Bruce Normand (SIMES Friday Seminar)

Date(s) - Nov 8 2013
11:00 AM - 12:30 PM

Shasta Room, Bldg. 40, Room 361


Quantum and Classical Criticality in the Quantum Antiferromagnet TlCuCl3

Bruce Normand

Professor of Physics, Renmin University of China

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.

We have achieved this control in a high-pressure, high-resolution neutron scattering experiment on the quantum dimer material TlCuCl3. Measurements of the magnetic excitation spectrum across the entire quantum critical phase diagram in pressure and temperature illustrate the similarities between quantum and thermal melting of magnetic order. They show the development of two types of criticality, quantum and classical, and we use both static and dynamic scaling properties to conclude that quantum and thermal fluctuations can behave largely independently near a QCP. We further demonstrate the critical nature of the unconventional longitudinal (“Higgs”) mode of the ordered phase by damping it thermally, and show that the same critical damping is a fundamental property of the magnetic excitations throughout the quantum critical regime.