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Submitted on: 06 April 1998.
Revised on: 15 April 1998.
Accepted on: 23 April 1998.
PHYSICAL REVIEW B
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Submitted on: 17 October 1998.
Revised on: 12 November 1998.
Accepted on: 02 December 1998.
PHYSICAL REVIEW B---AMER PHYSICAL SOC