Temperature dependence of polar phonons, plasma excitations, and effective charges below the semiconducting-metal phase transition of NbO2

Abstract

The temperature dependence of TO- and LO-phonon modes, decoupled from plasma background at high temperature and effective charges, is reported in the distorted-rutile semiconducting phase of Nb${\mathrm{O}}_2$, as obtained by infrared reflectivity spectroscopy. Results support recent conclusions about the predominance of electronic entropy to the transition.