Effect of Pressure on the Metal-Insulator Transition of the Cubic Pyrochlore Tl₂Ru₂O₇

Abstract The cubic pyrochlore Tl 2 Ru 2 O 7 undergoes concurrently a metal–insulator transition (MIT) and a first-order structural transition at T MIT ≈ 120 K, below which the system was found to form one-dimensional spin-one Haldane chains associated with an orbital ordering of Ru-4d electrons. With an aim to tune and access distinct ground states with strong entanglements of multiple degrees of freedom, i.e., spin, orbital, charge, and lattice, we utilize a high-pressure approach to regulate the MIT of this system. Our detailed resistivity ρ ( T ) measurements on the polycrystalline Tl 2 Ru 2 O 7 samples under various hydrostatic pressures indeed reveal an unusual evolution of the electronic ground states. At first, the MIT is suppressed monotonically from 120 K at ambient to about 70 K at 1.5 GPa and then vanishes suddenly at about 1.8 GPa without achieving a metallic ground state. Meanwhile, the system evolves into a semiconducting ground state with magnitude of ρ ( T ) in the entire temperature range enhanced gradually by further increasing pressure. Prior to the abrupt disappearance of MIT, a new electronic order manifested as a kink-like anomaly in ρ ( T ) emerges at T 0 > T MIT at 1.2 GPa and it continues to increase with pressure, producing a tricritical-point-like behavior in the T – P phase diagram of Tl 2 Ru 2 O 7 . The presence of two successive transitions at T 0 and T MIT in the pressure range 1.2–1.5 GPa indicates an inhomogeneous electronic state nearby the tricritical point. At P ≥ 3 GPa, another broad anomaly emerges in ρ ( T ) at T 1 > T 0 , and T 1 continuously increases with pressure, dividing the semiconducting ρ ( T ) into distinct thermally activated regions. These rich phenomena in the pressurized Tl 2 Ru 2 O 7 should originate from the complex interplay of strongly entangled multiple quantum degrees of freedom in the system near the localized-to-itinerant crossover regime.