DIRECT OBSERVATION OF SELF-LOCALIZED STATES IN THE MIR ABSORBENCY OF LA2-XSRXCUO4

Temperature-dependent absorbance measurements of a series of La2-xSrxC uO4 (LSCO) compounds with 0.03 less than or equal to x less than or eq ual to 0.17 are reported. An absorption band near 0.15 eV is directly observed in all underdoped samples. The differential absorbance A(T)-A (300 K) is maximal near the maximum of the absorbance band which is en hanced as the temperature is lowered and saturates at 50-100 K. The th ermal-activation energy is about 1/4 of the peak energy which is typic al for polaronic excitations. A quantitative analysis based on absorba nce measurements of two different host materials followed by an effect ive medium calculation demonstrates that direct observation of this ab sorption band is hardly possible in reflectance measurements. We concl ude that polarons are directly observed in underdoped metallic LSCO co mpounds, and show that the optical response is consistent with a multi -component behaviour.