  We show that the spin liquid phase of the half-filled Hubbard model on the
triangular lattice can be described by a pure spin model. This is based on a
high-order strong coupling expansion (up to order 12) using perturbative
continuous unitary transformations. The resulting spin model is consistent with
a transition from three-sublattice long-range magnetic order to an insulating
spin liquid phase, and with a jump of the double occupancy at the transition.
Exact diagonalizations of both models show that the effective spin model is
quantitatively accurate well into the spin liquid phase, and a comparison with
the Gutzwiller projected Fermi sea suggests a gapless spectrum and a spinon
Fermi surface.
