Recently a remarkable numerical study of the hubbard model on honeycomb lattice reveals a possible spin liquid phase in the vicinity of the Mott transition. What is this spin liquid phase? Are there measurable signatures of this spin liquid phase? In this blackboard talk, I will present our recent work with Yuan-Ming Lu on these issues. If time is allowed, I may also talk about a completely different subject: the weak indices and dislocations in a general topological band structures.