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 $T(>T_c$), 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 Pr${}_{2-x}$Ce${}_x$CuO${}_4$ and p–type Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ exhibit both similarities and striking differences. The Fermi surface is closed in overdoped metallic samples. In underdoped Pr${}_{2-x}$Ce${}_x$CuO${}_4$ samples, it clearly breaks into arcs, giving rise to a “pseudogap” signature. It is argued that such a signature is subtle in underdoped Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$.