Orbital Physics versus Spin Physics

摘要

To elucidate the similarities and differences between the physics displayed by orbital and spin degrees of freedom, we analyze an orbital-Hubbard model with two orbital flavors, corresponding to pseudospin 1/2, and contrast its behavior with that of the familiar (spin-1/2) Hubbard model. The orbital-Hubbard model describes a partly filled spin-polarized e_g band on a cubic lattice, as occurs in ferromagnetic manganites. We demonstrate that the absence of SU(2) invariance in orbital space has important implications ― superexchange contributes in all orbital ordered states, the Nagaoka theorem does not apply, and the kinetic energy is enhanced as compared with the spin case. As a result orbital-ordered states are destabilized by doping, and instead a strongly correlated orbital liquid with disordered orbitals is realized.