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PHYSICAL REVIEW B, vol. 37, no. 17, page. 10444 - 10447, 1988-06-15
PHYSICAL REVIEW A - ATOMIC, MOLECULAR, AND OPTICAL PHYSICS, vol. 37, no. 9, page. 3465 - 3468, 1988-03
PHYSICAL REVIEW B, vol. 35, no. 10, page. 5372 - 5374, 1987-04-01