1. Institut de Physique, B. 5, Université de Liége, B-4000 Liége, Belgium
2. Institut Montefiore, B.28, Université de Liége, B-4000 Liége, Belgium
3. Solid State Division, Solonos Str. 104, University of Athens, GR-10680 Athens, Greece
4. Department of Electrical and Computer Engineering, University of California, San Diego, Mail Code R-007, La Jolla, California 92093–0407, USA
5. Kernforschungszentrum Karlsruhe, Institut für Nukleare Festkörperphysik, Postfach 3640, D-7500 Karlsruhe, F.R. Germany