Pure Superfluid Research Articles

Superfluid Drain Vortex

Drain vortices are among the most common vortices observed in everyday life, yet their physics is complex due to the competition of vorticity’s transport and diffusion, and the presence of viscous layers and a free surface. Recently, it has become possible to study experimentally drain vortices in liquid helium II, a quantum fluid whose physics is characterised by the absence of viscosity and the quantisation of the circulation in the superfluid component. Using the Gross–Pitaevskii equation, we make a simple model of the problem which captures the essential physics ingredients, showing that the drain vortex of a pure superfluid consists of a bundle of vortex lines which, in the presence of a radial drain, twist, thus strengthening the axial flow into the drain.

P -wave superfluidity in mixtures of ultracold Fermi and spinor Bose gases

We reveal that the p-wave superfluid can be realized in a mixture of fermionic and F=1 bosonic gases. We derive a general set of the gap equations for gaps in the s- and p-channels. It is found that the spin-spin bose-fermi interactions favor the p-wave pairing and naturally suppress the pairing in the s-channel. The gap equations for the polar phase of p-wave superfluid fermions are numerically solved. It is shown that a pure p-wave superfluid can be observed in a well-controlled environment of atomic physics.

Multiple critical velocities in oscillatory flow of superfluidHe4due to quartz tuning forks

We report recent investigations into the transition to turbulence in superfluid $^4$He, realized experimentally by measuring the drag forces acting on two custom-made quartz tuning forks with fundamental resonances at 6.5 kHz and 55.5 kHz, in the temperature range 10 mK to 2.17 K. In pure superfluid in the zero temperature limit, three distinct critical velocities were observed with both tuning forks. We discuss the signicance of all critical velocities and associate the third critical velocity reported here for the first time with the development of large vortical structures in the flow, which thus starts to mimic turbulence in classical fluids. The interpretation of our results is directly linked to previous experimental work with oscillators such as tuning forks, grids and vibrating wires, focusing on the behavior of purely superfluid $^4$He at very low temperatures

Large quantum superpositions of a nanoparticle immersed in superfluid helium

Preparing and detecting spatially extended quantum superpositions of a massive object comprises an important fundamental test of quantum theory. These quantum states are extremely fragile and tend to quickly decay into incoherent mixtures due to the environmental decoherence. Experimental setups considered up to date address this threat in a conceptually straightforward way -- by eliminating the environment, i.e. by isolating an object in a sufficiently high vacuum. We show that another option exists: decoherence is suppressed in the presence of a strongly interacting environment if this environment is superfluid. Indeed, as long as an object immersed in a pure superfluid at zero temperature moves with a velocity below the critical one, it does not create, absorb or scatter any excitations of the superfluid. Hence, in this idealized situations the decoherence is absent. In reality the decoherence will be present due to thermal excitations of the superfluid and impurities contaminating the superfluid. We examine various decoherence channels in the superfluid $^4$He. It is shown that the total decoherence time can be as large as tens of seconds for a $10^6$ amu nanoparticle delocalized over $300$ nm in helium at $1$ mK. Performing interference experiments in superfluid helium can provide certain practical advantages compared to conventional schemes, e.g. compensation of gravity by the buoyancy force and effective cooling.

Elementary excitations in homogeneous superfluid neutron star matter: Role of the proton component

The thermal evolution of neutron stars depends on the elementary excitations affecting the stellar matter. In particular, the low-energy excitations, whose energy is proportional to the transferred momentum, can play a major role in the emission and propagation of neutrinos. In this paper, we focus on the density modes associated with the proton component in the homogeneous matter of the outer core of neutron stars (at density between one and three times the nuclear saturation density, where the baryonic constituents are expected to be neutrons and protons). In this region, it is predicted that the protons are superconducting. We study the respective roles of the proton pairing and Coulomb interaction in determining the properties of the modes associated with the proton component. This study is performed in the framework of the random phase approximation, generalized in order to describe the response of a superfluid system. The formalism we use ensures that the generalized Ward's identities are satisfied. An important conclusion of this work is the presence of a pseudo-Goldstone mode associated with the superconducting protons in neutron-star matter. Indeed, the Goldstone mode, which characterizes a pure superfluid, is suppressed in usual superconductors because of the long-range Coulomb interaction, which allows a plasmon mode. However, for the proton component of stellar matter, the Coulomb field is screened by the electrons and a pseudo-Goldstone mode occurs, with a velocity increased by the Coulomb interaction.

Phase separation in a two-species atomic Bose-Einstein condensate with an interspecies Feshbach resonance

We consider a mixture of two-species atomic Bose-Einstein codensates coupled to a bound molecular state at zero temperature via interspecies Feshbach resonance. The interspecies Feshbach coupling precludes the possibility of doubly mixed phases while enables not only the pure molecular superfluid but also the pure atomic superfluids to exist as distinct phases. We show that this system is able to support a rich set of phase separations, including that between two distinct mixed atom-molecule phases. We pay particular attention to the effects of the Feschbach coupling and the particle collisions on the miscibility of this multi-component condensate system.

