Emergent Interactions in Anisotropic Dipolar Magnets

by Mr. Tomer Dollberg

BGU

at Condensed Matter Seminar

Mon, 12 Jun 2023, 11:10
Sacta-Rashi Building for Physics (54), room 207

Abstract

Anisotropic dipolar magnets are materials in which a strong anisotropic crystal-field potential results in an Ising-like ground state doublet that is well separated from the first excited state. They are thus considered ideal systems to study the (dipolar) Ising model, and upon application of a transverse field B_x, the quantum (transverse-field) Ising model.
One notable example is the rare-earth material LiHoF4, which has been scrutinized experimentally, numerically and theoretically over many decades with the aim of understanding various collective magnetic phenomena.
Results from previous studies, using various Monte Carlo techniques and mean-field analyses, show a persistent discrepancy with experimental results for the B_x-T phase diagram. Namely, in the low-B_x regime, the experimental phase boundary separating the ferromagnetic and paramagnetic phases has a much smaller dependence on magnetic field compared to theoretical predictions.
We propose a mechanism that highlights the importance of quantum fluctuations, induced by the off-diagonal terms of the dipolar interaction, in determining the critical temperature of anisotropic dipolar magnets. Indeed, in a recent work [1], we numerically showed that the inclusion of such dipolar terms results in a significantly improved agreement with experimental data at zero and low transverse fields.
In this talk, we shall review previous numerical results and present a new analytical model where off-diagonal dipolar terms are shown to generate effective three-body interactions that disfavor ferromagnetic order. Thus, the same improved agreement with experiment is obtained analytically as was obtained numerically, now extendable to the full phase diagram.
We also address the existence of this mechanism in other anisotropic dipolar systems.

[1] T. Dollberg, J. C. Andresen, and M. Schechter, Phys. Rev. B 105, L180413 (2022).

Created on 07-06-2023 by Meidan, Dganit (dganit)
Updaded on 07-06-2023 by Meidan, Dganit (dganit)