1. Departamento de Física, Universidade Federal do Piauí, Teresina, PI, Brazil
2. Department of Physics, Binghamton University, State University of New York, Binghamton, New York 13902, United States
3. Departamento de Física, Universidade Federal do Ceará, P.O. Box 6030, CEP 60455-900, Fortaleza, Ceará, Brazil
4. Centre for the Physics of Materials and Department of Physics, McGill University, Montreal, QC, Canada