1. Institute of Nano-Physical Engineering, Kim Chaek University of Technology, Pyongyang 950003, North Korea
2. Department of Mechanical Engineering, Pyongyang University of Mechanical Engineering, Pyongyang 999093, North Korea
3. Department of Physics Science, Kim Chaek University of Technology, Pyongyang 950003, North Korea
4. Department of Mechanical Science, Kim Chaek University of Technology, Pyongyang 950003, North Korea