First- and Second-Order Transitions between Quantum and Classical Regimes for the Escape Rate of a Spin System

TL;DR

This paper investigates how the escape rate in a large-spin system transitions between quantum and classical regimes, revealing first- and second-order phase transitions depending on the transverse magnetic field strength, with implications for molecular magnet experiments.

Contribution

It introduces a novel analysis of first- and second-order transitions in the escape rate of a spin system based on the Hamiltonian with transverse field effects.

Findings

Crossover from thermal to quantum regime can be first or second order.

Transition order depends on the transverse field strength.

Predictions applicable to molecular magnets like Mn_12Ac.

Abstract

We have found a novel feature of the bistable large-spin model described by the Hamiltonian H = -DS_z^2 - H_xS_x.The crossover from thermal to quantum regime for the escape rate can be either first (H_x<SD/2) or second (SD/2<H_x<2SD) order, that is, sharp or smooth, depending on the strength of the transverse field. This prediction can be tested experimentally in molecular magnets like Mn_12Ac.