[1] Korea Res Inst Stand & Sci, Taejon 305600, South Korea
[2] Korea Univ, Dept Appl Phys, Chungnam 339800, South Korea
[3] Natl Nanofab Ctr, Taejon 305806, South Korea
[4] Korea Basic Sci Inst, Quantum Mat Res Team, Taejon 305333, South Korea
[5] Pohang Univ Sci & Technol, Dept Phys, Pohang 790784, South Korea
[6] Chonnam Natl Univ, Dept Phys, Kwangju 500757, South Korea
[7] Korea Univ, Sch Elect Engn, Seoul 136713, South Korea