1. Research Institute for Solid State Physics, H-525 Budapest, P. O. B. 49, Hungary, and Laboratoire de Physique des Solides Semicrystallins, IGA Department de Physique, Ecole Polytechnique Federale de Lausanne, 1015-Lausanne, Switzerland
2. Research Institute for Solid State Physics, H-1525 Budapest, P. O. B. 49, Hungary, and Institute of Physics of the Technical University, H-1521 Budapest, Hungary
3. Department of Physics, State University of New York at Stony Brook, NY 11794-3800, USA.
4. Laboratoire de Physique des Solides Semicrystallins, IGA Departement de Physique, Ecole Polytechnique Federale de Lausanne, 1015-Lausanne, Switzerland