Single-hole properties of the t-J model on the honeycomb lattice
  Motivated by the possible charge ordering in the recently discovered superconductor Na_xCoO_2 yH_2O, which at filling x=1/3 would correspond to a half-filled honeycomb lattice, we investigate the single-hole dynamics of the t-J model on this lattice. Using exact diagonalization, series expansion and the self-consistent Born approximation, we calculate the quasi-particle dispersion, bandwidth and residues and compare our findings with the well established results for the square lattice. Given the similarities between both lattices, it is not surprising that we find qualitatively similar features for the honeycomb lattice, namely an almost flat band along the edge of the magnetic Brillouin zone and well defined hole pockets around the corners. However, we report a considerable disagreement between the three methods used concerning the bandwidth and discuss possible origins of this discrepancy. 
Andreas Luscher, Andreas Laeuchli, Weihong Zheng, Oleg P. Sushkov