[1] Nanjing Univ, Natl Lab Solid State Microstruct, Nanjing 210093, Peoples R China
[2] Nanjing Univ, Dept Phys, Nanjing 210093, Peoples R China
[3] Guangxi Univ, Dept Phys, Nanning 530004, Peoples R China
[4] Univ N Carolina, Dept Phys & Astron, Chapel Hill, NC 27599 USA