E. A. Rietman

Cu-0 Superconductors: Through a Lens, but Darkly

The shock of superconducting transition temperatures (Tc’s) greater than 25 K in Cu-O based compounds1 has loosed a tidal wave of research. We have involved ourselves in a study of the optical properties of these materials, with particular emphasis on La2-xSrxCuO4 compounds. Recently we reported 2 reflectivity, R, spectra of La2-xSrxCuO4 for x = 0 and 0.175, and of Ba2YCu3O9-δ (δ~2.1) over a broad range of photon energy. The most striking observation was the profound deviation from the expected Drude-like R in the metallic compounds. Fitting R required that ∈ (ω) be dominated by a transition at energy 0.45 eV in La1.825Sr175CuO4 and 0.65 eV in Ba2YCu3O9-δ. From our fit we determined that the integrated oscillator strength in the 0.45 eV feature in La1.825Sr0.175CuO4 corresponded to a value of Neff m/m* of 2.5xl021cm-3. This density agrees well with the density of positive carriers (assuming one hole per Sr atom) of 1.9x 1021cm-3. Similar conclusions for La1.825Sr0.175CuO4 were reached by Herr et al.3 based upon Kramers-Kronig (KK) analysis of their R data.