1. Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019, Fred Hutchinson Cancer Research Center, Seattle, Washington 98019, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109, and Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden