Electronic band structure of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{\mathit{y}}$ (x=0.4 to 1.0) determined by infrared-induced intersite transitions

To investigate the band structure of nonmetallic ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Y}}_{\mathit{x}}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ (x=0.4–1.0), photon-induced hopping has been employed as a new method, in which transmission spectroscopy has been carried out from 10 to 12 000 ${\mathrm{cm}}^{\mathrm{-}1}$. The photon-induced-hopping conduction edge has been found at 5200 ${\mathrm{cm}}^{\mathrm{-}1}$. This edge does not shift with (x=0.4–1.0), whereas the absorption is proportional to 1-x. The charge-transfer absorption edge at 12 000 ${\mathrm{cm}}^{\mathrm{-}1}$ also does not shift with x. Using this method, the bandwidth has been determined to be 5200 ${\mathrm{cm}}^{\mathrm{-}1}$. Obvious midgap states have not been observed.