Can a local repulsive potential trap an electron?
  We study the classical dynamics of a charged particle in two dimensions, under the influence of a perpendicular magnetic and an in-plane electric field. We prove the surprising fact that there is a finite region in phase space that corresponds to the otherwise drifting particle being trapped by a local repulsive potential. Our result is a direct consequence of KAM-theory and, in particular, of Moser's theorem. We illustrate it by numerical phase portraits and by an analytic approximation to invariant curves. 
N. Berglund (EPF-Lausanne, Switzerland), Alex Hansen, E.H. Hauge   (University of Science and Technology, Trondheim, Norway)...