Yale University

Title: Breakdown of the Landau-Ginzburg-Wilson paradigm at quantum phase transitions

Abstract:

I present the theory of quantum phase transitions of Mott insulators between antiferromagnetically ordered and paramagnetic states in spatial dimensions greater than one. For Mott insulators with an even number of S=1/2 spins per unit cell, the Landau-Ginzburg-Wilson theory for the destruction of magnetic order agrees well with numerical and experimental studies. However, for Mott insulators with an odd number of S=1/2 spins per unit cell the situation is much more subtle. We argue that the spin Berry phases lead to the appearance of spontaneous bond order in the paramagnet. Further, the phases with magnetic and bond order are generically separated by a second order quantum critical point, in contradiction to the predictions of Landau-Ginzburg-Wilson theory. A theory for this novel critical point is presented, expressed in terms of emergent gauge excitations.