1. Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education & Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University 1 , Changsha 410082, China
2. Department of Physics, University of Guilan 2 , P.O. Box 41335-1914, Rasht, Iran
3. School of Physics, Institute for Research in Fundamental Sciences (IPM) 3 , P. O. Box 19395-5531, Tehran, Iran