  We formulate a U(1) gauge theory of the Hubbard model in the slave-rotor
representation. From this formalism it is argued that spin liquid phases may
exist near the Mott transition in the Hubbard model on triangular and honeycomb
lattices at half filling. The organic compound k-(BEDT-TTF)_2 Cu_2 (CN)_3 is a
good candidate for the spin liquid state on a triangular lattice. We predict a
highly unusual temperature dependence for the thermal conductivity of this
material.
