The kagome antiferromagnet: a chiral topological spin liquid ?

Abstract

Inspired by the recent discovery of a new instability towards a chiral phase of the classical Heisenberg model on the kagome lattice, we propose a specific chiral spin liquid that reconciles different, well-established results concerning both the classical and quantum models. This proposal is analyzed in an extended mean-field Schwinger boson framework encompassing time reversal symmetry breaking phases which allows both a classical and a quantum phase description. At low temperatures, we find quantum fluctuations favor this chiral phase, which is stable against small perturbations of second and third neighbor interactions. For spin-1/2 this phase may be, beyond mean-field, a chiral gapped spin liquid. Such a phase is consistent with Density Matrix Renormalization Group results of Yan et al. (Science 322, 1173 (2011)). Mysterious features of the low lying excitations of exact diagonalization spectra also find an explanation in this framework. Moreover, thermal fluctuations compete with quantum ones and induce a transition from this flux phase to a planar zero flux phase at a non zero value of the renormalized temperature (T/S^2), reconciling these results with those obtained for the classical system.