Raman study of the metal-insulator transition in pyrochloreMooxides

Abstract

Raman scattering spectra have been investigated for the correlated $4d$-electron system, ${R}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ ($R=\mathrm{Nd}$, $\mathrm{Sm}$, $\mathrm{Gd}$, $\mathrm{Tb}$,$\mathrm{Dy}$,${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}$), which undergoes a metal-insulator transition with changing the rare-earth ion $R$, or equivalently the one-electron bandwidth. It is found that several phonon peaks modulating the $\mathrm{Mo}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Mo}$ bond angle appear in the metallic phase ($R=\mathrm{Nd}$, $\mathrm{Sm}$, $\mathrm{Gd}$, ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}$), whereas they are remarkably suppressed in intensity in the insulating phase ($R=\mathrm{Tb}$,$\mathrm{Dy}$). This result indicates that the phonon modes of ${R}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ are coupled with the electron-hole excitation across the Fermi level, thus probing sensitively the low-energy charge dynamics in the vicinity of the bandwidth-control Mott transition.