1. Metallurgy Division National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, U.S.A.
2. Engineering and Applied Science Division, California Institute of Technology, 1200 E. California Blvd. MC 309-81, Pasadena, CA 91125, U.S.A.
3. Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, U.S.A.