Quantum Institute: Visitor Schedule

The Quantum Lunch is regularly held on Thursdays in the Theoretical Division Conference Room, TA-3, Building 123, Room 121.
For more information, contact Diego Dalvit.

Tuesday, December 9, 2008
12:30 PM - 2:00 PM

Speaker: Roger Melko, University of Waterloo

Technical Host: Matthew Hastings

TOPIC: Valence-bond solids and deconfined quantum criticality

Abstract
The modern theory of quantum phase transitions hinges upon the idea of an observable order parameter, related to broken symmetries of the ordered phases, whose fluctuations govern universal properties at criticality. This paradigm has recently been challenged by Senthil and coworkers, who proposed the existence of a "deconfined" quantum critical point (DQCP) - a generic continuous quantum phase transition separating phases with unrelated broken symmetries, and with exotic emergent properties. Prototypically, a DQCP is conjectured to occur between a Neel state and a valence-bond-solid (VBS) phase. Using extensive quantum Monte Carlo simulations, we examine two classes of 2D spin-1/2 models that contain Neel-VBS quantum phase transitions, which have been proposed recently as potential candidates for a DQCP. We focus on finite-temperature properties of the quantum critical region, including scaling behavior and the calculation of universal critical exponents. One case - an SU(2) Heisenberg model with four-spin exchange - stands out as our current best candidate for a DQCP; although this proposal is not without controversy.