Spin-gap phase of a quantum spin system on a honeycomb lattice
  We study a quantum spin system on a honeycomb lattice, when it includes frustration and distortion in antiferromagnetic (AF) exchange interactions. We transform the spin system onto a nonlinear sigma model (NLSM) in a new way preserving the original spin degrees of freedom. Assisted by a renormalization- group argument, the NLSM provides a ground-state phase diagram consisting of an ordered AF phase and a disordered spin-gap phase.   The spin-gap phase extends from a strong frustration regime to a strong distortion regime, showing that the disordered ground states are essentially the same in both the regimes. In the spin-half case, the spin-gap phase for the spin system on a honeycomb lattice is larger than that for the J1-J2 model on a square lattice. 
Ken'ichi Takano