Theory of anomalous magnon softening in ferromagnetic manganites

TL;DR

This paper develops a theoretical model explaining the anomalous magnon softening observed near the zone boundary in metallic manganites, highlighting the role of orbital fluctuations and electron-phonon interactions.

Contribution

It introduces a novel theory linking orbital degrees of freedom and Jahn-Teller phonons to magnon spectrum renormalization in colossal magnetoresistance manganites.

Findings

Charge and orbital-lattice fluctuations cause magnon softening.

The model explains experimental observations of magnon spectrum anomalies.

Orbital and phonon interactions are key to magnetic excitation behavior.

Abstract

In metallic manganites with low Curie temperatures, a peculiar softening of the magnon spectrum close to the magnetic zone boundary has experimentally been observed. Here we present a theory of the renormalization of the magnetic excitation spectrum in colossal magnetoresistance compounds. The theory is based on the modulation of magnetic exchange bonds by the orbital degree of freedom of double-degenerate e_g electrons. The model considered is an orbitally degenerate double-exchange system coupled to Jahn-Teller active phonons which we treat in the limit of strong onsite repulsions. Charge and coupled orbital-lattice fluctuations are identified as the main origin of the unusual softening of the magnetic spectrum.