Submillimeter and infrared spectroscopy on La0.85Sr0.15MnO3

The optical conductivity of La0.85Sr0.15MnO3 single crystals was studied by means of submillimeter and infrared spectroscopy for frequencies 4 < v < 1 5 000 cm(-1) and temperatures 10 K < T < 300 K. The submillimeter conductiv ity follows the temperature dependence of the de-data. The phonon spectrum of La0.85Sr0.15MnO3 changes considerably below T-C = 180 K revealing a stru ctural phase transition induced by charge or orbital order. At T = 10 K a n umber of phonon modes can be identified in addition to the room-temperature spectrum. The optical conductivity (sigma(1)) in the mid-infrared reveals the characteristics of small polaron absorption. Below the magnetic orderin g temperature the polaron binding energy is highly reduced, but the onset o f charge order interrupts the formation of free charge carriers with a Drud e-like behavior. The frequency and temperature dependence of sigma(1) in th is regime qualitatively resembles the small polaron predictions by Millis e t al.