Probing Quantum Turbulence in ^{4}He by Quantum Evaporation Measurements

Density functional theory of superfluid helium at finite temperatures

A density functional theory-based method is developed to describe the static and dynamic response of superfluid helium at finite temperatures. The model relies on the well-established 0 K Orsay-Trento functional, which has been extensively used to study the response of bulk superfluid helium as well as superfluid helium droplets. By including a phenomenological stochastic term in this model, it is possible to obtain thermodynamic equilibrium corresponding to a given temperature by propagating the system in imaginary time. The temperature dependence of thermodynamic quantities, such as the internal energy and entropy, is computed and is compared well with experimental reference data for the bulk liquid up to about 2 K, but begins to gradually deviate above that temperature. Inspection of pseudovorticity during real-time evolution of the system near 2 K reveals the presence of roton quasiparticles, which are suggested to be precursors for quantized vortex rings (Onsager's ghosts), as well as weaker analogs of extended vortex loops.

Reconnection scaling in quantum fluids

Superfluid helium exhibits topological defects in the form of line-like objects called quantum vortices. Reconnection occurs when two vortices collide and recoil by exchanging tails. We observe such a reconnection for nearly isolated conditions and find that the intervortex separation for a certain range scales closely as the square root of the time after reconnection and that the prefactor in the square-root law shows an analytical dependence on the reconnection angle. Reconnection is important because it provides a mechanism for energy dissipation which otherwise does not occur in the zero-temperature limit. The kinematics of reconnections are similar in systems of classical vortices, cosmic strings, magnetic flux tubes in plasmas, liquid crystals, and even DNA.

Fundamental to classical and quantum vortices, superconductors, magnetic flux tubes, liquid crystals, cosmic strings, and DNA is the phenomenon of reconnection of line-like singularities. We visualize reconnection of quantum vortices in superfluid ⁴He, using submicrometer frozen air tracers. Compared with previous work, the fluid was almost at rest, leading to fewer, straighter, and slower-moving vortices. For distances that are large compared with vortex diameter but small compared with those from other nonparticipating vortices and solid boundaries (called here the intermediate asymptotic region), we find a robust 1/2-power scaling of the intervortex separation with time and characterize the influence of the intervortex angle on the evolution of the recoiling vortices. The agreement of the experimental data with the analytical and numerical models suggests that the dynamics of reconnection of long straight vortices can be described by self-similar solutions of the local induction approximation or Biot–Savart equations. Reconnection dynamics for straight vortices in the intermediate asymptotic region are substantially different from those in a vortex tangle or on distances of the order of the vortex diameter.