1. Physics Department, Federal University of Paraíba, Caixa Postal 5008, 58059-900, João Pessoa, Paraíba, Brazil
2. Department of Chemistry and Physics, Federal University of Paraíba, Rodovia BR 079 - Km 12, 58397-000 Areia-PB, Brazil
3. Physics Department, Federal University of Lavras, Caixa Postal 3037, 37200-000 Lavras-MG, Brazil