Substrate Dependence of the Superfluid Onset of 4He Films

We have performed torsional oscillator measurements of ⁴ He films adsorbed on porous gold and gold preplated with Ar, Ne, D ₂ , HD, and H ₂ . On all substrates, the superfluid response is similar. The thickness of the nonsuperfluid layer as function of the substrate, however, increases monotonically with the well depth of the atom-substrate interaction potential.     

Characterization of MEMS Devices for the Study of?Superfluid Helium Films

Measurements on the mechanical properties of MEMS resonators were performed to characterize such devices at room temperature and low temperatures. Using state-of-the-art silicon integrated circuit technology, we have designed, fabricated, and manufactured resonators consisting of a pair of parallel plates with a well-defined gap whose size can be controlled with a high accuracy down to the sub-micron range. A full study of resonance properties at various pressures was performed at room temperature. We will discuss the details of design, fabrication, and operation. These studies open up a window of opportunities to look for novel phenomena in quantum fluids such as in superfluid ³He films.     

Refinement of the Dynamic Theory of Superfluid Helium Films

The standard theory of the dynamics of superfluid films, although in good qualitative agreement with experiment, is not in perfect quantitative agreement. Until now it has not been clear whether the disagreement is due to inaccuracies in the theory or because the experiments involve an extra physical effect not included in the theoretical model. Here we show how the theory can be refined: the ambiguity that arises over the difference between free and bound vortices in the superfluid system is avoided by using an ansatz in which all vortices are considered paired; the interaction between vortices is calculated using recently developed versions of the Kosterlitz recursion relations accurate to higher order in the fugacity; the linear response is calculated using accurate numerical techniques. We present a detailed comparison of the modified theory's predictions with recent experimental results. We find that, despite the changes in the theory, the agreement with experiment remains only qualitative, suggesting that the theory fails to capture some of the essential physics of experiments on Helium films.     

An Ellipsometric Study of the Oxidation of Thin Copper Films in Oxygen

The thermal oxidation of thin copper films deposited on single-crystal silicon was studied by ellipsometry. The optical properties of the oxide layers grown at 420–470 K over a period of up to 150 min were found to vary with layer thickness in a complex manner. The results were analyzed in terms of single- and two-layer models. The oxidation kinetics were shown to follow a parabolic rate law. The apparent activation energies for different stages of oxidation were evaluated with allowance for the self-organization of the interfacial layer.     

More about Rotons in Superfluid Helium 4

In a recent paper¹ it was argued that rotons in superfluid helium 4 are the soft modes announcing a charge density wave that leads to the crystal: rotons are a normal state property. A small superfluid condensate acts to hybridize quasiparticles and soft density fluctuations - hence a level repulsion that lowers the energy: superfluidity is energetically favourable. A shallow roton implies a very small condensate density, as found in He4: what we need is a saturation mechanism. The clue is depletion due to quantum fluctuations. In (1) we assumed that such a depletion was drawn from the condensate itself: superfluidity then disappears in the liquid if the roton gap is too small. Here we explore an alternate possibility: quantum fluctuations are drawn from the normal fluid. We reach the opposite conclusion: superfluidity persists down to the spinodal limit where the roton gap vanishes, with an unusual power law dependence. We briefly mention the possible extension of that argument to a frozen charge density wave: in a toy 1d model it might shed light on the features that favour supersolids.     

Adsorption of Thin Liquid Helium Films on Cs Measured via Photoelectron Tunneling

The alkali metals Cs and Rb are the only surfaces which are not wetted by superfluid ⁴ He below a certain temperature. In our experiments, using the photoelectron tunneling method, we can highly resolve the growth of the non-wetting thin-film state of ⁴ He on a quench-condensed Cs surface. It turns out that far from coexistence there is little adsorption of helium. In contrast, close to coexistence a rapid growth up to two monolayers of helium is observed, but the surface is still non-wet under the usual convention.     

Phonons Created in Superfluid Helium by a Thin Film Gold Heater

We show that a thin film gold heater, on polished sapphire, radiates phonons into liquid helium by both the acoustic and background channels. To analyse the measured angular distributions of phonons, at a pressure of 24 bar, we derive an expression for the phonon flux incident on a detector for a general spatial arrangement of heater and detector, when the phonons travel ballistically. We then develop an expression for the particular geometry of the experiments. We find that most of the energy flux, from the heater to the liquid helium, is transmitted by the background channel and this fraction increases with heater power. We discuss the implications for the Kapitza conductance problem.     

