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

TL;DR

This paper investigates how randomness affects spin excitations in double-exchange systems, revealing incoherence and localization phenomena through spectral function analysis, with implications for understanding colossal magnetoresistance materials.

Contribution

It introduces a spectral function approach to analyze spin dynamics in disordered double-exchange models, highlighting the role of spatial correlations in real materials.

Findings

Spin excitation energy is cosine-like at small q.

Linewidth exhibits a q-linear behavior near q=0.

Crossover to a q^2 dependence indicates a transition to a marginally-coherent regime.

Abstract

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 small wave number $q \sim 0$, the excitation energy is cosine-like and the linewidth shows a $q$-linear behavior. This indicates that the spin excitation becomes incoherent or localized near $q = 0$. Crossover takes place to marginally-coherent regime where both the excitation energy and the linewidth are proportional to $q^2$. 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.