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

Raman scattering spectra have been investigated for the correlated 4d-electron system, R2Mo2O7 (R=Nd, Sm, Gd, Tb,Dy,Nd1-xDyx), 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, Nd1-xDyx), 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.