1. Department of Physics, Harvard University, 17 Oxford St., Cambridge, Massachusetts 02138, USA
2. P. N. Lebedev Physical Institute of the RAS, Moscow 119991, Russia
3. GREMAN, UMR CNRS-7347, University F. Rabelais, 37200 Tours, France
4. L.F. Vereshchagin Institute for High Pressure Physics, RAS, Troitsk, Moscow 108840, Russia
5. Institute for Quantum Optics and Integrated Quantum Science and Technology (IQst), Ulm University, Albert-Einstein-Allee 11, D-89081 Ulm, Germany