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M. Ueta and N. Nagasawa: Proc. Oji Seminar on Physics of Highly Excited States in Solids, ed. M. Ueta and Y. Nishina, Lecture Notes in Physics 57 (Springer-Verlag, 1976) p. 1.
T. Itoh and T. Suzuki: J. Phys. Soc. Jpn. 45 (1978) 1939.