PHYSICAL REVIEW B, vol. 57, no. 4, page. R2053 - R2056, 1998-01-15
PHYSICAL REVIEW B, vol. 55, no. 11, page. 7047 - 7051, 1997-03-15
SURFACE SCIENCE, vol. 337, no. 1-2, page. L789 - L794, 1995-08-20
PHYSICAL REVIEW B, vol. 48, no. 7, page. 4755 - 4759, 1993-08-15