Controlling topological phases in quantum materials offers a route to explore emergent quantum states and develop devices with topologically protected carriers. Yet, few materials allow both efficient tunability and in situ electronic measurements. Here, we present our work on HfTe₅, a prototypical van der Waals material with exceptional topological tunability.
First, we apply a large, controllable uniaxial strain to induce a topological phase transition from a weak topological insulator (WTI) to a strong topological insulator (STI). This transition leads to a dramatic increase in resistivity of over 190,000% and results in surface-state-dominated transport at cryogenic temperatures.
Second, we find that the WTI phase of HfTe₅ supports zeroth Landau level physics at moderate magnetic fields. Fields above 10 T drive transitions to 1D Weyl modes and, under low carrier density, stabilize a spin-triplet excitonic insulator phase, enabled by strong electronic instabilities in quasi-1D systems. Notably, in the STI phase, this excitonic phase emerges at even lower fields.
Third, we explore thin HfTe₅ devices (<100 nm), where enhanced surface-to-bulk transport and correlated phenomena appear. These observations highlight HfTe₅ as a versatile platform for studying topological transitions and emergent correlated states.
Together, these results position HfTe₅ as a key material for advancing quantum device applications, from spintronics to fault-tolerant topological quantum computing.