[1] Stanford Univ, Dept Appl Phys, Geballe Lab Adv Mat, Stanford, CA 94305 USA
[2] Stanford Inst Mat & Energy Sci, SLAC Natl Accelerator Lab, Menlo Pk, CA 94025 USA
[3] Univ Tokyo, Dept Appl Phys, Bunkyo Ku, Tokyo 1138656, Japan
[4] Stanford Univ, Dept Phys, Stanford, CA 94305 USA
[5] Univ Bristol, Dept Phys, Bristol BS8 1TL, Avon, England