Submitted on: 28 May 2012.
Revised on: 11 August 2012.
Accepted on: 24 September 2012.
___________________Chalcogenide Letters---VIRTUAL CO PHYSICS SRL
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Submitted on: 05 January 2002.
Revised on: 28 April 2002.
Accepted on: 28 May 2002.
___________________Chalcogenide Letters---VIRTUAL CO PHYSICS SRL
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