LiHoF4 as a spin-half non-standard quantum Ising system

TL;DR

This paper investigates the effects of previously neglected off-diagonal dipolar interactions in LiHoF4, revealing their crucial role in accurately modeling its phase diagram and critical temperature behavior under transverse magnetic fields.

Contribution

We analytically derive an effective Hamiltonian including off-diagonal dipolar terms, improving the understanding of LiHoF4's quantum magnetic properties and phase diagram.

Findings

Off-diagonal dipolar terms significantly lower the zero-field critical temperature.

The critical temperature's dependence on transverse field is influenced by these dipolar interactions.

Comparison with Fe8 highlights the sensitivity of the mechanism to crystal structure.

Abstract

$\mathrm{LiHoF_{4}}$ is a magnetic material known for its Ising-type anisotropy, making it a model system for studying quantum magnetism. However, the theoretical description of $\mathrm{LiHoF_{4}}$ using the quantum Ising model has shown discrepancies in its phase diagram, particularly in the regime dominated by thermal fluctuations. In this study, we investigate the role of off-diagonal dipolar terms in $\mathrm{LiHoF_{4}}$, previously neglected, in determining its properties. We analytically derive the low-energy effective Hamiltonian of $\mathrm{LiHoF_{4}}$, including the off-diagonal dipolar terms perturbatively, both in the absence and presence of a transverse field. Our results encompass the full $B_{x}-T$ phase diagram, confirming the significance of the off-diagonal dipolar terms in reducing the zero-field critical temperature and determining the critical temperature's dependence on the transverse field. We also highlight the sensitivity of this mechanism to the crystal structure by comparing our calculations with the $\mathrm{Fe_{8}}$ system.