1. Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801, USA.
2. James Franck Institute, University of Chicago, 5640 Ellis Avenue, Chicago, IL 60637, USA.
3. AT&T Bell Laboratories, Murray Hill, NJ 07974, USA.
4. Department of Physics, Yale University, New Haven, CT 06511, USA.
5. Department of Physics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA.