1. Faculty of Physics, Babeş-Bolyai University, 400084 Cluj-Napoca, Romania
2. Chair in Hybrid Nanosystems, Nanoinstitute Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, 80539 Munich, Germany
3. State Key Laboratory of Powder Metallurgy, School of Physics and Electronics, Central South University, 410083 Changsha, China