Temperature dependence of the dielectric function of nonmetallic YBa 2Cu 3O x in the interband range from transmission and reflection measurements on thin films

Abstract

The interband dielectric function of nonmetallic YBa 2Cu 3O x at energies below 4.5 eV, for E ⊥ c polarisation, is obtain ed from transmission and reflection measurements on epitaxial thin films between 1.8 and 473 K. A significant temperature dependence is found, in particular, for the 1.75 eV and 4.1 eV features, and this is discussed in terms of electron-phonon interaction. For the 1.75 eV excitation, two models are used to analyse this dependence, one based on largely localised transitions (configuration- coordinate model) and one (recently introduced by Falck et al. [Phys. Rev. Lett. 69 (1992) 1109]) starting from delocalised excitations. Both models lead to a reasonable description of the data. For the 4.1 eV excitation, a configuration-coordinate analysis is presented. It is found that, for both excitations, one is dealing with rather strong coupling to phonons, probably from the whole phonon spectrum.