PHYSICAL REVIEW B, vol. 47, no. 21, page. 14020 - 14022, 1993-06-01
JOURNAL OF THE KOREAN PHYSICAL SOCIETY, vol. 26, no. 1, page. 64 - 68, 1993-02
PHYSICAL REVIEW B, vol. 47, no. 6, page. 2979 - 2982, 1993-02-01