1. Key Laboratory for Physical Electronics and Devices, Ministry of Education, School of Electronic Science and Engineering , Xi’an Jiaotong University , Xi’an 710049 , China
2. Institute of Spectroscopy , Russian Academy of Sciences , Troitsk , Moscow , 108840 , Russia
3. Departamento de Física and Centro de Física Teórica e Computacional, Faculdade de Ciências , Universidade de Lisboa , Campo Grande, Ed. C8 , Lisboa 1749-016 , Portugal