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We study the magnetic relaxation rate Gamma of the single-molecule magnet Mn12-tBuAc as a function of magnetic field component HT transverse to the molecule's easy axis. When the spin is near a magnetic quantum tunneling resonance, we find that Gamma increases abruptly at certain values of HT. These increases are observed just beyond values of HT at which a geometric-phase interference effect suppresses tunneling between two excited energy levels. The effect is washed out by rotating HT away from the spin's hard axis, thereby suppressing the interference effect. Detailed numerical calculations of Gamma using the known spin Hamiltonian accurately reproduce the observed behavior. These results are the first experimental evidence for geometric-phase interference in a single-molecule magnet with true four-fold symmetry.


Published in Physical Review Letters Vol. 110 no. 8, Feb. 2013

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