1. Departamento de Física, Universidade Federal do Rio Grande do Norte, Natal, RN 59078-970, Brazil
2. Applied Physics Department, University of Campinas, Campinas, SP 13083-970, Brazil
3. Departamento de Física, Universidade Federal de Pernambuco, Recife, PE 50670-901, Brazil
4. School of Physics, Trinity College Dublin, Dublin 2, Ireland
5. Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland
6. Departamento de Física, Universidade Federeal do Paraná, Curitiba, PR 81531-980, Brazil