Low Energy Excitations in La1.2Sr1.8Mn2O7 investigated by ellipsometry

The low-energy excitations of the bilayered manganite La1.2 Sr1.8 Mn2 O7 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 (Tc) of 125 K. Both the ab -plane and c -axis temperature-dependent conductivities have been determined. Essentially no temperature-dependent behavior is observed above Tc although below Tc 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 Tc, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase. © 2005 The American Physical Society.