Electron correlation and the metal-insulator transition of the pyrochlore molybdatesR2Mo2O7(R=Nd,Sm,Gd,Tb,Y)

Abstract

The bulk and surface electronic structures of pyrochlore-type molybdenum oxides ${R}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ ($R=\mathrm{Nd}$, Sm, Gd, Tb, and Y) have been studied by using photoelectron spectroscopy (PES) at high and low excitation energies. The spectra measured at different excitation energies were distinctly different in shape. PES spectra were found to depend on $R$. In particular, the high-resolution low-energy PES spectra near the Fermi level $({E}_{F})$ showed a noticeable change of the spectral intensity in accordance with the metal-insulator (M-I) transition as a function of $R$. We have deconvoluted the true bulk and surface spectra and demonstrated that the above-mentioned near-${E}_{F}$ feature reflects the bulk electronic states. Furthermore, we propose that the M-I transition can be understood as a Mott transition induced by the $R$ dependence of $U∕t$, the ratio between the Coulomb repulsion energy, and the hopping term through a comparison between the experimentally derived bulk electronic states and band-structure calculation.