Quantum Stabilization of the1/3-Magnetization Plateau inCs2CuBr4

We consider the phase diagram of a spatially anisotropic 2D triangular antiferromagnet in a magnetic field. Classically, the ground state is umbrellalike 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 spin density wave state separating the two chiral phases. Our analysis sheds light on several recent experimental findings for ${\mathrm{Cs}}_2{\mathrm{CuBr}}_4$.