1. Forschungszentrum Jülich, Institut für Festkörperforschung, Postfach 1913, D-52425 Jülich, Germany; Martin-Luther-Universität Halle-Wittenberg, Fachbereich Physik, D-06099 Halle, Germany; Department of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, U.K.; Hahn-Meitner Institut, D-14091 Berlin, Germany; Department of Applied Maths and Theoretical Physics, Centre for Mathematical Sciences, Wilberforce Road, Cambridge, CB3 0WA, U.K.; and Department of Applied Mathematics, University of Leeds,...