National Laboratory for Superconductivity;
Institute of Physics and National Laboratory for Condensed Matter Physics;
Chinese Academy of Sciences;
PO Box 603;
Beijing 100190;
Department of Physics;
The Pennsylvania State University;
University Park;
Pennsylvania 16802;
USA;
Department of Physics;
Peking University;
Beijing 100871;
Department of Materials Science and Engineering;
The Pennsylvania State University;
University Park;
Pennsylvania 16802;
USA;