1. Center for Quantum Nanoscience, Institute for Basic Science, Seoul 03760, Republic of Korea.
2. IBM Almaden Research Center, San Jose, CA 95120, USA.
3. Department of Physics, Ewha Womans University, Seoul 03760, Republic of Korea.
4. IV. Physical Institute, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany.
5. Institute of Physics, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
6. QuantaLab, International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga, 4715-310 Braga, Portugal.