The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. We used molecular beam epitaxy (MBE) to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe2) and a topological semimetal (ZrTe2). We demonstrated ferromagnetism and the anomalous Hall effect in CrTe2/ZrTe2 heterostructures down to one unit-cell (u.c.) thick 1T-CrTe2. We also demonstrated efficient current-driven spin-orbit torque (SOT) magnetization switching in these wafer-scale, full vdW topological semimetal/2D ferromagnet heterostructure devices up to about 100 K. [Y. Ou et al., Nature Communications 13, 2972 (2022)]

A collaboration with the Awschalom group (Chicago) produced a surprising discovery: illumination of topological insulator films on strontium titanate substrates provides a way to optically pattern persistently doped regions of the film. This provides a potentially powerful scheme for writing and erasing circuits that contain topological insulator devices [Yeats et al., Science Advances, 2015] . See also these press releases from Chicago and Penn State.

A collaborative experiment with the Ong group (Princeton) shows that at low temperature (T < 150 mK), the transition between Chern states in a quantum anomalous Hall insulator occurs via sudden macroscopic magnetization reversal events. These events are detected as sudden discrete jumps in the anomalous Hall resistance and their variation with temperature and chemical potential suggest a quantum tunneling out of a false vacuum [Liu et al., Science Advances, 2016].

The interface between an insulating ferromagnet (YIG) and a topological insulator provides an unambiguous platform for demonstrating that spin-charge conversion occurs in helical Dirac surface states [Wang et al., Physical Review Letters, 2016].

A collaboration with Dan Ralph at Cornell produced the first exciting   result in ‘topological spintronics,’ the demonstration of highly efficient spin-charge conversion at a topological insulator/ferromagnet interface [Mellnik et al., Nature (2014)] . Learn more in the press release.