PNNL

Abstract

Raman spectra of nickel cobalt oxide films with spinel crystal structure showed unexpected variations with film thickness. Combined with the results of studies as a function of incident laser power and temperature, from 25K to 800K, the results strongly point to a small polaron hopping model for electrical conductivity in these materials. During laser excitation, photons interact with valence electrons such that the carrier holes in these materials become localized or associated with the polarized environment created by the electron-phonen interaction (polaron). These interactions split the energy levels of the valence electrons contributing to band broadening well below room temperature.