Torsional Oscillator Study of Thin Helium Films on Hydrogen Substrates

We have made torsional oscillator measurements of thin helium films on a Mylar surface preplated with 2 and 5 layers of H ₂ . The minimum ⁴ He coverage needed for superfluidity, or inert layer, n _o on both surfaces is found to be 6 mole/m ² . This is equivalent to 1/2 of a monolayer at bulk liquid density. The superfluid transition in coverages above n _o is similar to that found on bare Mylar, exhibiting the standard Kosterlitz-Thouless characteristics. We find no anomaly in _s or the dissipation in the film that can be identified with a second transition reported in recent third sound experiments.     

Universal Features of the Superfluid Transition of Thin 4He Films on Mylar

Data from a series of torsional oscillator experiments performed on ⁴ He films on Mylar have been available for some time. However, up until now it was thought that disorder in the substrate leads to a wide variation in the results. Here the data is analysed in a new way which captures for the first time the universal features of the results. Comparisons are made with recent theoretical work which predicts universal behaviour in the superfluid film transition and with the results of similar experiments performed on a Grafoil substrate.     

Density Functional Theory of Freezing of Superfluid Helium 4

We discuss freezing of superfluid helium 4 in the frame of density functional theory. We first review the previous attempts made with different inputs for the liquid properties. We then present a new calculation based on a more recent input. Our resugt confirms the finding by Likos et al. [C. N. Likos, S. Moroni, and G. Senatore, Phys. Rev. B, 55, 8867 (1997)] that the solid is always too stable. However, a slight rescaling leads to freezing parameters and an equation of state for the solid consistent with the experiments. We discuss the physical origin of this rescaling.     

Measurements of Turbulence Onset and Dissipation in Superfluid Helium with a Silicon Double Paddle Oscillator

We have studied experimentally the response of a silicon single crystal double paddle oscillator submerged in superfluid helium from the lambda point to 1.55 K. Measuring the resonance frequency and dissipation on three modes of this high Q system allows us to study the dissipation at the onset of turbulence in the flow around the paddle. The critical velocity V _c for turbulence onset decreases with temperature. If we use the density of the normal component of the superfluid to obtain a Reynolds number Re associated with V _c we find a value which is largely temperature independent. This result is different from the behavior previously found by other authors below 1 K, where the quantized vorticity (extrinsic nucleation) is observed at velocities more than an order of magnitude greater. In our temperature range, we conclude that the transition is governed by the normal fraction acting as a classical fluid. The laminar regime shows a dissipation that is proportional to the viscous drag calculated by well known formulas for an object oscillating in a liquid. We also find a decrease in resonance frequency in the turbulent regime which is clearly observed but hard to reproduce from run to run.     

Superfluid 3He Thin Films with Specularly Reflecting Walls

We calculate the specific heat of superfluid ³ He film thinner than the coherence length. In such a thin film, quantization on the confined direction makes eigenstates discrete, thus there appears a set of Fermi disks which are just like subband. Because of both the discreteness and ABM-like energy gap, the specific heat exhibits Schottky type character, which becomes conspicuous in special thin region.     

Superfluid Transition of a 4He Thin Film Pressurized by Bulk Liquid 3He

We measured transverse acoustic impedance Z of normal fluid ³He at 46.6 MHz on a surface coated with a thin ⁴He film. The real component of the impedance, Z′, in the coated samples deviates from Z′ in the pure ³He in the low temperature region. Z′ on the coated samples is almost identical with Z′ in the pure sample at high temperature and gradually deviates below a particular temperature T _onset . T _onset  is possibly the superfluid onset temperature of the ⁴He film pressurized by the bulk liquid ³He. The gradual decrease in Z′ means that the superfluid component in ⁴He film increases gradually, which is expected from the dynamic KT transition at high frequency. The thicker is the film, the higher is the T _onset . The range of T _onset we observed was between 40 and 160 mK. This is much lower than that at the saturated vapor pressure. Suppression of T _onset achieved by the applied pressure from bulk liquid ³He was presumably caused by the dissolved ³He in the film, thickening of the inert layers and/or by the strong correlation effect. The result shows that the specularity of ³He quasiparticle scattering is strongly affected by superfluidity of the ⁴He film.     

