Quantum-Classical Crossover of the Escape Rate in a Spin System

Abstract

Quite generally, a system in a metastable well is rendered unstable by thermal and quantum fluctuactions. At high temperature, the former dominate the decayrate, whereas the latter do so at low temperatures. There is a narrow region of temperature where the system shows a crossover between the thermal and the quantum regimes. This crossover resembles a phase transition. It can be first-order (with an abrupt change of regime) or second-order (when the escape rate changes smoothly). More complicated behaviour is possible as well.

Spin systems provide a highly nontrivial example of the general theory. For these systems, the type of crossover can be tuned by an external field. Our predictions can be tested experimentallyin molecular nanomagnets, such as Mn12Ac and Fe8, and in single domain particles.