1. Department of Physics, University of California, San Diego, La Jolla, California 92093, United States
2. School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
3. Department of Physics and Astronomy, University of California, Los Angeles, California 90095, United States
4. Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, United States
5. Center for Memory and Recording Research, University of California, San Diego, La Jolla, California 92093, United States