Interaction of Helium Rydberg State Atoms with Superfluid Helium

The pair potentials between ground state helium and Rydberg He $^*(2s,2p,3s)$ atoms are calculated by the full configuration interaction electronic structure method for both the electronic singlet and the triplet manifolds. The obtained pair potentials are validated against existing experimental molecular and atomic data. Most states show remarkable energy barriers at long distances ( $R > 5$ Å), which can effectively stabilize He $^*$ against the formation of He $_2^*$ at low nuclear kinetic energies. Bosonic density functional theory calculations, based on the calculated pair potential data, indicate that the triplet ground state He $^*$ reside in spherical bubbles in superfluid helium with a barycenter radius of 6.1 Å at the liquid saturated vapor pressure. The pressure dependency of the relative He $^*$ $2s$ $^3S$ $\rightarrow $ $2p$ $^3P$ absorption line blue shift in the liquid was obtained through both the statistical line broadening theory as well as the dynamic adiabatic following method. The pronounced difference between the results from the static and dynamic models is attributed to the dynamic Jahn–Teller effect that takes places in the electronically excited state within the dephasing time of 150 fs. Transient non-thermalized liquid surroundings near He $^*$ may contribute to an artificial reduction in the absorption line blue shift by up to 30 cm $^{-1}$ .     

Thermal Conductance Study of Thin Superfluid Films Formed in Porous Glass With 1 Micrometer Pore Diameter

It has been shown that the limited solubility at low temperatures in binary system of cryocrystalsis due to the specificity of intermolecular interaction. A simple method which makes it possible to predict a type of a phase diagram has been proposed.     

Hysteretic Behavior of Superfluid Helium in Anopore

Superfluid ⁴ He exhibits hysteretic behavior due to capillary condensation in Anopore, a porous membrane containing mostly non-intersecting cylindrical pores. Because the pores are non-intersecting, we expect the system may behave similarly to a system of nearly non-interacting spins, where spinup is equivalent to a filled pore and a spin-down is equivalent to an empty pore. We show the global hysteresis loop as well as subloops, and compare the results to those predicted for a system of non-interacting spins.     

Modeling Kelvin Wave Cascades in Superfluid Helium

We study two different types of simplified models for Kelvin wave turbulence on quantized vortex lines in superfluids near zero temperature. Our first model is obtained from a truncated expansion of the Local Induction Approximation (Truncated-LIA) and it is shown to possess the same scalings and the essential behaviour as the full Biot-Savart model, being much simpler than the later and, therefore, more amenable to theoretical and numerical investigations. The Truncated-LIA model supports six-wave interactions and dual cascades, which are clearly demonstrated via the direct numerical simulation of this model in the present paper. In particular, our simulations confirm presence of the weak turbulence regime and the theoretically predicted spectra for the direct energy cascade and the inverse wave action cascade. The second type of model we study, the Differential Approximation Model (DAM), takes a further drastic simplification by assuming locality of interactions in k-space via using a differential closure that preserves the main scalings of the Kelvin wave dynamics. DAMs are even more amenable to study and they form a useful tool by providing simple analytical solutions in the cases when extra physical effects are present, e.g. forcing by reconnections, friction dissipation and phonon radiation. We study these models numerically and test their theoretical predictions, in particular the formation of the stationary spectra, and closeness of numerics for the higher-order DAM to the analytical predictions for the lower-order DAM.     

Van der Waals Effects at the Superfluid Transition of Confined 4Helium

We explore the role which the van der Waals field plays in the behavior of the superfluid density of confined helium near T. This effect can be calculated in the context of -theory. The influence of a solid wall is incorporated into the equation for the order parameter via a spatially-dependent transition temperature. The equation is solved numerically. We first calculate the scaling function without wall interactions for the case of 2D, 1D, and 0D dimensionality crossover. We find that upon introducing the van der Waals field, its influence is too small, and in the wrong direction to explain experimental observations.     

Density Functional Theory of the Interface between Solid and Superfluid Helium 4

No Heading We report a calculation of the equilibrium profile of the interface between superfluid helium 4 and its solid phase at 0 K. To calculate the excess energy of the interface, we use the density functional theory that allowed us to obtain the equilibrium density of both phases. We compare our result to the experimental and to a previous microscopic calculation.PACS numbers: 68.08.–p, 67.80.–s, 67.40.Kh.