U(1)-Symmetry breaking and violation of axial symmetry in TlCuCl3 and other insulating spin systems

TL;DR

This paper models the Bose-Einstein condensate of magnetic quasiparticles in insulating spin systems, showing how anisotropy causes symmetry violation and affects the energy spectrum, with implications for observing Goldstone modes.

Contribution

It introduces a phenomenological approach to describe symmetry breaking effects in magnetic condensates, explaining experimental features and predicting intrinsic instability of axial symmetry.

Findings

Anisotropy induces an energy gap in the spectrum.

The model reproduces known experimental results.

Goldstone modes are unlikely in real materials due to symmetry violation.

Abstract

We describe the Bose-Einstein condensate of magnetic bosonic quasiparticles in insulating spin systems using a phenomenological standard functional method for T = 0. We show that results that are already known from advanced computational techniques immediately follow. The inclusion of a perturbative anisotropy term that violates the axial symmetry allows us to remarkably well explain a number of experimental features of the dimerized spin-1/2 system TlCuCl3. Based on an energetic argument we predict a general intrinsic instability of an axially symmetric magnetic condensate towards a violation of this symmetry, which leads to the spontaneous formation of an anisotropy gap in the energy spectrum above the critical field. We, therefore, expect that a true Goldstone mode in insulating spin systems, i.e., a strictly linear energy-dispersion relation down to arbitrarily small excitations energies, cannot be observed in any real material.