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Quantum Magnets under Pressure

Note: Pizza served at 11:30 AM
Speaker: Bruce Normand, ETH Zurich, Switzerland
When: February 15, 2008 (Fri), 12:00PM to 01:00PM
Hosted by: Claudio Chamon

This event is part of the Condensed Matter Seminar Series. All seminars will take place in the Metcalf Science Center, Room 352 at noon, unless otherwise noted

Abstract: In recent inelestic neutron scattering experiments, Rueegg et al. investigated the pressure-induced magnetic quantum phase transition in the dimer spin system TlCuCl_3. The triplet excitation spectra were measured on both sides of the quantum critical point, and the emergence of a well-defined, massive amplitude mode, or longitudinal excitation of the magnetic moment, demonstrated in the ordered phase. We discuss the pressure-induced magnetic quantum phase transition and compare this with its field-induced counterpart. The splitting of the excitation mode gaps suggests a finite exchange anisotropy, which is shown to be of easy-plane type; its quasi-uniaxial nature is responsible for the presence of one effectively massless spin-wave mode in the ordered phase. Also fitted is the pressure-dependence of the interdimer coupling parameters required to explain the vanishing of the gap. The longitudinal mode arises naturally at the quantum phase transition, and is reproduced with no additional parameters.