PHYSICAL REVIEW LETTERS, vol. 70, no. 23, page. 3663 - 3666, 1993-06-07
PHYSICAL REVIEW LETTERS, vol. 72, no. 5, page. 794 - 794, 1994-01-31
PHYSICAL REVIEW B, vol. 51, no. 13, page. 8140 - 8151, 1995-04-01