Emergence of new quantum-critical magnetic ground state in pyrochlore iridate Sm₂Ir₂O₇

Pyrochlore iridates, comprised of two inter-penetrating magnetic lattices of corner-sharing tetrahedra, are a complex and varied group of compounds under investigation into their exotic magnetic and electronic properties across condensed-matter physics. These magnetic tetrahedral networks order in fascinating all-in-all-out or 2-in-2-out radial spin arrangements. The study of quantum criticality, and the tuning of metal-insulator transitions, are already fertile areas for uncovering new phases of matter and understanding of fundamental physics - criticality of such an exotic form of magnetism has richer potential still. We report magnetic, transport and magnetotransport measurements on single crystals of Sm₂Ir₂O₇, in particular using applied pressure to tune magnetic order in the system and mapping the full pressure-temperature phase diagram up to 87 kbar. The all-in-all-out long-range order of the Ir sub-lattice which governs the ambient-pressure behaviour is shown to be suppressed to a quantum critical point at a critical pressure of around 63 kbar. Rather than observing the metal-insulator transition in this compound give way to metallic behaviour as Ir order is removed, we report instead a secondary mechanism taking over the gapping of electrical transport, and the weak-insulator state persisting beyond the critical point. Within the Ir ordered phase at low pressures, we report a characteristic hysteresis in the magnetoresistance we equate to that previously seen in Ho₂Ir₂O₇, but with a far larger saturation field. Finally, in the vicinity of the quantum critical point we uncover the emergence of a new state described by a weaker hysteresis and a negative magnetoresistance. We suggest this to be due to the weak Sm-Sm interactions becoming dominant and driving a new form of order.