[1] Univ Nebraska, Dept Phys & Astron, Lincoln, NE 68588 USA
[2] Univ Nebraska, Nebraska Ctr Mat & Nanosci, Lincoln, NE 68588 USA
[3] Lawrence Berkeley Natl Lab, Mat Sci Div, Berkeley, CA 94720 USA
[4] UC Santa Cruz, Phys Dept, Santa Cruz, CA 95064 USA