1. Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.
2. 3. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany.
3. Chinese Academy of Sciences Key Laboratory of Microscale Magnetic Resonance and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China.
4. Department of Chemistry, Zukunftskolleg, and Konstanz Research School Chemical Biology, Universität Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany.
5. Institut de Physique, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.