1. Professor, Dept. of Applied Physics, Univ. of Granada, Campus Fuentenueva, 18071 Granada, Spain.
2. Professor, Dept. of Structural Mechanics and Industrial Constructions, Polytechnic Univ. of Madrid, C/José Gutierrez Abascal 2, 28006 Madrid, Spain.
3. Ph.D. Candidate, Dept. of Applied Physics, Univ. of Granada, Campus Fuentenueva, 18071 Granada, Spain (corresponding author).