The ground-state magnetic ordering of the spin-1/2 frustrated J1–J2 XXZ model on the square lattice

Using the coupled-cluster method for infinite lattices and the exact diagonalization method for finite lattices, we study the influence of an exchange anisotropy ∆ on the groundstate phase diagram of the spin-1/2 frustrated J1–J2 XXZ antiferromagnet on the square lattice. We find that increasing ∆ > 1 (i.e. an Ising type easy-axis anisotropy) as well as decreasing ∆ < 1 (i.e. an XY type easy-plane anisotropy) both lead to a monotonic shrinking of the parameter region of the magnetically disordered quantum phase. Finally, at ∆c ≈ 1.9 this quantum phase disappears, whereas in pure XY limit (∆ = 0) there is still a narrow region around J2 = 0.5J1 where the quantum paramagnetic ground-state phase exists. The interplay between frustration and quantum fluctuations in magnetic systems may lead to unusual quantum phases [1]. A canonical model to study these effects is the frustrated spin-1/2 J1–J2 antiferromagnet on the square lattice (J1–J2 model). This model has attracted a great deal of interest, see, e.g., Refs. [2–9]. The recent synthesis of magnetic materials that can be well described by the spin-1/2 J1-J2 model on the square lattice [10–12] has stimulated further interest in the model. For the isotropic spin-1/2 J1–J2 model there are two magnetically ordered ground state (GS) phases at small and at large J2 separated by an intermediate quantum paramagnetic phase (QPP) without magnetic long-range order (LRO) in the region Jc1 2 ≤ J2 ≤ Jc2 2 , where Jc1 2 ≈ 0.4J1 and Jc2 2 ≈ 0.6J1. The GS at J2 < Jc1 2 exhibits Néel LRO. The twofold degenerate GS at J2 > J c2 2 shows so-called collinear magnetic LRO. These two collinear GS’s are characterized by a parallel spin orientation of nearest neighbours in vertical direction and an antiparallel spin orientation of nearest neighbours in horizontal direction [collinear-columnar (CC) state] and vice versa (collinear-row state). The nature of the transition between the Néel phase and the QPP as well as the properties of the QPP are still under debate [6–9]. Several extensions of the J1–J2 model have been studied recently, see, e.g., Refs. [13–24]. For instance, it was found that by increasing the space dimension from D = 2 to D = 3 the intermediate QPP disappears [13–15]. Here we generalize the spin-1/2 J1-J2 model by including exchange anisotropy. Such an anisotropy is relevant experimentally as well as theoretically, since it is likely to be present in any real material. Its introduction also allows us to tune the strength of quantum fluctuations. Therefore, it may have a strong influence on the GS ordering [21,22,24]. −1.1 −1 −0.9 −0.8 −0.7 −0.6