Optical spectroscopy as a probe of gaps and kinetic electronic energy in p- and n-type cuprates

The real part of the optical in-plane conductivity of p- and n-type cuprates thin films at various doping levels was deduced from highly accurate reflectivity measurements. We present here a comprehensive set of optical spectral weight data as a function of the temperature T(>Tc), for underdoped and overdoped samples. The temperature dependence of the spectral weight is not universal. Using various cut-off frequencies for the spectral weight, we show that n-type Pr2−xCexCuO4 and p-type Bi2Sr2CaCu2O8+δ exhibit both similarities and striking differences. The Fermi surface is closed in overdoped metallic samples. In underdoped Pr2−xCexCuO4 samples, it clearly becomes discontinuous giving rise to a “pseudogap” signature. It is argued that such a signature is subtle in underdoped Bi2Sr2CaCu2O8+δ.