1. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
2. Institut Jean Lamour, UMR CNRS 7098, Université de Lorraine, 54011 Nancy Cedex, France
3. Departamento de Física, Universidade Federal de Viçosa, 36570-900 Viçosa, Minas Gerais, Brazil
4. Department of Mathematics, Iowa State University, Ames, Iowa 50011, USA
5. Ames Laboratory—USDOE, Iowa State University, Ames, Iowa 50011, USA