1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA
2. Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
3. Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
4. Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA