LAUSR

Studying the influence of broken time-reversal symmetry on topological materials is an important fundamental problem of current interest in condensed matter physics and its understanding could also provide a route toward proof-of-concept spintronic devices that exploit spin-textured topological states. Here we develop a new model quantum material for studying the effect of breaking time-reversal symmetry: a hybrid heterostructure wherein a ferromagnetic semiconductor Ga1−xMnxAs, with an out-of-plane component of magnetization, is cleanly interfaced with a topological insulator (Bi,Sb)2(Te,Se)3 by molecular beam epitaxy. Lateral electrical transport in this bilayer is dominated by conduction through (Bi,Sb)2(Te,Se)3 whose conductivity is a few orders of magnitude higher than that of highly resistive Ga1−xMnxAs. Electrical transport measurements in a top-gated heterostructure device reveal a crossover from weak antilocalization to weak localization as the temperature is lowered or as the chemical potential approaches the Dirac point. This is accompanied by a systematic emergence of an anomalous Hall effect. These results are interpreted in terms of the opening of a gap at the Dirac point due to exchange coupling between the topological insulator surface state and the ferromagnetic ordering in Ga1−xMnxAs. The experiments described here show that well-developed III–V ferromagnetic semiconductors could serve as valuable components of artificially designed quantum materials aimed at exploring the interplay between magnetism and topological phenomena.Topological insulators: Magnetism and surface statesA carefully engineered device allows the study of the role of time-reversal symmetry breaking in topological insulators. The topological properties and surface states of materials like bismuth selenide are protected by time-reversal symmetry, so it is crucial to understand how phenomena like ferromagnetism that break this symmetry will affect these states. Joon Sue Lee and collaborators from the Pennsylvania State University have created a device consisting of a topological insulator thin film grown on top of a ferromagnetic layer with tightly controlled doping in both layers. Quantum transport measurements show that the exchange interactions from the ferromagnet create a gap in the surface states at the interface while the far surface of the topological insulator remains gapless. Similar devices will enable detailed study of the fundamental role of symmetries in topological materials.