Effectiveness of classical spin simulations for describing NMR relaxation of quantum spins

TL;DR

This paper evaluates the accuracy of classical spin simulations in predicting NMR relaxation signals of quantum spins, identifying conditions where classical models are effective and providing semi-empirical criteria for their applicability.

Contribution

It systematically compares quantum and classical spin dynamics for NMR FID signals and establishes criteria for when classical simulations can reliably approximate quantum behavior.

Findings

Classical and quantum FIDs show excellent agreement in certain regimes.

Agreement quality decreases as interaction range and quantum effects increase.

Semi-empirical criteria predict when classical simulations are accurate.

Abstract

We investigate the limits of effectiveness of classical spin simulations for predicting free induction decays (FIDs) measured by solid-state nuclear magnetic resonance (NMR) on systems of quantum nuclear spins. The specific limits considered are associated with the range of interaction, the size of individual quantum spins and the long-time behavior of the FID signals. We compare FIDs measured or computed for lattices of quantum spins (mainly spins 1/2) with the FIDs computed for the corresponding lattices of classical spins. Several cases of excellent quantitative agreement between quantum and classical FIDs are reported along with the cases of gradually decreasing quality of the agreement. We formulate semi-empirical criteria defining the situations, when classical simulations are expected to accurately reproduce quantum FIDs. Our findings indicate that classical simulations may be a quantitatively accurate tool of first principles calculations for a broad class of macroscopic systems, where individual quantum microscopic degrees of freedom are far from the classical limit.