L. Liszkay1, C. Corbel1, P. Perez1, P. Desgardin2, M.-F. Barthe2, T. Ohdaira3, R. Suzuki3, P. Crivelli4, U. Gendotti4, A. Rubbia4, M. Etienne5, and A. Walcarius5
1DSM/IRFU and IRAMIS, CEA Saclay F-91191 Gif-sur-Yvette Cedex, France
2CNRS-CERI, 3A Rue de la Férollerie, F-45071 Orléans Cedex 2, France
3AIST, Tsukuba, Ibaraki 305-8568, Japan
4Institut für Teilchenphysik, ETHZ, CH-8093 Zürich, Switzerland
5LCPME, CNRS-Nancy-Université, 405 Rue de Vandoeuvre, F-54600 Villers-lès-Nancy, France
The positronium (Ps) is a bound state between an electron and its antiparticle, the positron. Producing clouds of positronium atoms in vacuum is a first condition to achieve new types of experiments in fundamental physics of gravity and antimatter. It also offers significant interest as a probe of porous materials at the nanometric scale. A unique collaboration involving among other, physicists from IRFU and IRAMIS at CEA-Saclay has been able to put the positronium production to a record level in stable and controlled conditions . This is an important step for the program to test the gravity of antimatter.
G. Lambert1,2,3, T. Hara2,4, D. Garzella1, T. Tanikawa2, M. Labat1,3, B. Carre1, H. Kitamura2,4, T. Shintake2,4, M. Bougeard1, S. Inoue4, Y. Tanaka2,4, P. Salieres1, H. Merdji1, O. Chubar3, O. Gobert1, K. Tahara2, M.-E. Couprie3
1Service des Photons, Atomes et Molécules, DSM/DRECAM, CEA-Saclay, 91191 Gif-sur-Yvette, France
2RIKEN SPring-8 Centre, Harima Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
3Groupe Magnétisme et Insertion, Synchrotron Soleil, L'Orme des Merisiers, Saint Aubin, 91192 Gif-sur-Yvette, France
4XFEL Project Head Office/RIKEN, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
1 Institut de Physique de Rennes, CNRS UMR 6251, Univ. Rennes 1, 35042 Rennes, France
2 CNRS, UMR 6251, IPR, 263 Avenue du Général Leclerc, 35042 Rennes Cedex, France.
3 Laboratoire Léon Brillouin, CEA-CNRS, CEA Saclay, 91191 Gif-sur-Yvette, France
4 Institut Laue-Langevin, 38042 Grenoble Cedex 9, France.
5 Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA.
6 Facultad de Ciencias, Universidad del Pais Vasco, Apdo 644, Bilbao, Spain.
One way to probe the structure of matter at the atomic scale is to use radiation diffraction (such as X-rays, or the wave associated with particles: neutrons, electrons ...). The presence of a long-distance order is then characterized by the presence of diffraction peaks, forming an image reflecting the symmetry of the object. This led to the discovery in the recent decades of non-periodic materials, but still with long range order as evidenced by discrete peaks in their diffraction spectrum. Physicists represent this type of crystals as periodic crystals but in a super-space (with dimension 3 + d, which represent the 3 usual dimensions of space and the dimension d of the internal space).
The systems understudy are aperiodic supramolecular model systems consisting of an urea single crystal (host structure) and alkane molecules (invited molecules), whose length is determined by the number nC of carbon atoms in the molecule (nC>7). Crystalline plane of urea have a structure of hexagonal symmetry. Along the C perpendicular axis, the structure exhibits a double helix forming channels in which alkane molecules may be inserted. At room temperature, these molecules are ordered along the C axis of the urea lattice, but according to an aperiodic order.
Neutron diffraction spectrum along the C* axis. Crossing the first critical temperature Tc1 superstructure rods (h = 1/2) along C* appear that indicates a change in the symmetry of the system. Along the rods, incommensurate discrete diffraction peaks are characteristic of the new aperiodic ordered structure. This mesh doubling is completely original because the observed phase transition between the two incommensurate phases can be only properly described by means of the 4-dimensional space. A second structural phase appears below Tc2.