Periodic Instantons and Quantum-Classical Transitions in Spin Systems

TL;DR

This paper investigates how spin systems transition from classical to quantum behavior at low temperatures, highlighting the role of periodic instantons and the nature of phase transitions influenced by effective mass variations.

Contribution

It introduces a detailed analysis of spin models with explicit calculations of instantons, revealing how effective mass dependence leads to first order quantum-classical transitions.

Findings

Systems with field-dependent effective mass show first order transitions.

Models with constant mass exhibit only second order transitions.

Explicit calculations of instantons are provided.

Abstract

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