Thermalization and many-body localization in systems under dynamic nuclear polarization

TL;DR

This paper investigates how dipolar interactions influence dynamic nuclear polarization, showing that nuclear hyperpolarization depends on the effective spin temperature and is affected by many-body localization transitions.

Contribution

It introduces an analytical method to estimate spin temperature and hyperpolarization, linking interaction strength, disorder, and localization effects in DNP systems.

Findings

Nuclear spins equilibrate to the electron spin temperature.

Hyperpolarization increases as interaction strength decreases.

Maximum polarization occurs near the many-body localization transition.

Abstract

We study the role of dipolar interactions in the standard protocol used to achieve dynamic nuclear polarization (DNP). In the so-called spin-temperature regime, where the interactions establish an effective thermodynamic behavior in the out-of-equilibrium stationary state, we provide numerical predictions for the level of hyperpolarization. We show that nuclear spins equilibrate to the effective spin-temperature established among the electron spins of radicals, as expected from the quantum theory of thermalization. Moreover, we present an analytical technique to estimate the spin temperature, and thus, the nuclear hyperpolarization in the steady state, as a function of interaction strength and quenched disorder. This reproduces both our numerical data and experimental results. Our central finding is that the nuclear hyperpolarization increases steadily upon reducing the interaction strength (by diluting the radical density). Interestingly, the highest polarization is reached at a point where the establishment of a spin temperature is just about to break down due to the incipient many-body localization transition in the electron spin system.