[en] The specular reflection of low energy phonons at the free surface of liquid ⁴He at ∼ 0.05 K has been measured for angles of incidence between 20 degrees and 80 degrees. The distribution of the incident beam with respect to energy and direction is obtained from the theory of phonon decay using the curvature of the phonon spectrum measured by Rugar and Foster. The average incident phonon energy, at a distance of 2.3 cm from the source, is ∼ 0.3 K, which corresponds to an effective beam temperature of about 0.1 K. The reflection data are compared with the theory of Saam, modified to take into account the thermal ripplons at the surface. The theory predicts that the deviations from specular reflection are too small to be measurable. However there should be a broadening of ∼ 1 degrees in the reflected image of the phonon source (a heater) due to phonon decay. In agreement with the theory, the reflection is specular within the angular accuracy of ≤ 2 degrees. The reflection coefficient is unity within the experimental error of about 10%. The experiment was performed on liquid with the free surface containing 0.034 of a monolayer of ³He, however no effect from the ³He impurity was observed

[en] Using Landau's theory of elementary excitations together with a simple model for the interactions between excitations, we calculate the thermodynamic properties of superfluid helium-4 at pressures below the saturated vapor pressure and above the normal freezing temperature

No abstract available

[en] A.F. Andreev first noted, in 1964, that quasiparticles reflected from a boundary in a superconductor could have all three velocity components change sign. Later, I. lguchi, G. Ihas, and others have calculated a similar effect for excitations in superfluid ⁴He. The authors have observed the Andreev reflection of rotons with positive group velocity (R⁺) to rotons with negative group velocity (R^-). Assuming conservation of energy and transverse momentum, rotons from point source (a heater) which are Andreev reflected at the free surface are focussed on to a line perpendicular to the surface and passing through the heater. A bolometer moving parallel to the surface, across the focussing line, detects a peak in the signal intensity. Both bolometer and heater (which can be used interchangeably) were made of graphite strips on which fibrous polyaniline was grown. The polyaniline was intended to increase the roton creation and detection efficiencies. The authors have not yet observed the reverse process, the reflection of R^- to R⁺

[en] The growth coefficient K (the velocity of growth per unit chemical potential difference) and the Onsager cross-coefficients b¹ and b², coupling growth and heat flow, are calculated for atomically rough surfaces of hcp ₄He crystals. The calculation is based on the premise, suggested by Andreev and Parshin, that growth is limited by the collision of phonons and rotons with the interface. The calculated K is compared with that obtained by Keshiskev et al. from the damping of melting-freezing waves. The theory assumes that the excitations are in the ballistic regime where their mean free path is large compared to the wavelength of the melting-freezing waves. In the experiment only the phonons satisfy this condition, yet the theory agrees with the data even when roton scattering is important. Irreversible thermodynamics requires that the cross coefficients b¹ and b² be equal. This is shown by direct calculation. The value of b¹ and b² depends on the ratio of two integrals over the phonon transmission coefficient and it is evaluated for two models of the transmission. The theory agrees fairly well with a recent measurement of b¹. A calculation of the dissipation in the hydrodynamic regime, where the free path is short, shows that the damping of melting-freezing waves should have a different dependence on frequency compared to the ballistic regime

[en] The excitations produced when a pulsed ⁴He atomic beam strikes the surface of liquid ⁴He held near T approximately OK have been measured as a function of direction in the liquid and time elapsed since the beam was generated. Comparison of the data with a theory which assumes that a single atom is converted into a single excitation indicates that the observed signals are due to ballistic phonons and rotons, although the experimental velocity and angular distributions of the excitations are displaced from the theoretical ones. The direction of the displacements suggests that multiple ripplon production occurs as an atom approaches the liquid surface, as predicted by Echenique and Pendry

No abstract available

[en] Measurements of surface tension and of the velocity of surface sound have shown that less than a monolayer of ³He adsorbed on the surface of superfluid ⁴He behaves like a two-dimensional gas of ³He ''quasiparticles'' with an effective mass M about 1.3 times the real mass m₃. The interaction potential between the surface quasiparticles has been found to be quite weak. When interpreted in terms of an ''effective interaction'' V/sup s/(q), the analog of the BBP² interaction V(q) between ³He dissolved in bulk ⁴He, the data give: V/sup s/(q = 0) = +(0.5 +- 2.0) x 10^-31 erg cm². The sign and magnitude of V/sup s/(q) are important because they determine whether the adsorbed ³He will become a two-dimensional BCS superfluid at a sufficiently low temperature. Some naive theoretical arguments are presented to augment information about V/sup s/(q) and the possibility of superfluidity

[en] Using a heater and bolometer as source and detector, the reflection of a pulsed beam of phonons at the free surface of liquid ⁴He at ∼0.05 K has been measured for angles of incidence between 30 degrees and 80 degrees. The energy distribution of the incident beam was obtained from the theory of phonon decay in a companion paper. The average incident phonon energy was ∼0.3 K, corresponding to an effective beam temperature of about 0.1 K. Both heater and bolometer were made from graphite resistor board with a layer of polyaniline fibers to increase the absorptivity for rotons. The angular distribution of the phonons from the heater is quite broad, approximately (cosⁿθ + cos^mθ)/2 with n∼0.65 and m∼3.4. However, the receiver response has a broad component with n ∼ 2.2 and an extraordinarily narrow one with m ∼ 106. In agreement with theory, the reflection appears to be specular within the accuracy of the experiment. The reflection coefficient is unity within the experimental error; the weighted mean value is 1.001 ± 0.025. During the experiment, the free surface became contaminated with ∼0.034 of a monolayer of ³He, but no effect from the ³He impurity was observed

[en] The properties of ³He films on a Nuclepore substrate have been measured by pulsed NMR at a Larmor frequency of 10 MHz between 1.3 and 4.2 K. The ³He film thickness was varied from 0.14 to 2 layers. The spin-spin relaxation time T₂ agrees well with previous measurements of ³He films on Mylar and Vycor glass at low temperatures. The spin-lattice relaxation time T₁ for submonolayer films shows a strong temperature dependence consistent with a thermally activated process. This behavior has not previously been observed on amorphous substrates. The spin diffusion coefficient was measured for the thickest films at 4.2 and 2.6 K and found to be consistent with free atom motion of the ³He in the vapor. In thin films or at low temperatures, the diffusion was too small to be observed. The magnetic coupling between the ³He nuclei in a film and the protons in the Nuclepore substrate was determined from the effect of the ³He on the proton-lattice relaxation time. It is about 100 times weaker than the interaction between ³He and the fluorine nuclei in a Teflon substrate