Different routes to spin gaps: Role of orbital ordering

Abstract

In many quantum spin systems, especially in low-dimensional ones, there exist states with spins forming singlets and with spin gaps. I consider several situations in which one gets such a state, paying main attention to the role of orbital occupation and orbital ordering. In particular, orbital ordering can reduce the effective dimensionality of the system and can lead to a Peierls-like state with the spin gap. It can also strongly influence the features of insulator-metal transitions in transition metal compounds, and can effectively remove frustrations in geometrically frustrated systems.