QFS2016 Book of Abstracts

Abstracts

O3.9 Multi-particle excitations in superfluid 4 He investigated as a function of pressure Beauvois K.(1, 2, 3), Campbell C. E.(4), Dawidowski J.(5), F˚ak B.(1, 6, 7), Godfrin H.(2, 3), Krotscheck E.(8, 9), Lauter H.-J.(10), Lichtenegger T.(8, 9), Ollivier J.(1), Sultan A.(2, 3, 7) 1) Institut Laue-Langevin, 6, rue Jules Horowitz, 38042 Grenoble, France 2) Univ. Grenoble Alpes, Inst NEEL, F-38000 Grenoble, France 3) CNRS, Inst NEEL, F-38042 Grenoble, France 4) School of Physics and Astronomy, Univ. of Minnesota, Minneapolis MN 55455, USA 5) Comisi´on Nacional de Energ´ıa At´omica and CONICET, Argentina 6) Univ. Grenoble Alpes, INAC-SPSMS, F-38000 Grenoble, France 7) CEA, INAC-SPSMS, F-38000 Grenoble, France 8) Department of Physics, University at Buffalo, SUNY Buffalo NY 14260, USA 9) Institute for Theoretical Physics, Johannes Kepler University, A4040 Linz, Austria 10) Spallation Neutron Source, ORNL, Oak Ridge, Tennessee 37831, USA The dynamic structure function S(q,E) of superfluid 4 He has been measured at very low temperatures for pressures up to 10 bars, in the multi-excitation regime. The neutron scattering measurements were performed at the Institut Laue-Langevin on the time-of-flight spectrometer IN5. Our measurements at saturated vapour pressure [1] display a very rich landscape of multi-excitations, including a “ghost phonon” and other remarkable features which are found to depend on pressure in agreement with the Dynamic Many-Body theory [2]. [1] K. Beauvois et al., arXiv:1605.02638v1, to be published. [2] C. E. Campbell, E. Krotscheck, and T. Lichtenegger, Phys. Rev. B 91, 184510 (2015). O3.10 Correlations in the low-density Fermi gas:Fermi-Liquid state, Dimerization, and BCS Pairing Fan, H.-H.(1, 2), Krotscheck E.(1, 2), Zillich, R. E.(2) (1) Department of Physics, University at Buffalo, SUNY Buffalo NY 14260, USA (2) Institute for Theoretical Physics, Johannes Kepler University, A4040 Linz, Austria We present ground state calculations for low-density Fermi gases the optimized Fermi-Hypernetted Chain integral equation method which provides, in the density regimes of interest, an accuracy better than one percent. As a function of density and/or coupling strength we encounter an instability of the normal state of the system which is characterized by a divergence of the scattering length indicating phonon-exchange driven dimerization. We then study, in the stable regime, the superfluid gap and its dependence on the density and the interaction strength. The most important finite-range corrections are a direct manifestation of the many-body nature of the system.

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