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 (lambda,alpha) space, where lambda=J(1)'/J(1) and alpha=J(2)/J(1). Depending on the values of lambda and alpha 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 lambda(1) < lambda < 1 (lambda(1) similar or equal to 0.62). (C) 2014 Published by Elsevier B.V.