Dynamic nuclear polarization and the paradox of Quantum Thermalization

TL;DR

This paper investigates how electron dipolar interactions influence the efficiency of Dynamic Nuclear Polarization (DNP), revealing two dynamical phases linked to quantum thermalization and ergodicity breaking.

Contribution

It uncovers the role of electron dipolar interactions in creating distinct dynamical phases in DNP, connecting polarization behavior to quantum thermalization phenomena.

Findings

Strong interactions lead to low effective electron spin temperature and high DNP efficiency.

Weak interactions result in a 'hole burning' polarization profile.

The two phases are related to ergodicity breaking in quantum many-body systems.

Abstract

Dynamic Nuclear Polarization (DNP) is to date the most effective technique to increase the nuclear polarization up to a factor $100,000$ opening disruptive perspectives for medical applications. In DNP, the nuclear spins are driven to an - out of equilibrium - hyperpolarized state by microwave saturation of the electron spins in interaction with them. Here we show that the electron dipolar interactions compete with the local magnetic fields resulting in two distinct dynamical phases: for strong interactions the electron spins equilibrate to an extremely low effective temperature that boosts DNP efficiency. For weak interaction this spin temperature is not defined and the polarization profile has an 'hole burning' shape characteristic of the non interacting case. The study of the many-body eigenstates reveals that these two phases are intimately related to the problem of thermalization in closed quantum systems where breaking of ergodicity is expected varying the strength of the interactions.