Abstract
In a two-dimensional (2D) classical fluid, a large-scale flow structure emerges out of turbulence, which is known as the inverse energy cascade where energy flows from small to large length scales. An interesting question is whether this phenomenon can occur in a superfluid, which is inviscid and irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly oblate geometry provide an experimental venue for studying 2D superfluid turbulence, but their full investigation has been hindered due to a lack of the circulation sign information of individual quantum vortices in a turbulent sample. Here, we demonstrate a vortex sign detection method by using Bragg scattering, and we investigate decaying turbulence in a highly oblate BEC at low temperatures, with our lowest being ~0.5Tc, where Tc is the superfluid critical temperature. We observe that weak spatial pairing between vortices and antivortices develops in the turbulent BEC, which corresponds to the vortex-dipole gas regime predicted for high dissipation. Our results provide a direct quantitative marker for the survey of various 2D turbulence regimes in the BEC system.
Highlights
Quantum turbulence (QT) is a state of chaotic flow in a superfluid
We examine the evolution of decaying 2D QT in a Bose-Einstein condensates (BECs) at low temperatures, with our lowest being ~0.5Tc, where Tc is the critical temperature of the trapped sample
Because of the Doppler effect, the scattering response is antisymmetric with respect to the Bragg beam axis [Fig. 2(b)]; the vortex sign can be determined from the position of the scattered atoms relative to the vortex core
Summary
Quantum turbulence (QT) is a state of chaotic flow in a superfluid. Because of its inviscidity and quantized circulation, QT constitutes a unique realm in turbulence research. It is well known that regarding the 2D turbulence of a classical hydrodynamic fluid, the kinetic energy flows toward large length scales, generating a large-scale flow structure due to small-scale forcing[7]. This phenomenon is qualitatively different from three-dimensional turbulence, where energy is dissipated at small length scales. In 2D, quantum vortices are topological point defects, and the turbulent superfluid can be depicted as a system of interacting ‘vortex’ particles This point-vortex picture was introduced by Onsager in his model, which presented a statistical description of classical 2D turbulence[24, 25]. The system eventually evolves toward a stationary ground state by decreasing both Ev and Nv via various dissipation mechanisms, such as sound radiation[27], mutual friction by coexisting thermal www.nature.com/scientificreports/
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