Gapped spin-liquid phase in the J1-J2 Heisenberg model by a bosonic resonating valence-bond ansatz

We study the ground-state phase diagram of the spin-1/2J(1)-J(2) Heisenberg model on the square lattice with an accurate bosonic resonating-valence-bond (RVB) wave function. In contrast to the RVB ansatz based on Schwinger fermions, the representation based on Schwinger bosons, supplemented by a variational Monte Carlo technique enforcing the exact projection onto the physical subspace, is able to describe a fully gapped spin liquid in the strongly frustrated regime. In particular, a fully symmetric Z(2) spin liquid is stable between two antiferromagnetic phases; a continuous transition at J(2) = 0.4J(1), when the Marshall sign rule begins to be essentially violated, and a first-order transition around J(2) = 0.6J(1) are present. Most importantly, the triplet gap is found to have a nonmonotonic behavior, reaching a maximum around J(2) = 0.51J(1), when the lowest spinon excitation moves from the Gamma to the M point, i.e., k = (pi, 0).