1. IBM Research Europe─Zurich, 8803 Rüschlikon, Switzerland
2. Fachbereich Physik, Universität Konstanz, D-78457 Konstanz, Germany
3. Laboratory for Solid State Physics, ETH Zürich, 8093 Zürich, Switzerland
4. Institute of Physics, Czech Academy of Sciences, 162 00 Prague, Czech Republic
5. Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain