Plane-polarized Raman continuum in the insulating and superconducting layered cuprates

Electronic properties of copper oxygen planes (and chains in Y-Ba-Cu-O) were studied with Raman spectroscopy of plane-polarized photons. The electronic continuum was found to be independent of doping in 2:1:4 and 1:2:3 materials at energies above ∼1000 cm-1. Temperature dependence at low energies differs significantly in undoped, lightly doped, and fully doped YBa2Cu3O6+x. A feature consistent with the superconducting gap was observed below Tc in YBa2Cu3O6.9 in all scattering geometries. However, the gaplike redistribution was not complete, with 40-60 % of states not shifted to higher energies at temperatures well below Tc. Above Tc the temperature dependence strongly depends on scattering geometry: the continuum is temperature independent (marginal-Fermi-liquid-like) in XX (x2) and X'X' (x2+y2) geometry; it has a Bose-factor temperature dependence in X'Y' (x2-y2) geometry, and a weak temperature dependence somewhat smaller than the Bose factor in YY (y2) geometry. A two-boson-like temperature dependence of the low-energy continuum is found in YBa2Cu3O6.1 and Sm2CuO4. It becomes one-particle-like in Y-Ba-Cu-O once small doping levels are introduced. Constraints these results place on theoretical models are discussed.