PHYSICAL REVIEW A, vol. 36, no. 3, page. 1387 - 1395, 1987-08-01
PHYSICAL REVIEW B, vol. 36, no. 7, page. 3638 - 3650, 1987-09-01
PHYSICAL REVIEW LETTERS, vol. 55, no. 19, page. 2051 - 2054, 1985-01
PHYSICAL REVIEW LETTERS, vol. 56, no. 26, page. 2880 - 2880, 1986-06-30