  A Mn4 single-molecule magnet displays asymmetric Berry-phase interference
patterns in the transverse-field (HT) dependence of the magnetization tunneling
probability when a longitudinal field (HL) is present, contrary to symmetric
patterns observed for HL=0. Reversal of HL results in a reflection of the
transverse-field asymmetry about HT=0, as expected on the basis of the
time-reversal invariance of the spin-orbit Hamiltonian which is responsible for
the tunneling oscillations. A fascinating motion of Berry-phase minima within
the transverse-field magnitude-direction phase space results from a competition
between noncollinear magnetoanisotropy tensors at the two distinct Mn sites.
