1. Institute for Theoretical Physics, TU Bergakademie Freiberg, Leipziger Straße 23, 09599 Freiberg, Germany
2. Institute for Theoretical Physics, Heidelberg University, Philosophenweg 19, 69120 Heidelberg, Germany
3. Institute for Theoretical Solid State Physics, RWTH Aachen University, 52056 Aachen, Germany