Low-energy excitations inLa1.2Sr1.8Mn2O7investigated by ellipsometry

Abstract

The low-energy excitations of the bilayered manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ have been explored by spectral ellipsometry from two faces of a single crystal over the range from 0.006 to 0.6 eV. This compound is a paramagnetic insulator at ambient temperature, with a transition to a ferromagnetic metal below a Curie temperature $({T}_{\mathrm{c}})$ of 125 K. Both the $ab$-plane and $c$-axis temperature-dependent conductivities have been determined. Essentially no temperature-dependent behavior is observed above ${T}_{\mathrm{c}}$ although below ${T}_{\mathrm{c}}$ both the phonon and electronic contributions are strongly temperature sensitive. The highest-frequency phonons, especially those involving Mn-O bond stretching, split and show frequency changes consistent with structural results in the literature, and furthermore there is clear evidence of an increase in electron-phonon coupling at and below ${T}_{\mathrm{c}}$. We interpret the temperature-dependent electronic spectral contribution in the light of recent calculations that indicate that a mixed phase exists in the doped manganites below ${T}_{\mathrm{c}}$, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase.