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Yasuhiko Takahashi
Institute of Applied Physics, University of Tsukuba, Tsukuba 305–8573
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Ken–ichi Ohshima
Institute of Applied Physics, University of Tsukuba, Tsukuba 305–8573
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Fujio P. Okamura
National Institute for Research in Inorganic Materials, Tsukuba 305–0044
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Noriaki Sato
Department of Physics, Tohoku University, Sendai 980–8578