1. Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
2. Institute of Physics, Johannes Gutenberg-University of Mainz, 55099 Mainz, Germany
3. Graduate School Materials Science in Mainz, Staudinger Weg 9, 55128 Mainz, Germany
4. Fachbereich Physik and Landesforschungszentrum, OPTIMAS, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany