1. Max-Planck-Institut für Physik (Werner-Heisenberg-Institut); Föhringer Ring 6 D-80805 Munich Germany
2. Department of Physics; Universidad de Oviedo; Avda. Calvo Sotelo 18 33007 Oviedo Spain
3. Department of Physics and Astronomy; University of Alabama; Tuscaloosa AL 35487 USA
4. National Center for Theoretical Sciences; Physics Division; No. 101, Section 2, Kuang Fu Road, Hsinchu, Taiwan 300, R. O. C.