[1] MIT, Dept Phys, Cambridge, MA 02139 USA
[2] Univ Texas Austin, Dept Phys, Austin, TX 78712 USA
[3] Cornell Univ, Dept Phys, Ithaca, NY 14850 USA
[4] Cornell Univ, Sch Appl & Engn Phys, Ithaca, NY 14850 USA
[5] Univ Washington, Dept Phys, Seattle, WA 98185 USA
[6] Univ Washington, Dept Mat Sci & Engn, Seattle, WA 98195 USA