1. Contribution from the T. J. Watson Research Center, Nanoscale Materials and Devices, IBM, 1101 Kitchawan Road, Route 134, P.O. Box 218, Yorktown Heights, New York 10598, Villanova University, 800 Lancaster Avenue, Villanova, Pennsylvania 19085, and Department of Applied Physics & Applied Mathematics, Columbia University, 200 SW Mudd Building, 500 West 120th Street, New York, New York 10027