Spectral Function Analysis on Spin Dynamics in Double-Exchange Systems with Randomness

Spin excitation spectrum is studied in the double-exchange model with randomness. Applying the spin wave approximation and the spectral function analysis, we examine excitation energy and linewidth using analytical as well as numerical methods. For a small wave number q ∼ 0, the excitation energy is cosine-like and the linewidth shows q-linear behavior. This indicates that the spin excitation becomes incoherent or localized near q = 0. Crossover takes place to the marginally coherent regime where both the excitation energy and the linewidth are proportional to q². The incoherence is due to local fluctuations of the kinetic energy of electrons. Comparison with experimental results in colossal magnetoresistance manganites suggests that spatially-correlated or mesoscopic-scale fluctuations are more important in real compounds than local or atomic-scale ones.