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PHYSICAL REVIEW B, vol. 61, no. 12, page. 8250 - 8255, 2000-03-15
PHYSICAL REVIEW B, vol. 61, no. 7, page. 4503 - 4506, 2000-02-15
PHYSICAL REVIEW B, vol. 61, no. 3, page. 1688 - 1691, 2000-01-15