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Physical review. B, Condensed matter

Suspension of superfluid helium using cesium-coated surfaces

Mark C. Williams, Clayton F. Giese, J. W. Halleý

Published Mar 1, 1996• 14 citations
Publisher

Abstract

We report results of an experiment which demonstrates that a layer of superfluid helium can be suspended over a cesium-coated orifice. By measuring the layer thickness with a capacitance bridge, we have shown in two runs that fluid layers up to 2 mm thick were suspended over a 70-\ensuremath{\mu}m-diam cesium-coated orifice in a platinum foil for over 2 h in a cryostat held at 1.2 K. The effect depends on the recently established fact that superfluid helium does not wet cesium-coated surfaces. As a consequence, superfluid helium is expected to form a stable meniscus across such a cesium-coated hole. The observed depths of suspended helium are consistent with a simple theoretical model based on this picture. We briefly discuss the possible application of this method to the performance of a proposed experiment to study quantum coherence in superfluid helium by directing pulsed beams of helium atoms at such a suspended layer of fluid. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physrevb.53.6627

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Physical Review Letters•Apr 3, 1995•54 citations
We report results of an experiment which demonstrates that a layer of superfluid helium can be suspended over a cesium-coated orifice. By measuring the layer thickness with a capacitance bridge, we have shown in two runs that fluid layers up to 2 mm thick were suspended over a 70-\ensuremath{\mu}m-diam cesium-coated orifice in a platinum foil for over 2 h in a cryostat held at 1.2 K. The effect depends on the recently established fact that superfluid helium does not wet cesium-coated surfaces. As a consequence, superfluid helium is expected to form a stable meniscus across such a cesium-coated hole. The observed depths of suspended helium are consistent with a simple theoretical model based on this picture. We briefly discuss the possible application of this method to the performance of a proposed experiment to study quantum coherence in superfluid helium by directing pulsed beams of helium atoms at such a suspended layer of fluid. \textcopyright{} 1996 The American Physical Society.
Two chambers filled with superfluid helium to different levels were connected by a small orifice (10-15 \ensuremath{\mu}m). The flow of the liquid through the orifice was modulated by a sound field. With one experimental apparatus only a continuous flow was observed. With a modified apparatus it was possible to obtain stable states with no net flow at finite level differences between the baths. The spacing between these states is inversely proportional to the sound frequency. It depends on the total height of the liquid in one of the baths and not on the level difference between them. We conclude that the stable states of zero flow result from ultrasonic standing waves of the superfluid in one of the chambers. A phase coupling or Josephson effect can be excluded as an explanation for these states.
We describe a measurement of the transmission probability of 4He atoms through a freely suspended slab of superfluid 4He at low temperatures. In our experiment the slab is realized by using an array of parallel cylindrical holes of diameter 51 microm in a glass disc of thickness 190 microm. By controlling the chemical potential, the holes can be made to fill or empty with liquid, and the surface curvature varied. We have measured the transmission of atom beams, generated by a thin-film heater and detected with a sensitive bolometer, through this structure. The results show that the dominant transmission channel is atom-R+ roton-atom with a probability p approximately 0.12 and that R+ rotons can undergo total internal reflection at the free liquid surfaces.
Low dimensional quantum fluids, where one can probe the effects of enhanced thermal and quantum fluctuations on macroscopic quantum wavefunctions, can be experimentally realized through transverse physical confinement of superfluid helium on scales smaller than the coherence length. Reaching this scale is difficult, requiring confinement in single or multiple pores with nanometer radii. Porous silicates such as MCM-41 have a pore radius larger than the coherence length of 4He, and in this work we systematically explore the possibility of pre-plating pores with different elements to reduce the pore size without localizing the confined superfluid. Through a direct solution of the few-body Schrodinger equation combined with quantum Monte Carlo simulations, we explore the behavior of helium confined inside cylindrical nanopores for a range of pre-plating elements, including rare gases and alkali metals. For rare gases, we find that helium remains strongly attracted to the pore walls and any atoms in the core form an incompressible liquid. For alkali metals such as Cs, weak interactions between helium and the pre-plating material prevent localization near the walls and enable delocalization in the pore center. Our results extend previous results for helium wetting on flat two dimensional coated substrates to the curved geometry inside nanopores, and demonstrate that alkali pre-plated nanopores may enable a tunable one-dimensional confined quantum liquid of helium.
We present a phenomenological analysis of an experiment to observe the condensate fraction of helium-4 by firing low-energy pulses of helium atoms at suspended droplets of the superfluid and observing the resulting emission of helium atoms from the fluid. Emission occurs through a conventional process in which rotons are produced and propagate to the other side of the droplet, and through a second process depending on the existence of the condensate. If A is the area of the incoming beam, the cross section scales with A2{\mathit{A}}^{2}A2 (first process) and A4{\mathit{A}}^{4}A4 (second process), respectively, for small A. Aspects of a fully many body calculation and the performance of such an experiment are briefly discussed.
When a chemical potential difference exists between the superfluid helium on two sides of an orifice, superfluid flow takes place at a critical velocity accompanied by the creation of quantized vortices. The frequency with which vortices are created is expected to be equal to the chemical potential difference (the analog of the ac Josephson frequency in superconducting tunneling). An experiment is described in which this frequency condition is verified by a method analogous to that used in superconductors. Experimental conditions were selected such that the gravitational head difference mgZ\mathrm{mgZ}mgZ was the dominant contribution to the chemical potential difference. ZZZ was measured by a capacitance technique. An ultrasonic transducer was used to modulate the flow through the orifice at the frequency $\ensuremath{\omega}$ so that the rate of creation of vortices was synchronized with the modulation frequency. The system was observed to exhibit dynamic stability at values of head difference $Z=\frac{n\ensuremath{\hbar}\ensuremath{\omega}}{{n}^{\ensuremath{'}}\mathrm{mg}}$ corresponding to nnn quantized vortices created every ${n}^{\ensuremath{'}}$ cycles of the modulation. The correspondence between theory and experiment is most convincing under experimental conditions such that the strength of the stability decreases rapidly with increasing values of the integer ${n}^{\ensuremath{'}}$. This was found to occur with small orifices and with moderate modulation amplitude, in agreement with the results of the analogous experiments in superconductors.
The parabolic superfluid meniscus has been seen for states in which only (1) the normal or (2) the superfluid component of 3{}^{3}3He- BBB rotates. A reduced, temperature-dependent meniscus (1) was formed at small speeds $\ensuremath{\Omega}\ensuremath{\lesssim}0.21$ rad ///s. A deeper-than-expected meniscus (2) was observed after a rapid halt of the cryostat, when the normal fluid stops during a short relaxation time, followed by slow decay of the superfluid circulation. The depth of the meniscus (2) was explained by a reactive radial force between the rotating superfluid and the stationary normal liquid.