Quantum spin glass and the dipolar interaction

TL;DR

This paper investigates how off-diagonal dipolar interactions influence the phase transition in dipolar systems modeled by the transverse field Ising model, showing that they prevent spin glass order at finite transverse fields.

Contribution

It demonstrates that off-diagonal dipolar interactions eliminate the spin glass phase at any finite transverse field, challenging previous interpretations of experimental data.

Findings

Off-diagonal dipolar interactions destroy spin glass order at finite fields.

Correlation length remains small near the crossover, consistent with experiments.

Crossover, not quantum criticality, explains experimental observations.

Abstract

Systems in which the dipolar energy dominates the magnetic interaction, and the crystal field generates strong anisotropy favoring the longitudinal interaction terms, are considered. Such systems in external magnetic field are expected to be a good experimental realization of the transverse field Ising model. With random interactions this model yields a spin glass to paramagnet phase transition as function of the transverse field. Here we show that the off-diagonal dipolar interaction, although effectively reduced, destroys the spin glass order at any finite transverse field. Moreover, the resulting correlation length is shown to be small near the crossover to the paramagnetic phase, in agreement with the behavior of the nonlinear susceptibility in the experiments on $\LHx$. Thus, we argue that the in these experiments a cross-over to the paramagnetic phase, and not quantum criticality, was observed.