1. Department of Physics
and Astronomy, West Virginia University, Morgantown, West Virginia 26506, United States
2. Physique Théorique des Matériaux, Université de Liège, B-4000 Sart-Tilman, Belgium
3. Centro de Investigación y Estudios Avanzados del IPN, MX-76230 Querétaro, México
4. Department of Material Science and Engineering, University of Wisconsin—Madison, Madison, Wisconsin 53706, United States