STATIC AND DYNAMIC PROPERTIES OF A DIMERIZED QUANTUM-SPIN CHAIN

Various static spin-correlation functions and the weights of the spin excitations, or collective normal modes, in the van Hove response func tion, observed in neutron scattering experiments, are calculated as a function of temperature for a model quantum-spin chain. The spins inte ract through an isotropic Heisenberg exchange that extends to nearest- neighbour spins. In our model, spins of magnitude 1/2 form a dimerized chain with alternating exchange interactions. We explore the static a nd dynamic properties for ferromagnetic and antiferromagnetic exchange interactions. Results for the various properties are shown to be exac t in the limit of a high temperature, and we argue that the results ar e very good at low temperatures. Unlike the case for linear spin-wave theory, the results are also exact in the limit of strong dimerization , i.e. non-interacting coupled pairs of spins.