Affiliation: | a National Institute of Materials Physics, P.O. Box MG-7, Magurele, RO-76900, Bucharest, Romania b National Institute of Lasers, Plasma and Radiation Physics, P.O. Box MG-26, Magurele, RO-76900, Bucharest, Romania c Aristotle University of Thessaloniki, Department of Physics, GR-54124 Thessaloniki, Greece |