1. Departamento de Física, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil
2. Escola de Engenharia Civil, Faculdade Meridional, 99070-220, Passo Fundo, RS, Brazil
3. Condensed Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University, S-75120 Uppsala, Sweden
4. Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden