1. 4. Physics Institute and Research Center SCoPE, Pfaffenwaldring 57, University of Stuttgart, 70569 Stuttgart, Germany
2. Kirchhoff Institute for Physics, University of Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
3. Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States.