1. Department of Energy Science, Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
2. Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, VA 23529, USA
3. School of Mechanical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea
4. Department of Physics, Yeungnam University, Gyeongsan 712-749, Republic of Korea
5. Department of Physics, Jeju National University, Jeju 690-756, Republic of Korea