1. Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221-0011
2. Department of Physics, Miami University, Oxford, Ohio 45056
3. Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia
4. Materials Engineering and Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia
5. Solid State Physics and the Nanometer Structure Consortium, P.O. Box 118, SE-221 00, Lund University, Lund, Sweden
6. Optical Science and Technology Center and the Department of Physics, University of Iowa, Iowa City, 54242