1. Department of Electrical and Computer Engineering, University of Thessaly, 38333 Volos, Greece
2. Department of Materials, Imperial College London, London SW7 2BP, UK
3. B. I. Verkin Institute for Low Temperature Physics and Engineering of National Academy of Science of Ukraine, 47 Nauki Ave., 61103 Kharkiv, Ukraine
4. Department of Physics, V. N. Karazin Kharkiv National University, 4 Svobody Sq., 61077 Kharkiv, Ukraine