Affiliation: | (1) Department of Applied Physics, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands;(2) Present address: Instituto di Fisica, Facoltà di Ingegneria, Università di Catania, viale A. Doria 6, I-95129 Catania, Italy;(3) Present address: Institut für Theoretische Festkörperphysik, Univesiität Karlsruhe, W-7500 Karlsruhe, Germany |