Demagnetization of Quantum Dot Nuclear Spins: Breakdown of the Nuclear Spin Temperature Approach

TL;DR

This study shows that nuclear spins in quantum dots cannot be described by a spin temperature due to strong quadrupolar interactions, leading to non-thermal states and very long relaxation times.

Contribution

It demonstrates the breakdown of the spin temperature concept in quantum dots caused by quadrupolar interactions and strain effects, revealing new nuclear spin dynamics.

Findings

Nuclear spins in quantum dots deviate from thermal equilibrium.

Quadrupolar interactions cause long nuclear spin relaxation times.

Magnetic field sweeps become non-adiabatic, causing polarization loss.

Abstract

The physics of interacting nuclear spins arranged in a crystalline lattice is typically described using a thermodynamic framework: a variety of experimental studies in bulk solid-state systems have proven the concept of a spin temperature to be not only correct but also vital for the understanding of experimental observations. Using demagnetization experiments we demonstrate that the mesoscopic nuclear spin ensemble of a quantum dot (QD) can in general not be described by a spin temperature. We associate the observed deviations from a thermal spin state with the presence of strong quadrupolar interactions within the QD that 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 quadrupolar interactions render magnetic field sweeps inherently non-adiabatic, thereby causing an irreversible loss of nuclear spin polarization.