Direct Spectroscopic Evidence for Berry-phase Interference in the Ni₄ Single-molecule Magnet

We present spectroscopic evidence for Berry-phase interference in the quantum tunneling of the spin in the Ni₄ single-molecule magnet. The signature of destructive Berry-phase interference for tunnel-split states is that the tunnel splitting goes to zero (is "quenched") for certain values of field when applied along the hard axis. In fixed frequency ESR, this shows up as two transitions for the same pair of energy eigenstates, one at a field slightly lower than quench field and one slightly higher. We performed parallel-mode electron-spin resonance with f = 3.78 GHz to drive transitions between tunnel-split states in this molecule. We monitor the behavior of these transitions as a function of transverse magnetic field and the angle φ between the field and the hard axis of the molecule. We observe the predicted quench behavior for an excited-state tunnel splitting in Ni₄ and can follow it as changing φ suppresses the interference effect. We find that the behavior is four-fold symmetric in φ, consistent with the expected symmetry of the system. Detailed simulations of all of the observed transitions reproduce the observations nicely.