Latest results on the anomalous Raman continuum in YBCO and in certain metals

Three new results are presented regarding the nearly flat electronic Raman continuum found in the cuprate superconductors. (1) Its properties are quite different for untwinnedYBa₂Cu₃O₇ in (bb) and (cc) polarization geometries. In (cc) the cross-section dσ/dω falls off with increasing ω, rather than gently rising as in (bb), and it shows a rather strong resonance towards 2.8 eV. (2) The continuum in YBa₂Cu₃O_x behaves differently with oxygen doping in (bb) relative to (aa), growing in strength with increasing x. [D. Reznik et al., this conference] (3) The T-dependence of the linewidth Γ of the 115 cm^-1 "Ba" phonon mode for 10<T<300K can be explained using a phenomenological model in which the phonon decays either into two phonons or into the electronic continuum; the cusp in Γ at T_c follows from that of dσ(T)/dω at T=T_c. Finally, we test suggestion that the Raman continuum results from Fermi surface nesting by studying the hcp metal Dy, which is nested. There the continuum is found to be less flat but somewhat similar to that of the cuprate superconductors.