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

The cubic pyrochlore Tl_2Ru_2O_7 undergoes concurrently a metal-insulator transition (MIT) and a first-order structural transition at T_\rm MIT\approx 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 \rho(T) measurements on the polycrystalline Tl_2Ru_2O_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 \rho(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 \rho(T) emerges at T_0 > T_\rm 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_2Ru_2O_7. The presence of two successive transitions at T_0 and T_\rm MIT in the pressure range 1.2-1.5 GPa indicates an inhomogeneous electronic state nearby the tricritical point. At P \geq 3 GPa, another broad anomaly emerges in \rho(T) at T_1 >T_0, and T_1 continuously increases with pressure, dividing the semiconducting \rho(T) into distinct thermally activated regions. These rich phenomena in the pressurized Tl_2Ru_2O_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.