Raman study of the metal-insulator transition in pyroclilore Mo oxides

Raman scattering spectra have been investigated for the correlated 4d-electron system, R₂Mo₂O₇ (Tf=Nd, Sm, Gd, Tb,Dy,Nd_1-xDy_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 Mo-O-Mo bond angle appear in the metallic phase (R=Nd, Sm, Gd, Nd_1-xJDy _x), whereas they are remarkably suppressed in intensity in the insulating phase (R=Tb,Dy). This result indicates that the phonon modes of R₂Mo₂O₇ 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.