1. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, Department of Physics, University of Texas, Arlington, Texas 76019, Advance Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, UNM/NSF Center for Micro-Engineered Materials, University of New Mexico, Albuquerque, New Mexico 87131, and Department of Chemistry, Brown University, Providence, Rhode Island 02912