PHYSICAL REVIEW LETTERS, vol. 82, no. 5, page. 968 - 971, 1999-02-01
PHYSICAL REVIEW B, vol. 57, no. 3, page. 1352 - 1355, 1998-01-15
PHYSICAL REVIEW B, vol. 55, no. 23, page. 15464 - 15466, 1997-06-15