1. Department of Applied Physics and Solid State Laboratory, Stanford University, Stanford, CA 94305, and Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309-0210, USA.
2. Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309-0210, and Solid State Laboratory, Stanford University, Stanford, CA 94305, USA.
3. Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309-0210, USA.
4. Physics Department, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.