Importance of the semimetallic state for the quantum Hall effect in HfTe5

At ambient pressure, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>HfTe</a:mi><a:mn>5</a:mn></a:msub></a:math> 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 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>HfTe</b:mi><b:mn>5</b:mn></b:msub></b:math> 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 ground state for the emergence of the QHE in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>HfTe</c:mi><c:mn>5</c:mn></c:msub></c:math> and thus favors a scenario based on a low carrier density metal in the quantum limit for the observed signatures of the quasiquantized 3D QHE. Published by the American Physical Society 2024