Crossover from thermal hopping to quantum tunneling in Mn_{12}Ac

TL;DR

This paper investigates the transition from thermal activation to quantum tunneling in Mn_{12}Ac, providing detailed calculations of decay rates and fluctuation contributions near the crossover temperature.

Contribution

It extends previous studies by including fluctuation modes and higher order couplings, offering a more accurate analysis of the crossover in molecular magnets.

Findings

Decay rate with dissipation can be accurately determined near the crossover temperature.

Fluctuation modes around the saddle point significantly affect the decay process.

Higher order couplings between dangerous fluctuation modes are essential for precise modeling.

Abstract

The crossover from thermal hopping to quantum tunneling is studied. We show that the decay rate $\Gamma$ with dissipation can accurately be determined near the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin (WKB) exponent, we also calculate contribution of the fluctuation modes around the saddle point and give an extended account of a previous study of crossover region. We deal with two dangerous fluctuation modes whose contribution can't be calculated by the steepest descent method and show that higher order couplings between the two dangerous modes need to be taken into considerations. At last the crossover from thermal hopping to quantum tunneling in the molecular magnet Mn_{12}Ac is studied.