The quantum J1−J′1−J2 spin-1/2 Heisenberg antiferromagnet: A variational method study

Abstract

The phase transition of the quantum spin-1/2 frustrated Heisenberg antiferroferromagnet on an anisotropic square lattice is studied by using a variational treatment. The model is described by the Heisenberg Hamiltonian with two antiferromagnetic interactions: nearest-neighbor (NN) with different coupling strengths J1 and J′1 along x and y directions competing with a next-nearest-neighbor coupling J2 (NNN). The ground state phase diagram in the (λ,α) space, where λ=J′1/J1 and α=J2/J1, is obtained. Depending on the values of λ and α, we obtain three different states: antiferromagnetic (AF), collinear antiferromagnetic (CAF) and quantum paramagnetic (QP). For an intermediate region λ1<λ<1 we observe a QP state between the ordered AF and CAF phases, which disappears for λ above some critical value λ1≃0.53. The boundaries between these ordered phases merge at the quantum critical endpoint (QCE). Below this QCE there is again a direct first-order transition between the AF and CAF phases, with a behavior approximately described by the classical line αc≃λ/2.