PHYSICAL REVIEW B, vol. 56, no. 8, page. 4825 - 4829, 1997-08-15
PHYSICS LETTERS A, vol. 226, no. 1-2, page. 17 - 21, 1997-02-10
MODERN PHYSICS LETTERS B, vol. 10, no. 9, page. 401 - 407, 1996-04-20