Electronic structure and polarons inCaMnO3−δsingle crystals: Optical data

Abstract

The ellipsometry measurements (in the energy range $E=0.5--5.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) on cleavage surfaces of single crystals, the measurements of the absorption and reflectance spectra $(E=0.04--0.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, electrical resistivity, and magnetic susceptibility were carried out for determination of peculiarities of the fundamental absorption spectrum and the conductivity mechanism in $\mathrm{Ca}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ single crystals. The energy of fundamental absorption edge associated with the direct transitions was determined as ${E}_{\mathrm{g}}=1.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The absorption band with two maxima at 2.2 and $3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ corresponding to the charge transfer transitions was revealed. The polaron band at $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was discovered and polaron parameters were determined. Peculiarities of IR absorption and dc conductivity of $\mathrm{Ca}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ in dependence on oxygen stoichiometry are determined by the superposition of hopping and band conductivity of spin-lattice polarons, and by additional localization of charge carriers under oxygen vacancies ordering.