1. Department of Physics, Center for Advanced Nanoscience, University of California, San Diego, California 92093, USA
2. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
3. Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands
4. Department of Electrical Engineering and Computer Science, University of Tennessee, Knoxville, Tennessee 37996, USA
5. IBM Research-Zurich, Rüschlikon, Zurich CH-8803, Switzerland
6. Department of Physics, Fudan University, Shanghai, China