报告题目：Large-range ordering and Onsager condensation of topological excitations in a two-dimensional superfluid far from equilibrium

报  告  人：Dr. Hayder Salman, University of East Anglia

报告时间：2019年4月9日（周二）下午15:30—17:00

报告地点：会议中心203室

报告摘要：

In 2D atomic BECs, quasi long-range ordering and phase coherence is understood to arise from the Berezinski-Kosterless-Thouless transition that is associated with the binding of vortex and anti-vortex pairs. This transition permits superfluid flows to emerge in such systems that can support ensembles of a large number of vortices and anti-vortices with a characteristic healing length that is small relative to the inter-vortex separation. Under such conditions, the emergent superfluid flow can exhibit quasi-classical hydrodynamic characteristics. In particular, like signed vortices can begin to cluster giving rise to a large scale coherent flow composed of many vortices within each cluster. This behaviour can be attributed to an emergent inverse energy cascade that leads to a so-called Onsager condensate. Therefore, in contrast to an inverse particle flux that leads to the emergence of a coherent matter wave, the 2D superfluid flow supports an inverse energy cascade that leads to spectral condensation of energy within the system.

We will report on numerical studies of the relaxation of a 2D ultracold Bose-gas from a nonequilibrium initial state consisting of vortices and antivortices in experimentally realizable square and rectangular traps. We demonstrate how vortex clustering that arises can be understood in terms of negative temperature states of a vortex gas and we show that, within the negative temperature regime, an order parameter emerges that is related to the formation of long-range correlations between vortices. It turns out that the order parameter corresponds to the streamfunction of the 2D flow field that is governed by a Boltzmann-Poisson equation [3]. This equation describes the emergent coherent flow associated with the spectral condensation of energy in 2D superfluid flows and is associated with the emergence of a mean rotational hydrodynamic flow with a non-zero coarse-grained vorticity field. Solutions of the Boltzmann-Poisson equation in a square domain reveal a diverse family of possible solutions. These mean-field predictions are verified through direct simulations of a point vortex gas and 2D simulations of the Gross-Pitaevskii equation. Due to the long-range nature of the Coulomb-like interactions in point vortex flows, the negative temperature states strongly depend on the shape of the geometry. As a further extension of these results, we analyse the spectra of the flow in the vortex clustered regime and relate these to the theory of non-thermal fixed points and the theory of Kraichnan for inverse energy cascades in 2D fluid turbulence.

报告人简介：

• 2009-now Lecturer and Senior Lecturer: University of East Anglia 

• 2007-2009 Postdoctoral Research  Associate: University of Cambridge

• 2003-2007 PostdoctoralResearch Fellow: University of North Carolina at Chapel Hill 

• 2002-2003 PostdoctoralResearch Associate: MIT

• 2001-2002 PostdoctoralResearch Associate: Brown University

• 1996-2001 PhD.Loughborough University, Aeronautical Engineering

• 1992-1996 MEng. Imperial College, Aeronautical Engineering