Schwinger-boson mean-field study of the J1−J2 Heisenberg quantum antiferromagnet on the triangular lattice

We use Schwinger-boson mean-field theory (SBMFT) to study the ground state of the spin-S triangular-lattice Heisenberg model with nearest (J1) and next-nearest (J2) neighbor antiferromagnetic interactions. Previous work on the S=1/2 model leads us to consider two spin-liquid Ansätze, one symmetric and one nematic, which upon spinon condensation give magnetically ordered states with 120∘ order and collinear stripe order, respectively. The SBMFT contains the parameter κ, the expectation value of the number of bosons per site, which in the exact theory equals 2S. For κ=1 there is a direct, first-order transition between the ordered states as J2/J1 increases. Motivated by arguments that in SBMFT, smaller κ may be more appropriate for describing the S=1/2 case qualitatively, we find that in a κ window around 0.6, a region with the (gapped Z2) symmetric spin liquid opens up between the ordered states. As a consequence, the static structure factor has the same peak locations in the spin liquid as in the 120∘ ordered state, and the phase transitions into the 120∘ and collinear stripe-ordered states are continuous and first order, respectively.