1. Department of Physics, Columbia University, New York, New York 10027, United States
2. Honda Research Institute USA, Inc., 70 Rio Robles, San Jose, California 95134, United States
3. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
4. Department of Physics, University of California, San Diego, La Jolla, California 92093, United States