Integrable models of interacting quantum spins with competing interactions

Europhys. Lett. 28 (1994) 363 We present a class of exactly solvable quantum spin models which consist of two Heisenberg-subsystems coupled via a long-range Lieb-Mattis interaction. The total system is exactly solvable whenever the individual subsystems are solvable and allows to study the effects of frustration. We consider (i) the antiferromagnetic linear chain and (ii) the Lieb-Mattis antiferromagnet for the subsystem-Hamiltonians and present (i) the complete ground-state phase diagram and (ii) the full thermodynamic phase diagram. We find a novel phase which exhibits {\it order from disorder} phenomena.