PHYSICAL REVIEW B, vol. 61, no. 12, page. 8250 - 8255, 2000-03-15
PHYSICAL REVIEW LETTERS, vol. 84, no. 8, page. 1724 - 1727, 2000-02-21
PHYSICAL REVIEW LETTERS, vol. 82, no. 5, page. 968 - 971, 1999-02-01