Spin-Current Order in Anisotropic Triangular Antiferromagnets

We analyze instabilities of the collinear up-up-down state of a two-dimensional quantum spin-$S$ spatially anisotropic triangular lattice antiferromagnet in a magnetic field. We find, within the large-$S$ approximation, that near the end point of the plateau, the collinear state becomes unstable due to the condensation of two-magnon bound pairs rather than single magnons. The two-magnon instability leads to a novel two-dimensional vector chiral phase with alternating spin currents but no magnetic order in the direction transverse to the field. This phase breaks a discrete $Z_2$ symmetry but preserves a continuous $U(1)$ one of rotations about the field axis.