"Ice states, cyclic exchange and fractional excitations in fermionic and spin models"

  The quarter-filled Hubbard model extended with nearest neighbor
repulsion on the pyrochlore lattice is an insulator for sufficiently
large couplings, with an extensive ground state degeneracy. The model
can be mapped to the ice- and the 6-vertex problems. The large degeneracy
in the ground state manifold can support topological excitations with
fractional charge. Similar excitations can arise in the Ising problem, as
well as in the classical Heisenberg model with  bilinear-biquadratic
terms.  Quantitative results are presented for the two-dimensional
 XXZ Heisenberg model, where we discuss the effects of quantum
fluctuations and the conditions for spinon deconfinement.