Theory of anomalous magnon softening in ferromagnetic manganites

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 magn etic excitation spectrum in colossal magnetoresistance compounds. The theor y is based on the modulation of magnetic exchange bonds by the orbital degr ee of freedom of double-degenerate e(g) electrons. The model considered is an orbitally degenerate double-exchange system coupled to Jahn-Teller activ e phonons which we treat in the limit of strong on-site repulsions. Charge and coupled orbital-lattice fluctuations are identified as the main origin of the unusual softening of the magnetic spectrum.