PERIODIC INSTANTONS AND QUANTUM-CLASSICAL TRANSITIONS IN SPIN SYSTEMS

Some models, allowing explicit calculation of periodic instantons and evaluation of their action, are studied with regard to transitions fro m classical to quantum behavior as the temperature is lowered and tunn eling sets in. It is shown that (beyond a critical value of a coupling ) the spin system considered acquires a first order transition as a re sult of the held dependence of its effective mass, whereas models with constant mass exhibit only second order transitions.