1. Condensed Matter Laboratory, Department of Physics, University of Colorado, Boulder, CO 80309, and Exxon Research and Engineering Company, Annandale, NJ 08801, USA.
2. Materials and Physics Departments and the Interdepartmental Program on Biochemistry and Molecular Biology, University of California, Santa Barbara, CA 93106, USA.
3. Exxon Research and Engineering Company, Annandale, NJ 08801, USA.
4. Condensed Matter Laboratory, Department of Physics, University of Colorado, Boulder, CO 80309, USA.