- Scholarpedia 5(9):8786 (2010)
- Physical Review Letters 75(19):3386+ (November 1995)
- Physical Review Letters 85(3):499+ (July 2000)
- (February 2009)
- (2009)cite arxiv:0902.0427 Comment: 11 pages .
- (2008)cite arxiv:0811.2381 Comment: 38 pages, 6 figures. v2: typos in eqns 2.14 and 2.20 corrected, improved section 8 and other minor changes .


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