Intrinsic instability at the Bose-Einstein condensation of bosonic quasiparticles in magnetic insulators

TL;DR

This paper investigates how explicit symmetry-breaking perturbations influence Bose-Einstein condensation in magnetic insulators, revealing intrinsic symmetry violation tendencies and explaining experimental anomalies in several spin systems.

Contribution

It introduces a modified energy functional accounting for symmetry-breaking perturbations, predicting spontaneous symmetry violation and first-order phase transitions in magnetic Bose-Einstein condensates.

Findings

Symmetry-breaking perturbations alter the condensation process.

Spontaneous axial symmetry violation occurs at the critical field.

Anisotropy gaps limit the lifetime of magnetic condensates.

Abstract

Starting from a phenomenological standard energy functional to describe the condensation of a dilute gas of bosonic quasiparticles in magnetic insulators, we find that the inclusion of a perturbation term that explicitly violates the axial symmetry significantly modifies the details of the resulting Bose Einstein condensation. Systems with an originally axial symmetry must show an intrinsic tendency to spontaneously violate this symmetry as soon as the condensation sets in, and the phase transition at the respective critical field may even become of first order. We can explain a number of features in the experimental data of various insulating spin systems, such as a slightly nonlinear magnetization near the critical field as well as hysteresis effects and peculiarities in the energy-level scheme of TlCuCl3. We also offer a consistent explanation for certain anomalies in the magnetocaloric effect and in the magnetization of BaCuSi2O6 by assuming a spontaneous violation of axial symmetry at the magnetic phase transition on an energy scale of about 1 micro-eV. The resulting anisotropy gap in the magnetic excitation spectrum, that inevitably forms at the critical field of all such systems, lifts the linear Goldstone mode and is therefore seriously limiting the lifetime of magnetic condensates to a few nanoseconds at most.