Affiliation: | Department of Applied Physics, Tokyo University of Education, 3-29-1 Otsuka, Bunkyo-ku, Tokyo 112, Japan Department of Physics, University of Tsukuba, Sakuramura, Ibaraki 300-31, Japan a Department of Applied Physics, Tokyo University of Education, 3-29-1 Otsuka, Bunkyo-ku, Tokyo 112, Japan b Department of Physics, University of Tsukuba, Sakuramura, Ibaraki 300-31, Japan |