Pressure-tuned insulator to metal transition in Eu ₂Ir ₂O ₇

We have studied the effect of pressure on the pyrochlore iridate Eu ₂Ir ₂O ₇, which, at ambient pressure, has a thermally driven insulator to metal transition at T _MI∼120K. As a function of pressure, the insulating gap closes, apparently continuously near P∼6GPa. However, rather than T _MI going to zero as expected, the insulating ground state crosses over to a metallic state with a negative temperature coefficient of resistivity, suggesting that these ground states have a novel character. The high-temperature state also crosses over near 6 GPa from an incoherent to a conventional metal, implying that there is a connection between the high- and the low-temperature states.