Optical absorption spectra and electronic structure of Bi2(La1−xSrx)2CuOy

Abstract The c -axis-oriented films of Bi 2 (La 1− x Sr x ) 2 CuO y (BLSCO) with a composition region 0.5≤ x ≤1.0 were grown on a single crystal MgO (100) substrate using the spin-coating pyrolysis method. Transmittance and reflectivity spectra were taken in the photon energy region from 2.5 to 13 meV and from 0.12 to 2.5 eV at room temperature. The absorption coefficient α was calculated from the film thickness, transmittance and reflectivity. The absorption spectra exhibited a constant edge of 1.2 eV which did not vary with strontium content x . This phenomenon demonstrates that the CT gap of BLSCO remains unchanged with the substitution of Sr for La. The absorption spectra in the infrared to visible light region were insensitive to the excess oxygen content, while those in the mid- and far-infrared regions showed strong dependence on the excess oxygen content. The absorption coefficient in the far-infrared region from 2.5 to 13 meV was very close to that calculated from the dc conductivity. The effective hole number calculated using the sum rule agreed with that calculated, assuming that one excess oxygen atom creates two holes in the composition region 0.5≤ x ≤0.8. The temperature dependence of conductivity showed the existence of an energy gap between the valence band and the acceptor level originated from the excess oxygen. An electronic model where the excess oxygen acts as the acceptor was proposed to interpret the observed results.