We have studied the interaction of metastable 4 He à 2 excimer molecules with quantized vortices in superfluid 4 He in the zero temperature limit. The vortices were generated by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96 AE 6 nm at a pressure of 0.1 bar and 27 AE 5 nm at 5.0 bar. We have also demonstrated that a moving tangle of vortices can carry the molecules through the superfluid helium. DOI: 10.1103/PhysRevLett.110.175303 PACS numbers: 67.25.dk, 47.27.Ài, 47.80.Jk Turbulence, the complex dynamics of systems with many degrees of freedom on a broad range of length scales, is common in nature. Its understanding is important for both fundamental science and technology. A special case is the hydrodynamics of superfluid liquids in the limit of zero temperature [1], which, while behaving as an ideal fluid, has a quantum constraint: vorticity is concentrated along the filamentary cores of quantized vortex lines, and the velocity circulation around any such line is equal to ¼ h=m 4 ¼ 1:00  10 À3 cm 2 s À1 (where h is the Planck constant and m 4 is the mass of a 4 He atom). Turbulence in such a system, known as quantum turbulence (QT), is a dynamic tangle of vortex lines.The characterization of classical turbulence is a formidable task: all the velocity field has to be visualized at once, and the most important regions are those of enhanced vorticity. Usually small passive tracers of flow are used [2]. The case for visualization of QT is different. To begin with, velocity tracers cannot be used as they are not entrained by the superfluid. On the other hand, small particles are attracted to the cores of quantized vortices, in which they can be trapped and then traced by optical means. This opens up an entirely new avenue for the visualization of turbulence. Mapping the field of vorticity, not velocity, has advantages for both the classical and quantum ranges of the QT spectrum. Within the former (coarse-grained flow on length scales greater than the mean separation between vortex lines), the regions of enhanced vorticity, i.e., those with an enhanced density of vortex lines, will be most visible. Within the latter (small length scales that resolve discrete vortex lines), one will be able to observe such processes as vortex reconnections, Kelvin waves, and the emission and absorption of small vortex loops-which are believed to be responsible for the quantum cascade of energy and control the dissipation of the vortex tangle [3,4].Micron-sized particles of solid hydrogen have already been used to tag vortex cores at high temperatures, T $ 2 K [5,6]; however, the invasive means of introduction and relatively large particle size preclude implementation of this technique for low temperatures and small length scales. Potentially ideal tracers would be metastable molecules He à 2 in the spin triplet state, which have a relatively long lifetime of ð13 AE 2Þ s [7]. They can be created in situ either by ionization in a strong laser field [8] or after rec...

Turbulence in a superfluid in the zero-temperature limit consists of a dynamic tangle of quantized vortex filaments. Different types of turbulence are possible depending on the level of correlations in the orientation of vortex lines. We provide an overview of turbulence in superfluid 4 He with a particular focus on recent experiments probing the decay of turbulence in the zero-temperature regime below 0.5 K. We describe extensive measurements of the vortex line density during the free decay of different types of turbulence: ultraquantum and quasiclassical turbulence in both stationary and rotating containers. The observed decays and the effective dissipation as a function of temperature are compared with theoretical models and numerical simulations.Superfluid helium is an ordered fluid with truly zero viscosity. Its local velocity is v s ðr; tÞ = Z m4 ∇θ, where θðr; tÞ is the macroscopically coherent phase of the complex order parameter (here we refer to the common isotope 4 He with atomic mass m 4 ). As a result, the velocity circulation around any closed contour through the superfluid is always quantized in units of κ ≡ h=m 4 . This allows stable topological defects-quantized vortex lines, i.e., vortices with precisely one quantum of circulation around their filamentary cores with a thickness of just a 0 ∼ 1 Å (inside which the order parameter is suppressed) (1, 2). Such a line can bend, move, and reconnect with another line when they come very close to each other. An array of vortex lines appears naturally as the equilibrium state of a superfluid rotating with the container (3-5). However, vortices can also form nonequilibrium tangles, known as "quantum turbulence" (QT) (6), that possesses excess energy. As in any nonequilibrium system, the presence and structure of the vortex tangle is a result of the history of generation of vortices and flow in the liquid.In contrast to vortex lines, particle-like excitations (phonons and rotons) are in thermal equilibrium, so their density increases with temperature. These scatter off the cores of vortex lines exerting the mutual friction force on the superfluid. At temperatures below about 0.5 K, the excitations are too depleted to affect vortex dynamics and can be neglected. Between about 0.5 and 0.7 K, they are nearly ballistic and can be treated as a small dissipative perturbation that damps Kelvin waves (helical perturbations of the shape of vortex lines) (2, 7) of wavelengths smaller than η q ðTÞ, which decreases with decreasing T (8). At higher temperatures, excitations form a fluid of low viscosity (the normal component). The normal component can move at velocity v n ðrÞ that is different from v s ðrÞ. The presence of two coexisting but weakly interacting (they only interact in the presence of quantized vortices) fluids, superfluid and normal (both of which can be independently turbulent), is a unique feature of superfluid helium. At high temperatures, Kelvin waves of all wavelengths are damped. With decreasing temperature between some 0.7 and 0.5 K, first larger...

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