Rejuvenation in a Quantum Spin Glass

We study the dynamic behavior of a quantum spin glass. The introduction of quantum fluctuations causes relaxation via tunneling rather than thermal fluctuations. We investigate this relaxation process in the dipole-coupled Ising spin glass, LiHo_0.2Y_0.8F₄, in a transverse magnetic field. We conduct a.c. susceptibility measurements

while performing a series of transverse field ramps analogous to conventional memory dip and negative temperature cycle procedures. We interpret the variation of the rate of relaxation with transverse field and the phenomenon of rejuvenation within a theoretical framework of quantum resonant tunneling. This theory is based on first principles and is more appropriate for quantum spin glasses than the usual heuristic description of a hierarchical free energy landscape.