1. Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA.
2. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA.
3. Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK.
4. School of Electrical Engineering, Korea University, Seoul 02841, South Korea.
5. School of Electrical & Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA.