[1] Okayama Univ Sci, Fac Sci, Dept Appl Phys, Ridaicho 1 1, Okayama 7000005, Japan
[2] Sichuan Univ, Coll Phys, Chengdu 610065, Peoples R China
[3] Asia Pacific Ctr Theoret Phys, Pohang 37673, South Korea
[4] Pohang Univ Sci & Technol, Dept Phys, Pohang 37673, South Korea