1. Universität Augsburg, Institut für Physik, Universitätsstrasse 1, 86135 Augsburg, Germany
2. Dipartimento di Fisica, Università di Camerino, 62032 Camerino, Italy
3. Frontier Research System, The Institute of Physical and Chemical Research (RIKEN), Wako‐shi, Saitama, 351‐0198, Japan
4. Center for Theoretical Physics, Department of Physics, University of Michigan, Ann Arbor, MI 48109‐1120, USA