Thin Superfluid Films Research Articles

Geometric Induction in Chiral Superfluids.

We explore the properties of chiral superfluid thin films coating a curved surface. Because of the vector nature of the order parameter, a geometric gauge field emerges and leads to a number of observable effects such as anomalous vortex-geometric interaction and curvature-induced mass and spin supercurrents. We apply our theory to several well-known phases of chiral superfluid ^{3}He and derive experimentally observable signatures. We further discuss the cases of flexible geometries where a soft surface can adapt itself to compensate for the strain from the chiral superfluid. The proposed interplay between geometry and chiral superfluid order provides a fascinating avenue to control and manipulate quantum states with strain.

Dynamical Mechanisms of Vortex Pinning in Superfluid Thin Films.

We characterize the mechanisms of vortex pinning in a superfluid thin film described by the two-dimensional Gross-Pitaevskii equation. We consider a vortex "scattering experiment" whereby a single vortex in a superfluid flow interacts with a circular, uniform pinning potential. By an analogy with linear dielectrics, we develop an analytical hydrodynamic approximation that predicts vortex trajectories, the vortex fixed point and the unpinning velocity. We then solve the Gross-Pitaevskii equation to validate this model, and build a phase portrait of vortex pinning. We identify two different dynamical pinning mechanisms marked by distinctive phonon emission signatures: one enabled by acoustic radiation and another mediated by vortex dipoles nucleated within the pin. Relative to obstacle size, we find that pinning potentials on the order of the healing length are more effective for vortex capture. Our results could be useful in mitigating the negative effects of drag due to vortices in superfluid channels, in analogy to maximizing supercurrents in type-II superconductors.

Non-stationary Thermal Electromotive Force Generated by Third Sound

It is predicted that oscillations of temperature during propagation of third sound in a thin superfluid film cause appearance of an alternating electric field in the surrounding space (a peculiar non-stationary thermoelectric effect). The magnitude of this field depends significantly on the substrate type and the method of its coating. It is shown that the differential thermal EMF (the ratio of electric potential amplitude to the film temperature amplitude) can exceed such one in metals and reach $10^{-4}$ V/K.

Characterization of Pyrolytic Graphite Sheet: A New Type of Adsorption Substrate for Studies of Superfluid Thin Films

We have measured surface morphology and gas adsorption characteristics of uncompressed pyrolytic graphite sheet (uPGS) which is a candidate substrate for AC and DC superflow experiments on monolayers of 4He below T = 1 K. The PGS is a mass-produced thin graphite sheet with various thicknesses between 10 and 100 {\mu}m. We employed a variety of measuring techniques such as imagings with optical microscope, SEM and STM, Raman spectroscopy, and adsorption isotherm. PGS has smooth and atomically-flat external surfaces with high crystallinity. Although the specific surface area (<0.1 m$^2$/g) is rather small, by making use of its smooth external surface, the thinnest uPGS of 10 {\mu}m thick is found to be suitable for the superflow experiments on the strictly two-dimensional helium systems.

Laser cooling and control of excitations in superfluid helium

Superfluidity is a quantum state of matter that exists macroscopically in helium at low temperatures. The elementary excitations in superfluid helium have been probed with great success using techniques such as neutron and light scattering. However, measurements of phonon excitations have so far been limited to average thermodynamic properties or the driven response far out of thermal equilibrium. Here, we use cavity optomechanics to probe the thermodynamics of phonon excitations in real time. Furthermore, strong light–matter interactions allow both laser cooling and amplification. This represents a new tool to observe and control superfluid excitations that may provide insight into phonon–phonon interactions, quantized vortices and two-dimensional phenomena such as the Berezinskii–Kosterlitz–Thouless transition. The third sound modes studied here also offer a pathway towards quantum optomechanics with thin superfluid films, including the prospect of femtogram masses, high mechanical quality factors, strong phonon–phonon and phonon–vortex interactions, and self-assembly into complex geometries with sub-nanometre feature size. It takes extreme sensitivity to measure the elementary excitations in liquid helium-4. An optomechanical cavity with a thin film of superfluid inside can be used to both observe and control phonons in real time.

