1. Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
2. Department of Physics, University of California, Berkeley, CA 94720, USA
3. Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada
4. Max Planck Institute of Microstructure Physics, 06120 Halle, Germany
5. Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45219, USA
6. Institute of Physics, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany