Event Details

The large family of half-Heusler compounds hosts a number of topological insulator materials and potential topological superconductors, making these compounds interesting candidates to study physical phenomena on the verge of a topological phase transition. Here we present first magnetotransport measurements on high-quality single crystals of HfNiSn, which according to density functional theory calculations is a nonmagnetic, topologically trivial semiconductor without a bulk band inversion. Our samples show unconventional transport properties already at moderately low temperatures T < 200 K. Instead of the thermal carrier freeze-out expected for a bulk semiconductor, electrical conduction in HfNiSn is increasingly dominated by metallic surface states, with a saturation of the longitudinal resistance and highly nonlocal transport. X-ray diffraction shows no structural transitions that could potentially lead to anisotropic conduction in this temperature regime. Magnetoresistance measurements are consistent with weak anti-localization, a signature of low-dimensional transport in a system with strong spin-orbit coupling. An anomalous transverse resistance at zero magnetic field possibly indicates chiral transport. This suspicion is confirmed by experimental evidence for time reversal symmetry (TRS) breaking: four-point measurements related by Onsager-Casimir symmetry show a measurable difference below ~ 30 K. This result is unexpected in the light of magnetic susceptibility and muon spin rotation measurements showing that HfNiSn is a diamagnetic compound with no magnetic moments and no evident magnetic order. The chiral component of the conductance depends very weakly on applied magnetic fields up to 9 T, however it is strongly temperature dependent and vanishes above ~ 30 K. The transverse conductance shows a nonlinear dc bias dependence where a moderate dc current of 1 mA leads to an 85% suppression of the transverse voltage. The nonlinearities in I(V) curves are suppressed above 30 K, the same energy scale established for the broken TRS. Despite being topologically trivial, the surface of HfNiSn appears to host chiral states with broken TRS, where the observed nonlinear transport could indicate collective phenomena.