# Unconventional Quantum Phase Transition in a Ring-Exchange Antiferromagnet

Preprint

**Published:**Monday, April 02, 2007

**Authors (4 total):** V. Kotov,
D. X. Yao,
A. Castro Neto,
D. K. Campbell

**Abstract:** We study the S=1/2 Heisenberg antiferromagnet on a square lattice with nearest-neighbor and plaquette ring exchanges; this model undergoes a quantum phase transition from a spontaneously dimerized phase to N\'eel order at a critical coupling. We show that as the critical point is approached from the dimerized side the system exhibits strong fluctuations in the dimer background, reflected in the presence of a low-energy singlet mode, with a simultaneous rise in the triplet quasiparticle density. We find that both singlet and triplet modes of high density condense at the transition, signaling restoration of lattice symmetry. These unconventional features are consistent with "deconfined quantum criticality". arXiv:0704.0114