University of Waterloo
200 University Avenue West
Waterloo, Ontario, Canada N2L 3G1
Phone: (519) 888-4567 ext 32215
Fax: (519) 746-8115
Department of Physics, Oxford University
Dr. Coldea's research group explores experimentally quantum properties of novel electronic and magnetic materials using neutron scattering and thermodynamic probes. The aim is to understand how electrons organize themselves in complex materials to lead to new properties.
Join us for a reception prior to the talk at 3:30pm in STC 0040
Continuous phase transitions between distinct phases of matter at zero temperature are driven by (quantum) critical fluctuations that involve the macroscopic system as a whole. In this regime the microscopic constituents of the material - electrons or spins - correlate their motion with one another so strongly that one can no longer describe the collective behaviour in terms of individual electrons or spins. Instead new types of quasiparticles appear that involve the coherent motion of all constituents. This talk will explore some of the challenges and opportunities to realize and probe experimentally the regime of quantum criticality. This will be illustrated with inelastic neutron scattering measurements of the spin dynamics in a quasi-one-dimensional easy-axis (Ising) magnet driven to a quantum critical point separating spontaneous magnetic order and paramagnetic phases by an applied transverse magnetic field. Near the transition field the experiments observe a continuum of excitations with linearly-dispersive lower boundaries, a characteristic signature of the predicted Dirac quasiparticles (massless Ising quarks) of the quantum critical point. At lower fields a spectrum of quantum resonances is observed, attributed to confinement bound states (Ising mesons) stabilized by mean-field effects of the weak interchain couplings. The frequency ratio of the lowest two bound states approaches the golden mean close to the transition, as predicted by theories of emergent E8 symmetry for weakly-perturbed criticality.