Photoemission study of electronic structure and metal-semiconductor transition in pyrochlore-typeTl2Ru2O7

Abstract

The electronic structure of pyrochlore-type ${\mathrm{Tl}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7},$ which undergoes a metal-semiconductor transition at ${T}_{t}\ensuremath{\sim}120\mathrm{K},$ has been studied by ultraviolet photoemission spectroscopy (UPS) and inverse-photoemission spectroscopy [bremsstrahlung isochromat spectroscopy (BIS)]. The temperature dependence of the UPS intensity around the Fermi level ${(E}_{F}),$ which is largely due to the Ru $4d$ states, reflects the transition. In the BIS spectra, changes occur across the metal-semiconductor transition not only near ${E}_{F}$ but also several eV above ${E}_{F}$ where the Ru $4d$ ${e}_{g}$ states and the Tl $6s$ states are dominant. It is therefore concluded that the transport properties across the metal-semiconductor transition in ${\mathrm{Tl}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ are mainly determined by the Ru ${4dt}_{2g}$ states around ${E}_{F}$ but that the changes in the Tl $6s$ states also make contributions to the transition.