报告题目：Largerange ordering and Onsager condensation of topological excitations in a twodimensional 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 longrange ordering and phase coherence is understood to arise from the BerezinskiKosterlessThouless transition that is associated with the binding of vortex and antivortex pairs. This transition permits superfluid flows to emerge in such systems that can support ensembles of a large number of vortices and antivortices with a characteristic healing length that is small relative to the intervortex separation. Under such conditions, the emergent superfluid flow can exhibit quasiclassical 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 socalled 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 Bosegas 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 longrange correlations between vortices. It turns out that the order parameter corresponds to the streamfunction of the 2D flow field that is governed by a BoltzmannPoisson 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 nonzero coarsegrained vorticity field. Solutions of the BoltzmannPoisson equation in a square domain reveal a diverse family of possible solutions. These meanfield predictions are verified through direct simulations of a point vortex gas and 2D simulations of the GrossPitaevskii equation. Due to the longrange nature of the Coulomblike 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 nonthermal fixed points and the theory of Kraichnan for inverse energy cascades in 2D fluid turbulence.
报告人简介：

2009now Lecturer and Senior Lecturer: University of East Anglia

20072009 Postdoctoral Research Associate: University of Cambridge

20032007 PostdoctoralResearch Fellow: University of North Carolina at Chapel Hill

20022003 PostdoctoralResearch Associate: MIT

20012002 PostdoctoralResearch Associate: Brown University

19962001 PhD.Loughborough University, Aeronautical Engineering

19921996 MEng. Imperial College, Aeronautical Engineering