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 ${\mathrm{cm}}^{\mathrm{-}1}$. Temperature dependence at low energies differs significantly in undoped, lightly doped, and fully doped ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$. A feature consistent with the superconducting gap was observed below ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ 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 ${\mathit{T}}_{\mathit{c}}$. Above ${\mathit{T}}_{\mathit{c}}$ the temperature dependence strongly depends on scattering geometry: the continuum is temperature independent (marginal-Fermi-liquid-like) in XX (${\mathit{x}}^2$) and X’X’ (${\mathit{x}}^2$+${\mathit{y}}^2$) geometry; it has a Bose-factor temperature dependence in X’Y’ (${\mathit{x}}^2$-${\mathit{y}}^2$) geometry, and a weak temperature dependence somewhat smaller than the Bose factor in YY (${\mathit{y}}^2$) geometry. A two-boson-like temperature dependence of the low-energy continuum is found in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.1}$ and ${\mathrm{Sm}}_2$ ${\mathrm{CuO}}_4$. 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.