Resonant Tunneling and Memory Erasure in a Quantum Spin Glass

Spin glasses exhibit complex dynamics, including memory, aging, and rejuvenation. Here we explore a quantum spin glass, the dipole-coupled Ising magnet LiHo_0.2Y_0.8F₄, where those dynamics are driven by quantum fluctuations created by a transverse magnetic field. Unlike classical spin glasses, where the behavior is driven by thermal fluctuations and can be understood as a hierarchy of energies, the quantum system is driven by inelastic resonant tunneling processes involving individual spin flips or multi-spin coflips. Random fields are created by a combination of the disordered set of spins, the off-diagonal components of the dipole interactions, and the external transverse field. These random fields change which particular sets of spins are in resonance with each other, leading to pronounced history dependence as the spin configuration changes. We explore this via a.c. susceptibility measurements while performing the quantum equivalents of conventional memory dip and negative temperature cycle protocols, observing aging curves which move well beyond the envelope of reference curves and a negative effective time corresponding to erasure of memory. A first-principles theory calculation based on a resonant tunneling picture provides a quantitative description of the observed behavior including the field and temperature dependence of relaxation rates.