Metallic ferromagnetism in a single-band model. IV. Effect of pair hopping

The Coulomb-exchange matrix element between electrons in nearest-neighbor Wannier orbitals J has recently been proposed to play a fundamental role in metallic ferromagnetism. In addition to exchanging electrons on nearest-neighbor sites, it gives rise to a pair-hopping contribution to the Hamiltonian. Here we examine the effect of the pair-hopping term on the tendency to ferromagnetism. It is stressed that the important role that these interactions play in ferromagnetism results from their momentum dependence in the solid-state environment. We find that the tendency to ferromagnetism is further enhanced by the pair-hopping term, and the parameter range where unsaturated ferromagnetism occurs is enlarged. Its effect becomes small for large on-site repulsion U. The combination of exchange and pair hopping gives rise to bond-charge repulsion, and metallic ferromagnetism can be understood as arising from the tendency of electrons to avoid this direct Coulomb interaction.