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 < 15 000 cm(-1) and temperatures 10 K < T < 300 K. The submillimeter conductivity follows the temperature dependence of the de-data. The phonon spectrum of La0.85Sr0.15MnO3 changes considerably below T-C = 180 K revealing a structural phase transition induced by charge or orbital order. At T = 10 K a number 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 ordering temperature the polaron binding energy is highly reduced, but the onset of charge order interrupts the formation of free charge carriers with a Drude-like behavior. The frequency and temperature dependence of sigma(1) in this regime qualitatively resembles the small polaron predictions by Millis et al.