Critical thermalization of a disordered dipolar spin system in diamond

TL;DR

This study demonstrates critical thermalization behavior in a large disordered spin system in diamond, revealing slow relaxation dynamics and power-law decay influenced by disorder and interactions, challenging traditional statistical mechanics assumptions.

Contribution

It provides experimental evidence of critical thermalization in a three-dimensional disordered dipolar spin system, supported by a resonance counting theory accounting for disorder and interactions.

Findings

Observation of slow, sub-exponential relaxation dynamics.

Identification of a power-law decay regime with disorder-dependent exponents.

Late-time behavior influenced by many-body interactions.

Abstract

Statistical mechanics underlies our understanding of macroscopic quantum systems. It is based on the assumption that out-of-equilibrium systems rapidly approach their equilibrium states, forgetting any information about their microscopic initial conditions. This fundamental paradigm is challenged by disordered systems, in which a slowdown or even absence of thermalization is expected. We report the observation of critical thermalization in a three dimensional ensemble of $\sim 10^6$ electronic spins coupled via dipolar interactions. By controlling the spin states of nitrogen vacancy color centers in diamond, we observe slow, sub-exponential relaxation dynamics and identify a regime of power-law decay with disorder-dependent exponents; this behavior is modified at late times owing to many-body interactions. These observations are quantitatively explained by a resonance counting theory that incorporates the effects of both disorder and interactions.