1. Institute of Applied Physics, Eberhard Karls University of Tuebingen, Auf der Morgenstelle 10, D-72076, Tuebingen, Germany
2. Natural and Medical Sciences Institute at the University of Tuebingen, Markwiesenstr. 55, D-72770 Reutlingen, Germany
3. School of Physics and Electronics, Hunan University, Changsha, Hunan 410082, China
4. Institute of Physical and Theoretical Chemistry, Eberhard Karls University of Tuebingen, Auf der Morgenstelle 18, D-72076, Tuebingen, Germany