Mailing address: Solid-State Theory Division, Institute for Theoretical Physics, Technical University of Braunschweig, Physics Center, Mendelssohnstr. 3, 38106 Braunschweig, Germany, Phone: +49 531 391 5181, Fax: +49 531 391 5833, EMail: w.brenig[at]tu-bs.de [at]=@