Breakdown of the nuclear-spin-temperature approach in quantum-dot demagnetization experiments

The physics of interacting nuclear spins arranged on a crystalline lattice is generally described using a thermodynamic framework(1) and the concept of spin temperature. In the past, experimental studies in bulk solid-state systems have proven this concept to be not only correct(2,3) but also vital for the understanding of experimental observations(4). Here we show, using demagnetization experiments, that the concept of spin temperature in general fails to describe the mesoscopic nuclear-spin ensemble of a quantum dot. We associate the observed deviations from a thermal spin state with the presence of strong quadrupolar interactions within the quantum dot, which cause significant anharmonicity in the spectrum of the nuclear spins. Strain-induced, inhomogeneous quadrupolar shifts also lead to a complete suppression of angular-momentum exchange between the nuclear-spin ensemble and its environment, resulting in nuclear-spin relaxation times exceeding an hour. Remarkably, the position-dependent axes of the quadrupolar interactions render magnetic-field sweeps inherently non-adiabatic, thereby causing an irreversible loss of nuclear-spin polarization.