Anomalous pressure dependence of phonon line widths in metallic pyrochlore iridates (Eu$_{1-x}$Bi$_x$)$_2$Ir$_2$O$_7$: crossover from incoherent to coherent metal

Abstract

Pyrochlore iridate (Eu$_2$Ir$_2$O$_7$) shows an interesting metallic phase, termed as incoherent metal, above the metal–insulator transition temperature, with high resistivity and positive temperature coefficient of resistivity (TCR). On increasing the pressure, there is a crossover at pressure P* to a coherent metal phase with positive TCR. We have investigated this crossover using high-pressure Raman spectroscopy and X-ray diffraction for a series of samples (Eu$_{1-x}$Bi$_x$)$_2$Ir$_2$O$_7$ (x = 0, 0.02, 0.035 and 0.1) up to $\sim$20 GPa. X-ray diffraction data show a clear change in the pressure dependence of Ir–Ir bond distance at P*. The line width of the A$_{1g}$ phonon shows an anomalous decrease with pressure up to P*, without significant change in the line width of the other three Raman modes with symmetries $T^1_{2g}$, $E_{g}$ and $T^2_{2g}$. We attribute these results to the decreasing contribution of electron–phonon coupling in the incoherent metal till P*.