Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3

TL;DR

This paper investigates quantum phase transitions in TlCuCl_3 induced by magnetic field and pressure, using a bond-operator model to describe the phases and excitations, and compares predictions with experimental data.

Contribution

It introduces a continuous bond-operator framework to describe pressure- and field-induced quantum phase transitions in TlCuCl_3, linking them to magnon Bose-Einstein condensation.

Findings

Bond-operator model accurately describes magnetization and magnon dispersion.

Pressure and magnetic field induce similar quantum phase transitions.

Predictions for pressure-dependent measurements are provided.

Abstract

Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers. Interdimer superexchange interactions give a three-dimensional magnon dispersion and a spin gap significantly smaller than the dimer coupling. This gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a magnetic field of 5.6T, offering a unique opportunity to explore the both types of quantum phase transition and their associated critical phenomena. We use a bond-operator formulation to obtain a continuous description of all disordered and ordered phases, and thus of the transitions separating these. Both pressure- and field-induced transitions may be considered as the Bose-Einstein condensation of triplet magnon excitations, and the respective phases of staggered magnetic order as linear combinations of dimer singlet and triplet modes. We focus on the evolution with applied pressure and field of the magnetic excitations in each phase, and in particular on the gapless (Goldstone) modes in the ordered regimes which correspond to phase fluctuations of the ordered moment. The bond-operator description yields a good account of the magnetization curves and of magnon dispersion relations observed by inelastic neutron scattering under applied fields, and a variety of experimental predictions for pressure-dependent measurements.