1. Department of Physics, University of South Florida, Tampa, Florida 33620, United States
2. Departamento de Física, Universidad de Oviedo, Oviedo E-33007, Spain
3. BCMaterials Edificio No. 500, Parque Tecnológico de Vizcaya, Derio 48160, Spain
4. Servicios Científico-Técnicos, Universidad de Oviedo, Oviedo E-33006, Spain
5. Department
of Electricity and Electronics, University of Basque Country (UPV/EHU), Leoia 48940, Spain
6. Department of
Applied Physics II, University of Basque Country (UPV/EHU), Leoia 48940, Spain
7. Departamento de Física & IUTA, EPI, Universidad de Oviedo, Gijón E-33203, Spain