1. Physics Department, Lehigh University, 16 Memorial Drive East, Bethlehem, Pennsylvania 18015, Center for Advanced Materials and Nanotechnology, Lehigh University, 5 East Packer Avenue, Bethlehem, Pennsylvania 18015, IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York 10598, and Ioffe Institute, 26 Polytekhnicheskaya, St. Petersburg, 194021, Russia