1. Department of Mathematics and Physics, Hirosaki University, Hirosaki, Aomori 036-8561, Japan
2. Department of Applied Physics, Okayama University of Science, Okayama 700-0005, Japan
3. Max-Planck-Institute für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
4. Institute für Theoretische Physik, ETH Zürich, 8093 Zürich, Switzerland