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Submitted on: 12 October 2009.
Revised on: 18 November 2009.
Accepted on: 22 December 2009.
___________________INDIAN JOURNAL OF PURE & APPLIED PHYSICS
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Submitted on: 20 August 1998.
Revised on: 05 December 1998.
Accepted on: 03 January 1999.
___________________INDIAN JOURNAL OF PURE & APPLIED PHYSICS
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