Laboratory;
of;
Solid;
State;
Microstructures;
and;
College;
of;
Engineering;
and;
Applied;
Sciences,;
Nanjing;
University,;
Nanjing;
210093,;
China;
of;
Physics,;
University;
of;
Arkansas,;
Fayetteville,;
Arkansas;
72701,;
USA;