1. Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, Behlen Laboratory of Physics, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588-0111, Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, North Dakota, 58102, and Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Highway, Baton Rouge, Louisiana 70806