1. Graduate School of Science and Technology, Niigata University, Niigata 950-2181
2. Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568
3. Department of Physics, University of California, San Diego La Jolla, CA 92093-0360, U.S.A.
4. Faculty of Integrated Arts and Sciences, The University of Tokushima, Tokushima 770-8502
5. Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397