Low-Energy Excitations in La1.2Sr1.8Mn2O7 Investigated by Ellipsometry

The low-energy excitations of the bilayered manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 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{sub 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{sub c} although below T{sub 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 Tc. 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{sub c}, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase.