Anomaly in Spin Excitation Spectrum of Double-Exchange Systems with Randomness

Abstract

Spin excitation spectrum of the double-exchange model is studied in the presence of randomness. Spin wave approximation in the ground state shows that randomness significantly modifies the spectrum from the cosine-like one in the pure system to that with anomalies such as broadening, anticrossing and gap opening. The origin of anomalies is speculated to be the modulation of effective ferromagnetic coupling by the Friedel oscillation. These anomalies qualitatively reproduce the spin excitation spectrum in colossal magnetoresistance manganites whose Curie temperatures are relatively low. Our results suggest that randomness control is an important notion to understand the effects of A-site substitution which has previously been understood as the bandwidth control.