1. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
2. Institute of Applied Physics, TU Dresden, Dresden 01069, Germany
3. National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing, Jiangsu 210093, China
4. Department of Applied Physics, Stanford University, Stanford, California 94305, USA and Department of Physics, Stanford University, Stanford, California 94305, USA