Variational Monte Carlo study of a gapless spin liquid in the spin-1/2 XXZ antiferromagnetic model on the kagome lattice

By using the variational Monte Carlo technique, we study the spin-1/2 XXZ antiferromagnetic model (with easy-plane anisotropy) on the kagome lattice. A class of Gutzwiller projected fermionic states with a spin Jastrow factor is considered to describe either spin liquids [with U(1) or Z(2) symmetry] or magnetically ordered phases [with q = (0,0) or q = (4 pi/3,0)]. We find that the magnetic states are not stable in the thermodynamic limit. Moreover, there is no energy gain to break the gauge symmetry from U(1) to Z(2) within the spin-liquid states, as previously found in the Heisenberg model. The best variational wave function is therefore the U(1) Dirac state, supplemented by the spin Jastrow factor. Furthermore, a vanishing S = 2 spin gap is obtained at the variational level, in the whole regime from the XY to the Heisenberg model.