Frustrated spin- 12 J1 - J2 isotropic XY model on the honeycomb lattice

We study the zero-temperature ground-state (GS) phase diagram of a spin-half J1-J2 XY model on the honeycomb lattice with nearest-neighbor exchange coupling J1>0 and frustrating next-nearest-neighbor exchange coupling J2≡κJ1>0, where both bonds are of the isotropic XY type, using the coupled cluster method. Results are presented for the GS energy per spin, magnetic order parameter, and staggered dimer valence-bond crystalline (SDVBC) susceptibility, for values of the frustration parameter in the range 0≤κ≤1. In this range, we find phases exhibiting, respectively, Néel xy planar [N(p)], Néel z-aligned [N(z)], SDVBC, and Néel-II xy planar [N-II(p)] orderings. The Néel-II states, which break the lattice rotational symmetry, are ones in which the spins of nearest-neighbor pairs along one of the three equivalent honeycomb directions are parallel, while those in the other two directions are antiparallel. The N(p) state, which is stable for the classical version of the model in the range 0≤κ≤16, is found to form the GS phase out to a first quantum critical point at κc1=0.216(5), beyond which the stable GS phase has N(z) order over the range κc1<κ<κc2=0.355(5). For values κ>κc2, we find a strong competition to form the GS phase between states with N-II(p) and SDVBC forms of order. Our best estimate, however, is that the stable GS phase over the range κc2<κ<κc3≈0. 52(3) is a mixed state with both SDVBC and N-II(p) forms of order; and for values κ>κc3 is the N-II(p) state, which is stable at the classical level only at the highly degenerate point κ=12. Over the range 0≤κ≤1, we find no evidence for any of the spiral phases that are present classically for all values κ>16, nor for any quantum spin-liquid state.