More Evidence for a Distribution of Tunnel Splittings in Mn$_{12}$-acetate

TL;DR

This study provides evidence that Mn$_{12}$-acetate molecules exhibit a broad distribution of tunnel splittings, challenging the assumption of uniformity and highlighting the role of anisotropy variations in quantum tunneling behavior.

Contribution

The paper demonstrates that the distribution of tunnel splittings in Mn$_{12}$-acetate is broad and fits a Gaussian profile, offering new insights into molecular magnetic quantum tunneling.

Findings

The fraction of molecules remaining after resonance sweeps indicates a broad distribution of tunnel splittings.

A Gaussian distribution of the anisotropy parameter fits the experimental data well.

Dipolar shuffling effects are negligible in explaining the observed distribution.

Abstract

In magnetic fields applied parallel to the anisotropy axis, the magnetization of Mn$_{12}$ has been measured in response to a field that is swept back and forth across the resonances corresponding to steps $N=4,5,...9$. The fraction of molecules remaining in the metastable well after each sweep through the resonance is inconsistent with expectations for an ensemble of identical molecules. The data are consistent instead with the presence of a broad distribution of tunnel splittings. A very good fit is obtained for a Gaussian distribution of the second-order anisotropy tunneling parameter $X_E=-\ln(\mid E\mid/2D)$. We show that dipolar shuffling is a negligible effect which cannot explain our data.