Effective field theory of chiral spin liquid between ordered phases in a kagome antiferromagnet.

We propose in this work an effective field theory description of the chiral spin liquid state in a Heisenberg spin system on a kagome lattice. To this end, we derive the low-energy effective theory of a kagome (isotropic) Heisenberg antiferromagnet around its ordered ground states found numerically and show that quantum fluctuations induced by further neighbor spin exchanges are equally strong as those from the first neighbor. We use a chiral order parameter theory to argue for the occurrence of finite temperature chiral symmetry breaking transition into a chiral ordered state in a kagome antiferromagnet with further neighbor spin exchange interactions. We compute the chiral symmetry breaking term in the effective ground state energy and show that chiral spin liquid necessarily occurs in the ground state of a kagome antiferromagnet with the first three nearest-neighbor spin exchange interactions. Finally, we consider the quantum criticality of the kagome antiferromagnet and show that a Chern-Simons term emerges naturally at the transition between the two ordered states that satisfy an appropriate 'matching condition' that we derive, providing an explanation for why chiral spin liquid could occur at the transition between appropriate ordered states. This emergent Chern-Simons term is the low energy effective theory of the chiral spin liquid state, where the chirality is the immediate consequence of the breaking of discrete symmetries by this topological field theory.