Vortex Rings in Superfluid 3He–B at Low Temperatures

Turbulence in classical fluids has far-reaching technological implications but is poorly understood. A better understanding might be gained from studying turbulence in quantum systems. In a pure superfluid (at low temperatures), there is no viscosity and vortex lines are quantised. Quantum turbulence consists of a tangle of quantised vortex lines which interact via their self-induced flow. We have recently developed techniques for detecting vortices in superfluid 3He–B in the low temperature limit. We find that the transition to turbulence from a moving grid occurs by the entanglement of emitted vortex rings. Here, we discuss the propagation of the ballistic vortex rings emitted at low grid velocities. We have measured the temperature at which the rings decay before reaching the detectors. Our results, at two different pressures, confirm that the vortex rings decay in accordance with mutual friction.

Nonlinear optics: Shocking superfluids

How shock waves travel through a superfluid provides clues to understanding the deeper nature of Bose–Einstein condensation. An optical analogue that behaves as a pure superfluid could tell us what these clues mean.

MONOLAYER CHARGED QUANTUM FILMS: A QUANTUM SIMULATION STUDY

We use path-integral Monte Carlo (PIMC) to study the effects of adding a long-range repulsive Coulomb interaction to the usual Van der Waals interaction between two atoms of a submonolayer quantum film such as helium on graphite or a pure two-dimensional superfluid. Such interactions frustrate or compete with the natural tendency of the system for phase separation namely to form a macroscopic liquid or solid phase. We find that as a function of the relative strength of the long-range repulsion, surface coverage and temperature, the system undergoes a series of transformations, including a triangular Wigner-like crystal of clusters, a charge stripe-ordered phase and a fluid phase. The goal of these studies is to understand the role of quantum fluctuations when such competing interactions appear together with formation of preexisting electron pairs as might be the case in cuprate superconductors.

Instability of trajectories of solid particles around vortex lines

Progress in implementing the particle image velocimetry visualization technique in liquid helium has stimulated interest in understanding the dynamics of micron-size solid particles in a superfluid. We show that at sufficiently low temperatures, in the limit of a pure superfluid, the trajectories of small, neutrally buoyant, solid particles are unstable and deviate from the trajectories of superfluid particles, even in the simplest case of the motion around a single stationary straight vortex line. The result also applies to classical Euler flows. The implications of this result for the visualization of turbulent superflow at very low temperatures are discussed.

Two-fluid hydrodynamic modes in a trapped superfluid gas

In the collisional region at finite temperatures, the collective modes of superfluids are described by the Landau two-fluid hydrodynamic equations. This region can now be probed over the entire BCS-Bose-Einstein-condensate crossover in trapped Fermi superfluids with a Feshbach resonance, including the unitarity region. Building on the approach initiated by Zaremba, Nikuni, and Griffin in 1999 for trapped atomic Bose gases, we present a variational formulation of two-fluid hydrodynamic collective modes based on the work of Zilsel in 1950 developed for superfluid helium. Assuming a simple variational Ansatz for the superfluid and normal fluid velocities, the frequencies of the hydrodynamic modes are given by solutions of coupled algebraic equations, with constants only involving spatial integrals over various equilibrium thermodynamic derivatives. This variational approach is both simpler and more physical than a direct attempt to solve the Landau two-fluid differential equations. Our two-fluid results are shown to reduce to those of Pitaevskii and Stringari for a pure superfluid at T=0.

Determination of the Neutron Lifetime Using Magnetically Trapped Neutrons.

We report progress on an experiment to measure the neutron lifetime using magnetically trapped neutrons. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering 0.89 nm neutrons in isotopically pure superfluid 4He. Neutron decays are detected in real time using the scintillation light produced in the helium by the beta-decay electrons. The measured trap lifetime at a helium temperature of 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron lifetime. This is attributed to the presence of neutrons with energies higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an trap lifetime, consistent with the currently accepted value of the free neutron lifetime.

Phase diagram of the superfluid phases of3Hein 98% aerogel

The phase diagram of the superfluid phases of $^{3}$He in 98% aerogel was determined in the range of pressure from 15 to 33 bars and for fields up to 3 kG using high-frequency sound. The superfluid transition in aerogel at 33.4 bars is field independent from 0 to 5 kG and shows no evidence of an $A_{1}-A_{2}$ splitting. The first-order transition between the A and B-phases is suppressed by a magnetic field, and exhibits strong supercooling at high pressures. We show that the equilibrium phase in zero applied field is the B-phase with at most a region of A-phase {\small $\alt$} 20 $\mu$K just below T${_c}$ at a pressure of 33.4 bars. This is in contrast to pure $^{3}$He which has a large stable region of A-phase and a polycritical point. The quadratic coefficient for magnetic field suppression of the AB-transition, $g_{a}(\beta)$, was obtained. The pressure dependence of $g_{a}(\beta)$ is markedly different from that for the pure superfluid, $g_{0}(\beta)$, which diverges at a polycritical pressure of 21 bars. We compare our results with calculations from the homogeneous scattering model for $g_{a}(\beta)$, defined in a Ginzburg-Landau theory in terms of strong-coupling parameters $\beta$. We find qualitatively good agreement with the experiment if the strong-coupling corrections are rescaled from known values of the $\beta$'s for pure $^{3}$He, reduced by the suppression of the superfluid transition temperature. The calculations indicate that the polycritical pressure in the aerogel system is displaced well above the melting pressure and out of experimental reach. We cannot account for the puzzling supercooling of the aerogel AB-transition in zero applied field within the framework of known nucleation scenarios.

