Orbital degeneracy, Hund's coupling, and band ferromagnetism: effective quantum parameter, suppression of quantum corrections, and enhanced stability

摘要

An effective quantum parameter is obtained for the band ferromagnet in termsof orbital degeneracy and Hund's coupling. This quantum parameter determines,in analogy with 1/N for the generalized Hubbard model and 1/S for quantum spinsystems, the strength of quantum corrections to spin stiffness and spin-waveenergies. Quantum corrections are obtained by incorporating correlation effectsin the form of self-energy and vertex corrections within aspin-rotationally-symmetric approach in which the Goldstone mode is explicitlypreserved order by order. It is shown that even a relatively small Hund'scoupling is rather efficient in strongly suppressing quantum corrections,especially for large N, resulting in strongly enhanced stability of theferromagnetic state. This mechanism for the enhancement of ferromagnetism byHund's coupling implicitly involves a subtle interplay of lattice,dimensionality, band dispersion, spectral distribution, and band fillingeffects.