Bandlike electrical transport inPr1−xCaxMnO3manganites

The conductivity and dielectric permittivity spectra of polycrystalline manganites $\mathrm{P}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ with $x=0.3$, 0.4, and 0.5 have been measured in a broad frequency range ($5--3000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) down to low temperatures. From the dispersion analysis the existence of a Drude component can be established that is associated with small polarons coherently moving within a band several meV wide. The temperature-dependent conductivity shows an activated behavior with an activation energy of approximately 165 meV above 170 K and 4.8 meV below 120 K. The change in the activation energy is assigned to the onset of magnetic order at 120 K. In all three samples an absorption band is discovered at $40--60\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ that is associated with the transition between Stark-split $\mathrm{P}{\mathrm{r}}^{3+}$ electron states which gain optical activity due to coupling to acoustical phonons.