1. Institute of Micro- and Nanotechnologies, Technical University Ilmenau, P.O. Box 100565, 98684 Ilmenau, Germany, Department of Physics, and Instrumental Analysis and Research Center, Shanghai University, Shanghai 200444, China, and Fraunhofer-Institut für Angewandte Festkörperphysik (IAF), Tullastrasse 72, 79108 Freiburg, Germany