Nucleation and interfacial coupling between pure and dirty superfluid phases of 3He.

The nucleation of the first-order phase transition of superfluid 3He-B from superfluid 3He-A is quite remarkable since it requires a seed of the order of a micron. We have studied this nucleation for 3He confined to a very dilute silica aerogel. This dirty superfluid behaves in a manner similar to previous reports for the pure superfluid. But we have discovered a novel magnetically driven nucleation switch acting on the pure superfluid- B phase. Last, we find the surprising result that the proximity effect between the pure and dirty superfluids at their interface is insufficient to nucleate the B phase in either superfluid.

Superfluid turbulence at zero temperature

Little is known about the turbulent flow of pure superfluid. We are investigating this problem numerically by following the motion of a complex tangle of vortex filaments evolving according to the Biot–Savart equation of motion, with the assumption that vortex lines will reconnect upon crossing. We measure the complexity of this vortex tangle by recording the number of closed vortex loops in the tangle and by determining the number of links between these vortex loops. These values change in a characteristic manner as the vortex tangle evolves from the initial conditions (random vortex rings).

Motion of Quantized Vortices as Elementary Objects

The general local, nondissipative equations of motion for a quantized vortex moving in an uncharged laboratory superfluid are derived from a relativistic, co-ordinate invariant framework having vortices as its elementary objects in the form of stable topological excitations. This derivation is carried out for a pure superfluid with an isotropic gap at the absolute zero of temperature, on the level of a hydrodynamic, collective co-ordinate description. In the formalism, we use as fundamental ingredients that particle number as well as vorticity are conserved and that the fluid is perfect. No assumptions are involved as regards the dynamical behaviour of the order parameter. The interaction of the vortex with the background fluid, representing the Magnus force, and with itself via phonons, giving rise to the hydrodynamic vortex mass, are separated. For a description of the motion of the vortex in a dense laboratory superfluid such as helium II, two limits have to be considered: The nonrelativistic limit for the superfluid background is taken, and the motion of the vortex is restricted to velocities much less than the speed of sound. The canonical structure of vortex motion in terms of the collective co-ordinate is used for the quantization of this motion.

Pressure dependence of vortex tunneling in He II

Measurements of the rate ν at which negative ions nucleate charged vortex rings in isotopically pure superfluid 4 He for electric fields E , temperatures T and pressures P within the ranges 10 3 ≤ E ≤ 10 6 Vm −1 , 75 ≤ T ≤ 500 mK, 12 ≤ P ≤ 23 bar show that the energy barrier impeding the tunneling process is strongly dependent on pressure. Minima observed in ν ( T ) are ascribed to phonon damping.

Superfluid transport and its applications in space

Various transport modes in superfluid helium are discussed in this paper. They include zero net mass flow (ZNMF) and finite mass flow (FMF) for pure superfluid and normal fluid flow. An attempt is made to characterize these transport modes in a common frame of reference. Two dimensionless numbers are used, namely the dimensionless heat flux number and the dimensionless driving force number. The equations are generalized by the use of a characteristic length so that they can be applied to the transport of He II in any geometry. The theories are then extended to applications in space. In particular, fountain effect pumps (FEPs) and superfluid management at zero g by vapour—liquid phase separators (VLPSs) will be discussed in detail. While transport in a phase separator is close to that of ZNMF, the flow in a FEP belongs to the FMF mode. The transport modes of the above systems using porous media are found to be strongly size dependent. For VLPSs, the heat rejection rate is proportional to the square root of plug permeability, K P. As in the case of FEPs, the volumetric flow rate is inversely proportional to K P. These findings are supported by ZNMF data in capillaries and FMF (gravitational flow) data in millipores, respectively.

Pressurized Superfluid 4He Acoustic Microscopy

An acoustic microscope has been developed which makes use of pressurized super fluid helium as an acoustic coupling medium. Acoustic micrographs were taken using pure super fluid 4He under saturated vapor pressure at 0.2 K and under 25 atm at 0.2 K and 1.9 K, for the frequency of 400 MHz. By pressurizing liquid 4He, the acoustic attenuation decreases, the efficiency of the antireflection coating increases, and the power level of saturation caused by nonlinearity of the super fluid 4He is raised. A good signal-to-noise ratio of imaging is thus obtained.

Turbulence in Pure Superfluid Flow

A series of experiments is described which provide an unambiguous description of the steady-state properties of turbulence in pure superfluid flow. The turbulence is qualitatively different from that observed in counterflow but comparable to the homogeneous turbulence described by theory.