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Physica B Condensed Matter

Measurements of the atomic kinetic energy of 4He close to the superfluid transition

J. Mayers, F. Albergamo, David N. Timms

Published Mar 1, 2000• 15 citations
Publisher
DOI
10.1016/s0921-4526(99)01336-8

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Self-collimation in an atomic beam evaporated from a superfluid 4He film

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Using a scaling Monte Carlo method, we have simulated the scattering in a pulsed atomic beam evaporated from a superfluid 4He film. The simulation assumes that the atoms leaving the surface have the equilibrium Maxwellian distribution at the temperature of the film T. This means that the initial particle flux varies as cos straight theta (Lambert's law). We find that the effect of atomic scattering just above the film is to bias the flux in favor of the forward (small straight theta) direction, in agreement with the experiment of Eckardt et al. The simulation predicts that the deviation from Lambert's law grows rapidly with increasing heat input Q and decreasing pulse length. At the same time, the average kinetic energy per particle in the forward direction is enhanced relative to the global average, 2k(B)T. The distribution with respect to speed is narrower than Maxwellian in the forward direction but broader at large angles. We find that a beam that has passed through a slit is slightly narrower than in a ballistic calculation with no collisions. This effect seems to saturate at values of Q that correspond to 30 or 40 collisions per atom.

Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics•Dec 1, 2000•1 citation
The binding energy of a hydrogen atom to the surface of liquid 4He has been calculated to be at least 0.6K by Mantz and Edwards. This means that the properties of adsorbed spin-polarized hydrogen (H↑), a 2D Bose fluid, may soon be observed experimentally. Since 4He is used as a neutral coating for vessels to contain H↑ gas, adsorption onto the 4He surface may enhance the rate of recombination into H2 molecules. For given temperature and density in the gas, the amount of H↑ adsorbed depends strongly on the H↑-H↑ interaction on the 4He surface. We estimate the interaction in a simple approximation, and find that it limits the surface density to not more than 1014 cm-2. Nevertheless this density should be sufficient for a 2D superfluid transition to be observed, under conditions close to those achieved experimentally.
We review fundamental problems involved in liquid theory including both classical and quantum liquids. Understanding classical liquids involves exploring details of their microscopic dynamics and its consequences. Here, we apply the same general idea to quantum liquids. We discuss momentum condensation in liquid helium which is consistent with microscopic dynamics in liquids and high mobility of liquid atoms. We propose that mobile transit atoms accumulate in the finite-energy state where the transit speed is close to the speed of sound. In this state, the transit energy is close to the oscillatory zero-point energy. In momentum space, the accumulation operates on a sphere with the radius set by interatomic spacing and corresponds to zero net momentum. We show that this picture is supported by experiments, including the measured kinetic energy of helium atoms below the superfluid transition and sharp peaks of scattered intensity at predicted energy. We discuss the implications of this picture including the macroscopic wave function and superfluidity.
Using a scaling Monte Carlo method, we have simulated the scattering in a pulsed atomic beam evaporated from a superfluid 4He film. The simulation assumes that the atoms leaving the surface have the equilibrium Maxwellian distribution at the temperature of the film T. This means that the initial particle flux varies as cos straight theta (Lambert's law). We find that the effect of atomic scattering just above the film is to bias the flux in favor of the forward (small straight theta) direction, in agreement with the experiment of Eckardt et al. The simulation predicts that the deviation from Lambert's law grows rapidly with increasing heat input Q and decreasing pulse length. At the same time, the average kinetic energy per particle in the forward direction is enhanced relative to the global average, 2k(B)T. The distribution with respect to speed is narrower than Maxwellian in the forward direction but broader at large angles. We find that a beam that has passed through a slit is slightly narrower than in a ballistic calculation with no collisions. This effect seems to saturate at values of Q that correspond to 30 or 40 collisions per atom.