1. Department of Physics and Astronomy, Brigham Young University, Provo, Utah 84602, USA
2. Department of Physics Applied to Electronics, Universidad de Puerto Rico-Humacao, Humacao, Puerto Rico 00792, USA
3. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany
4. Institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany