[1] Univ Basque Country, Dept Elect & Elect, Leioa 48940, Spain
[2] Univ Oviedo, Dept Fis, E-33007 Oviedo, Spain
[3] Inst Postosina Invest Cient & Tecnol AC, Div Mat Avanzados, San Luis Potosi 78216, Mexico
[4] Univ Oviedo, EPI, Dept Fis, Gijon 33203, Spain