Doping dependent optical properties ofBi2Sr2CaCu2O8+δ

Abstract

We report on the ab-plane reflectance of the high temperature superconductorBi2Sr2CaCu2O8+δ (Bi-2212). Measurements on samples spanning the doping range from underdoped withTc = 67 K (UD), to optimallydoped with Tc = 96 K (OPT),to overdoped with Tc = 60 K (OD) were made from room temperature down to the superconducting state regime. Themeasured reflectance data were analysed to extract the optical conductivity and the realand imaginary parts of the free carrier optical self-energy. We get an estimate of the dcresistivity from the low frequency extrapolation of the optical conductivity and thesuperfluid density from the imaginary part of the optical conductivity. The conductivitysum rule can be related to the changes of the kinetic energy of the system. When thissystem becomes a superconductor, the kinetic energy decreases in the underdoped samplesand increases in the overdoped ones. The optical self-energy, obtained from the extendedDrude model, is dominated by two channels of interaction: a sharp mode and a broadbackground. The amplitude of the mode is strongly doping and temperature dependentwhereas the background decreases weakly with doping and is nearly temperatureindependent.