1. Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, United States
2. Department of Engineering Physics, University of Wisconsin, Madison, Wisconsin 53706, United States
3. Lawrence Livermore National Laboratory, Livermore, California 94551, United States
4. Wigner Research Centre for Physics, Institute for Solid State Physics and Optics, PO. Box
49, H-1525 Budapest, Hungary
5. Department of Atomic Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rakpart 3., H-1111 Budapest, Hungary
6. Instituto de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago, Chile