A structural study of a two-dimensional electrolyte by Monte Carlo simulations.

Properties of superconducting and superfluid thin films, modeled as a two-dimensional classic Coulomb fluid, are connected to the molecular structure of the system. Monte Carlo simulations to explore structural properties and ordering in the classical two-dimensional Coulomb fluid were performed. The density dependence of translational order parameters at various temperatures and cluster distribution below and above the Kosterlitz-Thouless line were studied, and the percolation temperature threshold was determined. Results show that one could detect the insulator-conductor transition by observing the translational order parameters, average cluster number, or mean cluster size besides dielectric constant and dipole moment of the system.

Strang splitting methods for a quasilinear Schrödinger equation: convergence, instability, and dynamics

We study the Strang splitting scheme for quasilinear Schrodinger equations. We establish the convergence of the scheme for solutions with small initial data. We analyze the linear instability of the numerical scheme, which explains the numerical blow-up of large data solutions and connects to the analytical breakdown of regularity of solutions to quasilinear Schrodinger equations. Numerical tests are performed for a modified version of the superfluid thin film equation.

Strang splitting methods for a quasilinear Schrödinger equation: convergence, instability, and dynamics

We study the Strang splitting scheme for quasilinear Schr\"odinger equations. We establish the convergence of the scheme for solutions with small initial data. We analyze the linear instability of the numerical scheme, which explains the numerical blow-up of large data solutions and connects to analytical breakdown of regularity of solutions to quasilinear Schr\"odinger equations. Numerical tests are performed for a modified version of the superfluid thin film equation.

Fast atoms and negative chain-cluster fragments from laser-induced Coulomb explosions in a super-fluid film of ultra-dense deuterium D(−1)

Fragments from laser-induced Coulomb explosions (CE) in a thin super-fluid film of ultra-dense deuterium D(−1) on a vertical surface are now observed by both negative and positive time-of-flight mass spectrometry. The so-called normal phase of the super-fluid is probably associated with D4 clusters and gives only neutral atomic fragments with a kinetic energy from the CE of 945 eV. The super-fluid phase is associated with long chain clusters D2N with N deuteron pairs and gives cluster fragments by CE mainly with a kinetic energy of 315 eV from the central cleavage in a neutral, positive or negative form. This indicates that the chain clusters are standing perpendicularly to the surface. The fragment charge state is influenced by the external field, which indicates efficient charge transfer processes.

Torsional Oscillator measurements of multiply connected monolayer 4He films under high speed rotation up to 4.0 rev./s

We are studying the physical phenomena of superfluid thin films adsorbed on multiply connected surface of porous glasses under high speed rotation. The combination of thin superfluid 4He films on large pore porous surfaces and high-speed rotation gives rise to a new quantum vortex excitation, where vortex lines are pinned on the center of pores and have the “core” at the pore. This excitation has been predicted and named as a “pore vortex”. We are trying to measure the behavior of pore vortices using the Torsional Oscillator (TO) technique and we have succeeded in making measurements up to the high-speed rotation of 4.0 rev./s. Here we report the actual design of the TO set up.

Dry vortices in thin helium films

A very thin superfluid film on a weakly attractive surface (such as rubidium metal) should display an unusual vortex structure, with a dry core larger than the standard core radius λ. In this structure, the capillary energies are slightly increased, but the vortex kinetic energy is reduced.

Images and nonlocal vortex pinning in thin superfluid films

For thin films of superfluid adsorbed on a disordered substrate, we derive within a mean field (Hartree) description of the condensate the equation of motion for a vortex in the presence of a random potential. The compressible nature of the condensate leads to an effective pinning potential experienced by the vortex which is nonlocal, with a long-range tail that smooths out the random potential coupling the condensate to the substrate. We interpret this nonlocality in terms of images, and relate the effective potential governing the dynamics to the pinning energy arising from the expectation value of the Hamiltonian with respect to the vortex wave function.

