First-order transition on the frustrated spin-1/2 Heisenberg ferromagnet on an anisotropic square lattice

We have studied the quantum spin-1/2 frustrated Heisenberg model with two ferromagnetic interactions: nearest-neighbor (NN) with different coupling strengths J ₁ and J 1 ′ along x and y directions, respectively, competing with a next-nearest-neighbor (NNN) with coupling J ₂ . Using the effective-field theory we obtain the ground-state phase diagram in the ( λ, α) space, where λ = J 1 ′ / J 1 and α= J ₂/ J ₁. Depending on the values of λ and α we observe three different states: ferromagnetic (F), collinear ferromagnetic (CF) and quantum paramagnetic (QP). We observe a QP state between the ordered F and CF phases in the region λ ₁< λ<1 ( λ ₁≃0.62).