1. Bremen Center for Computational
Materials Science, University of Bremen, Am Fallturm 1, D-28359, Bremen, Germany
2. Department of Electronic Engineering, University of Rome “Tor Vergata”, via
del Politecnico 1, I-00133 Roma, Italy
3. Institute for Theoretical Physics, University of Bremen, Otto-Hahn-Allee 1, D-28359 Bremen, Germany