Quantum stabilization of 1/3-magnetization plateau in Cs_2CuBr_4

We consider the phase diagram of a spatially anisotropic 2D triangular antiferromagnet in a magnetic field. Classically, the ground state is umbrella-like for all fields, but we show that the quantum phase diagram is much richer and contains a 1/3 magnetization plateau, two commensurate planar states, two incommensurate chiral umbrella phases, and, possibly, a planar state separating the two chiral phases. Our analysis sheds light on several recent experimental findings for the spin-1/2 system Cs_2CuBr_4.