1. Department of Physics, Oregon State University, Corvallis, Oregon 97331, USA
2. Department of Physics, Linfield College, McMinnville, Oregon 97128, USA
3. Energy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa 904-0495, Japan