Submitted on: 01 January 2007.
Revised on: 03 February 2007.
Accepted on: 03 March 2007.
___________________CURRENT APPLIED PHYSICS---ELSEVIER
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Submitted on: 03 October 2007.
Revised on: 15 December 2007.
Accepted on: 16 January 2008.
___________________CURRENT APPLIED PHYSICS---ELSEVIER
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Submitted on: 20 January 2007.
Revised on: 26 April 2007.
Accepted on: 15 May 2007.
___________________CURRENT APPLIED PHYSICS---ELSEVIER
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