Magnetic polarons and the metal-semiconductor transitions in (Eu,La)B-6 and EuO: Raman scattering studies - art. no. 174412

We present inelastic light scattering measurements of EuO and Eu1-xLaxB6 (x =0, 0.005, 0.01, 0.03, and 0.05) as functions of doping, B isotope, magneti c field, and temperature. Our results reveal a variety of distinct regimes as a function of decreasing T. (a) a paramagnetic semimetal regime, which i s characterized by a collision-dominated electronic scattering response who se scattering rate Gamma decreases with decreasing temperature, (b) a spin- disorder scattering regime, which is characterized by a collision-dominated electronic scattering response whose scattering rate Gamma scales with the magnetic susceptibility; (c) a magnetic polaron regime, in which the devel opment of an H=0 spin-flip Raman response betrays the formation of magnetic polarons in a narrow temperature range above the Curie temperature T-C; an d (d) a ferromagnetic metal regime, characterized by a flat electronic cont inuum response typical of other strongly correlated metals. By exploring th e behavior of the Raman responses in these various regimes in response to c hanging external parameters, we are able to investigate the evolution of ch arge and spin degrees of freedom through various transitions in these mater ials.