1. Department of Physics, Federal University of Rio Grande do Norte, 59078-970 Natal, RN, Brazil
2. Department of Physics, State University of Rio Grande do Norte, 59610-210 Mossoró, RN, Brazil
3. Department of Science and Technology, State University of Rio Grande do Norte, 59610-210 Mossoró, RN, Brazil
4. Department of Science and Technology, Federal Rural University of the Semiarid Region, 59515-000 Angicos, RN, Brazil