1. Department of Electrical and Electronic Engineering, Miyazaki University, 1‐1 Gakuen Kibanadai‐nishi, Miyazaki 889‐2192, Japan
2. Istituto per lo Studio di Nuovi Materali per l'Elettronica (IME) del CNR, Via Arnesano, I‐73100 Lecce, Italy
3. Department of Electrical and Electronic Engineering, Tottori University, 4‐101 Koyama‐cho minami, Tottori 680‐8552, Japan
4. Department of Applied Physics, Okayama University of Science, 1‐1 Ridai‐cho, Okayama 700‐0005, Japan
5. INFM, and Dipartimento di Ingegneria dell'Innovazione, Università di Lecce, Via Arnesano, I‐73100 Lecce, Italy