Quantum and classical criticality in a dimerized quantum antiferromagnet

TL;DR

This study uses neutron scattering on TlCuCl3 to explore quantum and classical criticality, revealing how quantum and thermal fluctuations independently influence magnetic order near a quantum critical point.

Contribution

It provides direct experimental evidence of quantum and classical criticality behaviors and their largely independent nature in a dimerized quantum antiferromagnet.

Findings

Quantum and thermal fluctuations can behave independently near a QCP.

The longitudinal 'Higgs' mode is thermally damped, confirming its critical nature.

Development of two types of criticality, quantum and classical, with distinct scaling properties.

Abstract

A quantum critical point (QCP) is a singularity in the phase diagram arising due to quantum mechanical fluctuations. The exotic properties of some of the most enigmatic physical systems, including unconventional metals and superconductors, quantum magnets, and ultracold atomic condensates, have been related to the importance of the critical quantum and thermal fluctuations near such a point. However, direct and continuous control of these fluctuations has been difficult to realize, and complete thermodynamic and spectroscopic information is required to disentangle the effects of quantum and classical physics around a QCP. Here we achieve this control in a high-pressure, high-resolution neutron scattering experiment on the quantum dimer material TlCuCl3. By measuring the magnetic excitation spectrum across the entire quantum critical phase diagram, we illustrate the similarities between quantum and thermal melting of magnetic order. We prove the critical nature of the unconventional longitudinal ("Higgs") mode of the ordered phase by damping it thermally. We demonstrate the development of two types of criticality, quantum and classical, and use their static and dynamic scaling properties to conclude that quantum and thermal fluctuations can behave largely independently near a QCP.