Evidence for Geometric-Phase Interference in a Mn$_{12}$-Acetate Single-Molecule Magnet*

Recent work by our group has shown evidence for geometric-phase interference between tunneling paths in the Mn$_{12}$ $^t$BuAc single-molecule magnet, the first observation of this effect in a system that has true four-fold rotational symmetry [1]. This effect was not previously observed in the bellwether Mn$_{12}$Acetate molecule, presumably because of solvent disorder inherent to the crystal. Here we report measurements on a crystal of Mn$_{12}$Acetate$\,\cdot\,$MeOH, which crystallizes without solvent disorder and therefore preserves the molecule's four-fold symmetry. The relaxation rate $\Gamma$ as a function of transverse field $H_T$ exhibits structure indicative of interference between tunneling paths similar to that found in [1]. This suggests that the solvent disorder, and not the larger dipole interactions found in Mn$_{12}$Acetate, is the most important factor in suppressing the interference effect. \\ \smallskip [1] S. T. Adams et al., Phys. Rev. Lett., {\bf 110}, 087205 (2013).