[1] SLAC Natl Accelerator Lab, Stanford Inst Mat & Energy Sci, Menlo Pk, CA 94025 USA
[2] Stanford Univ, Dept Phys, Geballe Lab Adv Mat, Stanford, CA 94305 USA
[3] Stanford Univ, Dept Appl Phys, Stanford, CA 94305 USA
[4] Univ Calif Berkeley, Lawrence Berkeley Lab, Berkeley, CA 94720 USA
[5] Chinese Acad Sci, Beijing Natl Lab Condensed Matter Phys, Beijing 100190, Peoples R China
[6] Chinese Acad Sci, Inst Phys, Beijing 100190, Peoples R China