GENERALIZED DEFORMED OSCILLATOR FOR VORTICES IN SUPERFLUID FILMS

The algebra of observables of a system of two identical vortices in a superfluid thin film is described as a generalized deformed oscillator with a structure function containing a linear (harmonic oscillator) term and a quadratic term. In contrast to the deformed oscillators occurring in other physical systems (correlated fermion pairs in a single-j nuclear shell, Morse oscillator), this oscillator is not amenable to perturbative treatment and cannot be approximated by quons. From the mathematical viewpoint, this oscillator provides a novel boson realization of the algebra su (1,1).

Weak electrolytes, Brownian motion, vortices in superfluid films, and Odins Aker

A brief sketch of the author's days at Yale as Lars Onsager's last physics student is followed by the contributions of the Onsager school to our current understanding of persistent current decays and vortex pinning in very thin superfluid films. The resulting theory is an interplay of three subjects that were dear to Onsager's heart: electrolytes, vortices in superfluids, and Brownian motion. The discussion also surveys a topic of current interest, the role played by defects and boundaries in producing the “stiffness” that characterizes superfluids. The article ends with a few words about the author's connection to Norway.

Superfluid helium on solid hydrogen

Using third sound, we have found that helium atoms can form a superfluid in direct contact with the crystalline surface of solid hydrogen. An anomalous phase marked by a second line of transitions similar to the Kosterlitz-Thouless transition has also been discovered for thin superfluid films on the H 2 substrate for T< T KT. This article compares these measured third sound velocities, in the ordinary superfluid phase, to a theoretical calculation, and the results from other work. The possible causes and interpretations for the new transition are discussed.

Precision specific-heat studies of thin superfluid films.

We report precision specific-heat measurements for two-dimensional helium films in Anopore membranes and in Millipore filter paper. Above the transition temperature and for films less than a superfluid layer thick, we find a distinct specific-heat peak which can be understood in terms of the Kosterlitz-Thouless vortex unbinding mechanism. Although our results are well described by the planar theory, transition broadening becomes evident for thicker films.

Phase slippage in thin superfluid films

We have extended the Ambegaokar, Halperin, Nelson and Siggia theory of vortex de-pairing in two-dimensional superfluids to describe how the current of free vortices varies with superfluid velocity, v s, in the steady state, for temperatures just above the Kosterlitz-Thouless transition. To do so we have solved exactly the Fokker-Planck equation which describes the current of vortices produced at one edge of the film. We are able to calculate how a persistent current decays, and the thermal resistance of a film as a function of heater power. Superconducting films are considered as a charged, two-dimensional superfluid, and the theory is applied to them.

3He-induced free vortices in thin superfluid films

We report analysis of measurements of the thermal conductance of 3He-4He mixture films near the Kosterlitz-Thouless transition. It is found that on the superfluid side of the transition our data can be understood by the existence of free vortices induced by the addition of 3He to 4 He films.

3He-induced free vortices in thin superfluid films

3He-induced free vortices in thin superfluid films

Zero-gravity experiments on superfluid helium aboard a rocket

Superfluid experiments in the zero-gravity state were performed with a cryostat aboard the rocket S-520. Observation of the dynamic behaviour of liquid helium (He ll), verification of a porous-plug phase separator with a controlling heater, investigation of thermal behaviour of superfluid thin films and a superfluid heat pipe, and the noise test of a cooled FET pre-amplifier for i.r. detectors were performed. The experiments showed that the initial bulk fluid motions were rather quickly damped and a stable liquid containment in the cryostat was achieved. Very good temperature uniformity was attained throughout the vessel. The porous-plug was found to work as well as predicted in the zero-gravity state. The adsorbed film became thicker in the reduced gravity state than on the ground.