1. Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, U.S.A.
2. Departments of Physics and Electrical Engineering, University of Colorado Denver, Denver, CO 80217, U.S.A.
3. National Laboratory for Superconductivity, Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, P.R. China