Affiliation: | (1) Department of Mathematical Physics, University College, Dublin 4, Ireland;(2) Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China;(3) Present address: Institute for Theoretical Physics, University of Cologne, D-5000 Köln 41, Germany;(4) Institute for Theoretical Physics, University of Cologne, D-5000 Köln 41, Germany |