Optical investigations on $Y_{2-x}B{i}_{x}R{u}_2{O}_7$: Electronic structure evolutions related to the metal-insulator transition

Abstract

Optical conductivity spectra of cubic pyrochlore $Y_{2-x}B{i}_{x}R{u}_2{O}_7$ (0.0$\le${\it x}$\le$2.0) compounds are investigated. As a metal-insulator transition (MIT) occurs around {\it x}$=$0.8, large spectral changes are observed. With increase of {\it x}, the correlation-induced peak between the lower and the upper Hubbard bands seems to be suppressed, and a strong mid-infrared feature is observed. In addition, the $p-d$ charge transfer peak shifts to the lower energies. The spectral changes cannot be explained by electronic structural evolutions in the simple bandwidth-controlled MIT picture, but are consistent with those in the filling-controlled MIT picture. In addition, they are also similar to the spectral changes of Y${}_{2-x}$Ca${}_x$Ru${}_2$O${}_7$ compounds, which is a typical filling-controlled system. This work suggests that, near the MIT, the Ru bands could be doped with the easily polarizable Bi cations.

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