Tunneling and inversion symmetry in single-molecule magnets: the case of the Mn12 wheel molecule

TL;DR

This study investigates how various interactions, especially inversion symmetry breaking and Dzyaloshinski-Moriya interactions, influence quantum tunneling in a Mn12 wheel molecule, revealing the necessity of symmetry breaking to explain experimental results.

Contribution

It demonstrates that breaking molecular inversion symmetry is essential to account for observed tunneling behaviors and explores the effects of DM interactions using microscopic and simplified models.

Findings

Inversion symmetry breaking is crucial for explaining tunneling experiments.

DM interactions influence Berry phase minima and quantum interference.

Significant reorientation of DM vectors is needed, not attributable to disorder.

Abstract

We present a detailed study of the influence of various interactions on the spin quantum tunneling in a Mn12 wheel molecule. The effects of single-ion and exchange (spin-orbit) anisotropy are first considered, followed by an analysis of the roles played by secondary influences, e.g. disorder, dipolar and hyperfine fields, and magnetoacoustic interactions. Special attention is paid to the role of the antisymmetric Dzyaloshinski-Moriya (DM) interaction. This is done within the framework of a 12-spin microscopic model, and also using simplified dimer and tetramer approximations in which the electronic spins are grouped in 2 or 4 blocks, respectively. If the molecule is inversion symmetric, the DM interaction between the dimer halves must be zero. In an inversion symmetric tetramer, two independent DM vectors are allowed, but no new tunneling transitions are generated by the DM interaction. Experiments on the Mn12 wheel can only be explained if the molecular inversion symmetry is broken, and we explore this in detail using both models, focussing on the asymmetric disposition and rounding of Berry phase minima associated with quantum interference between states of opposite parity. A remarkable behavior exists for the `Berry phase zeroes' as a function of the directions of the internal DM vectors and the external transverse field. A rather drastic breaking of the molecular inversion-symmetry is required to explain the experiments; in the tetramer model this requires a reorientation of the DM vectors on one half of the molecule by nearly 180 degrees. This cannot be attributed to sample disorder. These results are of general interest for the quantum dynamics of tunneling spins, and lead to some interesting experimental predictions.