QFS2016 Book of Abstracts

Abstracts

O4.8 Magnus force on vortex in superfluids without Galilean invariance: BEC in optical lattice Sonin, E.B. Racah Institute of Physics, Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel Broken Galilean invariance suppresses the transverse Magnus force on a vortex as demonstrated by well-known examples of superfluids without Galilean invariance: the Josephson-junction array and impure superconductors. Another example of a superfluid without Galilean invariance is BEC in optical lattices, where up to now vortex dynamics is in early stages of investigation and usually uses the Bose-Hubbard model. This report addresses the general case of a superfluid in a periodic potential based on the Bloch band theory and the momentum balance. The continuous approximation restores translational invariance broken by the periodic potential, but the theory is not Galilean invariant. Noether’s theorem provides the conservation law for quasimomentum, while the balance for true momentum is derived from the equations of motion. The calculations of the transverse forces on the vortex, the Magnus and the Lorentz forces, require in general the analysis of the balance of true momentum. While the developed theory yields the same Lorentz force, which was known before, a new expression for the Magnus force was obtained. The theory demonstrates how a small Magnus force emerges if the particle-hole symmetry is broken. The theory was applied to the Bose-Hubbard model close to the phase transition “superfluid-Mott insulator”. There is an area in the phase diagram, where the Magnus force has an inverse sign with respect to that expected from the sign of velocity circulation. The present theory predicts the Magnus force much weaker than obtained in previous estimations, which did not check momentum balance. O4.9 Vortex reconnections and rebounds in trapped Bose Einstein condensates L. Galantucci(1), S. Serafini(2), F. Dalfovo(2), G. Lamporesi(2), G. Ferrari(2), C. F. Barenghi(1) (1) Joint Quantum Centre and School of Mathematics and Statistics, Newcastle University (2) INO-CNR BEC Center and Dipartimento di Fisica, Universita‘ di Trento We perform a numerical study of two-vortex interactions in trapped elongated Bose-Einstein condensates in the T=0 limit. We observe different vortex interactions regimes depending on the vortex orientations and their relative velocity: unperturbed orbiting, bounce dynamics, single and double reconnection events. The key ingredients driving the dynamics are the anti-parallel preferred alignment of the vortices and the impact of density gradients. The results are confirmed by ongoing experiments in Trento performed employing a real-time imaging technique [1]. [1] Serafini et al., Phys. Rev. Lett., 115, 170402 (2015)

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