Hopf term and the effective Lagrangian for the Skyrmions in a   two-dimensional electron gas at small g-factor
  We study interacting electrons in two dimensions moving in the lowest Landau level under the condition that the Zeeman energy is much smaller than the Coulomb energy and the filling factor is one. In this case, Skyrmion quasiparticles play an important role. Here, we present a simple and transparent derivation of the corresponding effective Lagrangian. In its kinetic part, we find a non-zero Hopf term the prefactor of which we determine rigorously. In the Hamiltonian part, we calculate, by means of a gradient expansion, the Skyrmion-Skyrmion interaction completely up to fourth order in spatial derivatives. 
W. Apel (1) and Yu. A. Bychkov (2,1) ((1) Physikalisch-Technische   Bundesanstalt, Germany and (2) L.D. Landau Institute for...