Optical conductivity spectra and electronic structure of Bi2Sr2(Y1-xCax)Cu2Oy system

Abstract In order to obtain information on the electronic structure of high T c superconductors in the normal state, highly oriented Bi 2 Sr 2 (Y 1− x Ca x )Cu 2 O y films with the compositions of x =0–1.0 were grown on an MgO single crystal substrate by the spin-coating pyrolysis method. Both transmittance and reflectivity were measured in the photon energy region up to 2.5 eV and the optical conductivity was calculated without making any assumptions. The optical conductivity in the mid-infrared region, where the photon energy is less than 1.25 eV, increased proportionally with the excess oxygen content. The effective hole number obtained for the photon energy of 1.25 eV using the sum rule was in good agreement with that calculated assuming that one excess oxygen atom creates two holes. Based on these facts, it was concluded that the absorption in the mid-infrared region was caused by the electron excitation from the valence band to the acceptor level produced by excess oxygen. The CT gap was found to be 1.25 eV and the width of the valence band was estimated to be approximately 1.4 eV. A semiconductor model in which the excess oxygen acts as an acceptor was proposed for the interpretation of the optical and d.c. conductivities of the Bi 2 Sr 2 (Y 1− x Ca x )Cu 2 O y system.