Normal-state gap transition in Cu-O superconductors

Abstract

The reflectivity spectra of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ for x=0 and 0.175, and ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_{\mathrm{c}}$ ${\mathrm{O}}_{9\mathrm{-}\mathrm{\delta }}$ (δ∼2.1) were measured over a broad frequency range, from 0.005 to 3 eV. In the normal state of these compounds the reflectivity is temperature independent and significantly smaller than expected from the Drude model. We describe a dielectric function which reproduces the main features of the spectra by supplementing the Drude contribution with a strong transition across a gap.