1. a Department of Physics and Astronomy , Clemson University , Clemson , South Carolina , 29631 , U.S.A.
2. b Department of Theoretical Physics , The University , Newcastle upon Tyne , NE1 7RU , England
3. c Department of Mechanics , College of Engineering, State University of New York at Stony Brook , Stony Brook , New York , 11790 , U.S.A.