[1] Univ Sci & Technol China, Dept Phys, Hefei 230026, Anhui, Peoples R China
[2] Inst Nucl Res ATOMK, POB 51, Debrecen, Hungary
[3] Inst Mat, Natl Key Lab Surface Phys & Chem, POB 9071, Jiangyou 621907, Sichuan, Peoples R China
[4] Univ Sci & Technol China, Hefei Natl Res Ctr Phys Sci Microscale, Hefei 230026, Anhui, Peoples R China