Photoemission study of electronic structure and metal-semiconductor transition in pyrochlore-type Tl₂Ru₂O₇

The electronic structure of pyrochlore-type Tl₂Ru₂O₇, which undergoes a metal-semiconductor transition at T_t∼120 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 EF 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 Tl₂Ru₂O₇ 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.