Modeling the mid-infrared optical gap in La2−xSrxCuO4

Abstract

In this work, we used a periodic lattice potential in order to model the infrared optical data of the high-temperature superconductor La_2−xSr_xCuO₄. This potential consists of a two-dimensional array of double-well potentials, which simulate the CuO₂ layers. It is obtained by assembling Cu-O-Cu units rather than Cu and O single atoms in the tight-binding approach. A gap separating two energy bands can be obtained and is used to fit the infrared (IR) optical gap of this cuprate. We derived the dielectric function and showed that in the classical limit it reduces to the one consisting of a Drude term plus a number of lorentz components, equivalent to the dielectric function used empirically by several authors in their fits of the reflectivity. By refitting available reflectance data, we deduced a simple law for the doping dependence of the optical gap in La_2−xSr_xCuO₄. In the present study, we argue that the optical gap is distinct from the pseudogap or the two-magnon gap, because it characterizes La_2−xSr_xCuO₄ for all doping regimes.