurs only in the semimetallic phase at low pressures.

A Lifshitz transition is observed within the insulating phase near 2 GPa.

Abstract

At ambient pressure, HfTe$_{5}$ is a material at the boundary between a weak and a strong topological phase, which can be tuned by changes in its crystalline structure or by the application of high magnetic fields. It exhibits a Lifshitz transition upon cooling, and three-dimensional (3D) quantum Hall effect (QHE) plateaus can be observed at low temperatures. Here, we have investigated the electrical transport properties of HfTe$_{5}$ under hydrostatic pressure up to 3 GPa. We find a pressure-induced crossover from a semimetallic phase at low pressures to an insulating phase at about 1.5 GPa. Our data suggest the presence of a pressure-induced Lifshitz transition at low temperatures within the insulating phase around 2 GPa. The quasi-3D QHE is confined to the low-pressure region in the semimetallic phase. This reveals the importance of the semimetallic groundstate for the emergence of the QHE in HfTe$_{5}$ and thus favors a scenario based on a low carrier density metal in the quantum limit for the observed signatures of the quasi-quantized 3D QHE.