Affiliation: | (1) Institute for Physics, University of Mainz, D-55099 Mainz, Germany, and;(2) Institute for Theoretical Physics, University of Saarland, D-66041 Saarbrücken, Germany;(3) Present address: Theoretical Physics, University of Konstanz, D-78457 Konstanz, Germany;(4) Erich Schmid Institute of Materials Science, Austrian Academy of Sciences and University of Leoben, Jahnstr. 12, A-8700 Leoben, Austria;(5) Department of Physics and Mathematics, Rutgers University, Piscataway, New Jersey, 08855-0849 |