Skyrme and Meron crystals in quantum Hall ferromagnets

The ground state of a two-dimensional electron gas at Landau level filling factor nu near 1 is studied in the Hartree-Fock approximation. For Zeeman coupling different from zero, the ground state is a Skyrme crystal with both spontaneous long range order in the charge density and spontaneous long range order in the transverse spin density. The energy of the Skyrme crystal is lowest for a square lattice structure with opposing postures for topological er;citations on opposite sublattices. We discuss interpretations of our results in terms of non-linear a and generalized spin models for quantum Hall ferromagnets. For zero Zeeman coupling, we find that the ground state can be interpreted as a meron crystal with two interpenetrating sublattices, each supporting quasiparticles with charge e/2.