1. Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Department of Biophysics and Physics of Complex Systems, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, and Federal Institute for Materials Research and Testing, Richard-Willstaetter-Strasse 11, D-12489 Berlin, Germany.