Spin Tunneling in Magnetic Molecules: Quasisingular Perturbations and Discontinuous SU(2) Instantons

TL;DR

This paper explores the role of discontinuous semiclassical paths in spin tunneling phenomena in magnetic molecules, revealing how higher-order anisotropy terms influence tunneling and quenching points, with quantitative agreement with experiments.

Contribution

It demonstrates the importance of quasisingular perturbations and discontinuous instantons in understanding spin tunneling in magnetic molecules like Fe8, introducing an accurate formula for quenching point spacing.

Findings

Discontinuous paths are essential for understanding quenched spin tunneling.

Fourth order anisotropy acts as a singular perturbation affecting tunneling.

Quantitative agreement with experimental field dependence and quenching points.

Abstract

Spin coherent state path integrals with discontinuous semiclassical paths are investigated with special reference to a realistic model for the magnetic degrees of freedom in the Fe8 molecular solid. It is shown that such paths are essential to a proper understanding of the phenomenon of quenched spin tunneling in these molecules. In the Fe8 problem, such paths are shown to arise as soon as a fourth order anisotropy term in the energy is turned on, making this term a singular perturbation from the semiclassical point of view. The instanton approximation is shown to quantitatively explain the magnetic field dependence of the tunnel splitting, as well as agree with general rules for the number of quenching points allowed for a given value of spin. An accurate approximate formula for the spacing between quenching points is derived.