1. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6487, United States
2. Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 37673, Republic of Korea
3. Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States
4. Department of Physics and the Quantum Theory Project, University of Florida, Gainesville, Florida 32611, United States