1. Institute for Solid State Physics and Optics, Wigner Research Centre for Physics of the Hungarian Academy of Sciences, P. O. B. 49, H-1525, Budapest, Hungary
2. Department of Physics, Tohoku University, Sendai 980-8578, Japan
3. Theoretical Physics III, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg, Germany