1. Microsystems Engineering, Science and Applications Sandia National Laboratories Albuquerque NM 87123 USA
2. Department of Physics Temple University Philadelphia PA 19122‐1801 USA
3. Neural Exploration and Research Laboratory Sandia National Laboratories Albuquerque NM 87123 USA
4. Department of Electrical and Computer Engineering The University of Texas at Austin Austin TX 78712 USA
5. Department of Physics New York University New York NY 10003 USA
6. Department of Electrical Engineering and Computer Science University of Tennessee Knoxville TN 37996 USA