Terahertz spectroscopy of low-energy excitations inEu0.42Sr0.58MnO3

Abstract

The origin of low-energy excitations in the polycrystalline $\mathrm{E}{\mathrm{u}}_{0.42}\mathrm{S}{\mathrm{r}}_{0.58}\mathrm{Mn}{\mathrm{O}}_{3}$ manganite is explored by terahertz and infrared spectroscopies. The transition from the charge and/or orbital to antiferromagnetic order (${T}_{\mathrm{N}}=130\ensuremath{-}140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is seen as smooth (spread over a temperature range from 50 to 100 K) decrease of free-carrier conductivity with the activation energy change from 86 meV (high temperatures) to 0.52 meV (low temperatures). A broad relaxation is observed at around $10\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ which we associate with the dynamics of free charge carriers in the presence of random localizing potential. We discover an absorption resonance at $\ensuremath{\approx}50\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ below ${T}_{\mathrm{N}}$ that is assigned to the formation of hybridized ``acoustic phonon-magnon'' quasiparticles.