1. Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599
2. North Carolina State University, Department of Electrical and Computer Engineering, Raleigh, North Carolina 27695, and University of North Carolina, Department of Physics and Astronomy, Chapel Hill, North Carolina 27599, and Microelectronics Center of North Carolina, Research Triangle Park, North Carolina 27709
3. Microelectronics Center of North Carolina, Research Triangle Park, North Carolina 27709