Quantum spin liquid in a spin-1/2 XY model with four-site exchange on the kagome lattice

We study the ground-state phase diagram of a two-dimensional kagome-lattice spin-1/2 XY model (J) with a four-site ring-exchange interaction (K) using quantum Monte Carlo simulations. We find that the superfluid phase, existing in the regime of small ring exchange, undergoes a direct transition to a Z(2) quantum spin liquid phase at (K/J)(c) approximate to 22, which is related to the phase proposed by L. Balents, M. P. A. Fisher, and S. M. Girvin [Phys. Rev. B 65, 224412 (2002)]. The quantum phase transition between the superfluid and the spin liquid phase has exponents z and nu falling in the three-dimensional XY universality class, making it a candidate for an exotic XY* quantum critical point, mediated by the condensation of bosonic spinons.