1. Institute of Nano-Physical Engineering, Kim Chaek University of Technology, Pyongyang, D. P. R. Korea
2. Faculty of Mechanics, Kim II Sung University, Pyongyang, D. P. R. Korea
3. Faculty of Physics Science, Kim Chaek University of Technology, Pyongyang, D. P. R. Korea
4. Department of Engineering Machine, Pyongyang University of Mechanical Engineering, Pyongyang, D. P. R. Korea