1. Max Planck Institute for Solid State Research, Heisenbergstraβe 1, 70569 Stuttgart (Germany), Max Planck Institute for Metals Research, Heisenbergstraβe 3, 70569 Stuttgart (Germany), and Institut de Physique des Nanostructures, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne (Switzerland)