We study the fate of spin-$1/2$ spiral-ordered two-dimensional quantum antiferromagnets that are disordered by quantum fluctuations. A crucial role is played by the topological point defects of the spiral phase, which are known to have a $Z_2$ character. Previous works established that a nontrivial quantum spin-liquid phase results when the spiral is disordered without proliferating the $Z_2$ vortices. Here, we show that when the spiral is disordered by proliferating and condensing these vortices, valence-bond solid ordering occurs due to quantum Berry phase effects. We develop a general theory for this latter phase transition and apply it to a lattice model. This transition potentially provides a new example of a Landau-forbidden deconfined quantum critical point.

• Received 2 December 2010

DOI:https://doi.org/10.1103/PhysRevB.84.104430