Affiliation: | (1) Institute for Solid State Physics, Eötvös Univerity, P.O. Box 327, H-1445 Budapest, Hungary;(2) Central Research Institute for Physics, P.O.Box 49, H-1525 Budapest, Hungary;(3) Sektion Physik, Karl-Marx-Universität Leipzig, Karl-Marx-Platz, GDR-7010 Leipzig, German Democratic Republic |