  In this paper, we study the non-magnetic insulator state near Mott transition
of 2D pi-flux Hubbard model on square lattice and find that such non-magnetic
insulator state is quantum spin liquid state with nodal fermionic excitations -
nodal spin liquid (NSL). When there exists small easy-plane anisotropic energy,
the ground state becomes Z2 topological spin liquid (TSL) with full gapped
excitations. The U(1)times U(1) mutual--Chern-Simons (MCS) theory is obtained
to describe the low energy physics of NSL and TSL.
