1. Research Institute for Solid State Physics and Optics, H-1525 Budapest, P. O. Box 49, Hungary
2. Department of Physics, FJFI ČVUT, Břehová 7, 115 19 Praha 1 — Staré Město, Czech Republic
3. Institut für Angewandte Physik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany
4. Eötvös University, Pázmány P. stny. 1A, H-1117 Budapest, Hungary