M ar 2 00 6 Temperature dependence of the spectral weight in p – and n – type cuprates : a study of normal state partial gaps and electronic kinetic energy

The optical conductivity of CuO2 (copper-oxygen) planes in p and n–type cuprates thin films at various doping levels is deduced from highly accurate reflectivity data. The temperature dependence of the real part σ1(ω) of this optical conductivity and the corresponding spectral weight allow to track the opening of a partial gap in the normal state of n-type Pr2−xCexCuO4 (PCCO), but not of p–type Bi2Sr2CaCu2O8+δ (BSCCO) cuprates. This is a clear difference between these two families of cuprates, which we briefly discuss. In BSCCO, the change of the electronic kinetic energy Ekin – deduced from the spectral weight– at the superconducting transition is found to cross over from a conventional BCS behavior (increase of Ekin below Tc) to an unconventional behavior (decrease of Ekin below Tc) as the free carrier density decreases. This behavior appears to be linked to the energy scale over which spectral weight is lost and goes into the superfluid condensate, hence may be related to Mott physics.