Phase diagram of the frustrated quantum-XY model on the honeycomb   lattice studied by series expansions: Evidence for proximity to a bicritical   point
  We study the nearest-neighbor exchange ($J_1$) and second-neighbor exchange ($J_2$) XY antiferromagnet on the honeycomb lattice using ground state series expansions around N{\'e}el, columnar and dimer phases. The conventional two-sublattice XY N{\'e}el order at small $J_2$ vanishes at $J_2/J_1=0.22\pm 0.01$ in agreement with results from Density Matrix Renormalization Group (DMRG) studies. Near the transition, we find evidence for an {\it approximate} emergent symmetry between XY and Ising degrees of freedom, namely the nearest-neighbor Ising and XY spin correlations become nearly equal. This suggests that the system is close to a bicritical point separating XY and Ising orders. At still larger $J_2/J_1$ the columnar and dimer energies are found to be nearly degenerate. At even larger $J_2$ the columnar phase is obtained. The ground state energies in all three phases are in good agreement with the values found in the DMRG studies. 
Jaan Oitmaa and Rajiv R. P. Singh
cond-mat.str-el