1. International Institute for Applied Systems Analysis, Schlossplatz 1, A-2361 Laxenburg, Austria, Institute of Physics, Tampere University of Technology, P. O. Box 692, FI−33101 Tampere, Finland, Laboratory of Physics and Helsinki Institute of Physics, Helsinki University of Technology, P. O. Box 1100, FI-02015 HUT, Finland, Department of Physics/DEAS, Harvard University, McKay Laboratory, 40 Oxford Street, Cambridge, Massachusetts 02138, Department of Life Science and Chemistry, Roskilde University, P. O...