Magnetotransport of pyrochlore iridate Sm2Ir2O7 across a pressure-induced quantum-critical phase boundary*

Rare-earth pyrochlore iridates host two interlocking magnetic sublattices of corner-sharing tetrahedra and can harbour a unique combination of frustrated moments, exotic excitations and highly correlated electrons. We have measured the transport and magnetotransport properties of single-crystal Sm2Ir2O7 up to and beyond the pressure-induced quantum critical point (QCP) for all-in-all-out (AIAO) Ir order at pc = 63 kbar previously identified by resonant X-ray scattering. Contrary to prior predictions, we do not find a crossover from insulating to metallic behavior at low temperatures above pc. Instead, the metal-insulator transition temperature, which tracks the decrease in the AIAO ordering temperature for pressures up to 30 kbar, begins to increase under further application of pressure, pointing to the presence of an as-yet unidentified localization mechanism. The magnetotransport does track the suppression of Ir magnetism, however, with a strong hysteresis observed only within the AIAO phase boundary, similar to that found for Ho2Ir2O7 and attributed to plastic deformation of Ir domains. Around pc we find the emergence of a new type of electronic behavior, characterized by a negative magnetoresistance with small hysteresis at the lowest temperatures, and hysteresis-free positive magnetoresistance above 5 K. A Weyl semimetal phase is predicted to occur in the vicinity of the QCP. The temperature dependence of our low-temperature transport data, however, are found to be best described by a model consistent with a Weyl semimetal across the entire pressure range.