paper

Ground-state phases of the frustrated spin-1/2 ---- Heisenberg ferromagnet () on the honeycomb lattice with

arXiv:1109.6229 · doi:10.1103/PhysRevB.85.085115

Abstract

We study the ground-state (gs) properties of the frustrated spin-1/2 ---- Heisenberg model on a honeycomb lattice with ferromagnetic (FM) nearest-neighbor () exchange and frustrating antiferromagnetic (AFM) next-nearest-neighbor () and next-next-nearest-neighbor () exchanges, for the case . We use the coupled cluster method in high orders of approximation, complemented by the exact diagonalization of a lattice with 32 sites, and calculate the gs energy, magnetic order parameter, and spin-spin correlation functions. We find a quantum phase transition between regions characterized by FM order and a form of AFM ("striped") collinear order at . We compare results for the FM case (with ) to previous results for the corresponding AFM case (with ). While the magnetic order parameters behave similarly for the FM and the AFM models for large values of the frustration parameter , there are considerable differences between them for . For example, the quasiclassical collinear magnetic long-range order for the AFM model (with ) breaks down at , whereas the "equivalent" point for the FM model (with ) occurs at . Unlike in the AFM model (with ), where a plaquette valence-bond crystal phase intrudes between the two corresponding quasiclassical AFM phases (with Néel and striped order) for , with , we find no clear indications in the FM model for an intermediate magnetically disordered phase between the phases exhibiting FM and striped order. Instead, the evidence points strongly to a direct first-order transition between the two ordered phases of the FM model.

21 pages, 6 figures (a & b)

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