1. Department of Preventive and Restorative Dental Sciences, University of California, San Francisco, Box 0758, 707 Parnassus Ave., San Francisco, CA 94143-0758, USA;
2. Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; and
3. Departments of Materials Science and Engineering and Physics, North Carolina State University, Raleigh, NC 27695, USA