[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 authors compare the wetting behavior of helium isotopic mixtures on the weak binding substrates cesium, rubidium, potassium and sodium. The ability to vary the chemical potentials of ³He and ⁴He independently as well as the substrate potential leads to a wide variety of interesting new phenomena. The complete phase diagrams will be presented. Among the new phenomena predicted are a prewetting transition in the superfluid film under bulk concentrated ³He on Na, and the absence of a superfluid film at the surface of Cs, Rb and K, leading to the possibility of forming supersaturated solutions of ⁴He in ³He. Possible experimental tests are discussed

[en] The Zharkov-Silin Fermi Liquid theory of solutions of ⁴He in normal (non-superfluid) liquid ³He is reviewed and slightly extended. The theory is expected to be valid only below ∼ 0.1 K, and it predicts that there should be a hundred-fold increase in the diffusion coefficient as the temperature is lowered into this region. The limited range of validity explains the apparent disagreement between the recent very low temperature measurements of the phase separation line by Nakamura et al. and extrapolations from higher temperatures. In the low temperature experiments the ⁴He concentration X₄ is so small that there is no macroscopic phase separation, only a gradual thickening of the ⁴He-rich film on the walls. The authors confirm that the phase separation temperature T_ps(X₄) including the new data show that the ⁴He effective mass m₄^* is close to, and may be equal to, the bare mass m₄. The difference in binding at zero pressure between ⁴He in liquid ⁴He and in liquid ³He is (E₄₄-E₄₃)/k_B = (0.21 + 0.03/-0.01) K. Using the volume measurements of Laheurte to calculate the pressure dependence of E₄₃ indicates that the difference in binding has a minimum of (0.0 ± 0.2) K near ∼ 11 atm. This implies that the solubility of ⁴He in ³He is enhanced in this region of pressure. The behavior of the spinodal line at low temperature, and the possibility of observing Bose condensation in a metastable solution of ⁴He in liquid ³He are also discussed. 31 refs., 6 figs

[en] The Zharkov-Silin Fermi Liquid theory of solutions of ⁴He in non-superfluid liquid ³He has been applied to the recent phase separation data of Nakamura et al. At zero pressure, the difference in binding between a ⁴He atom in liquid ⁴He and in liquid ³He is smaller than previous estimates, and the ⁴He effective mass is close to the bare mass. The volume measurements of Laheurte show that the difference in binding has a minimum near ∼11 atm. This implies an enhanced solubility of ⁴He in ³He below 0.1 K at this pressure, although there is experimental evidence that the solubility at 0 K remains zero

[en] The authors examine the effect of ³He impurities on the wetting behavior of ⁴He on cesium, predicting a phase diagram which includes reentrant wetting transitions. This phase diagram is shown to be very sensitive to effects such as a theoretically predicted bound state of ³He at the liquid-cesium interface, and the contact angle may be sensitive to interesting temperature dependences of the helium-cesium surface tension resulting from surface rotons or Rayleigh waves

[en] The authors give a brief survey of experiments that have been performed to study the nucleation of bubbles (cavitation) in liquid helium at negative pressures. There have been two principal motivations for research in this field. Because all impurities (except ³He) freeze out of the liquid at low temperatures, it is possible to prepare helium with a much higher purity than ordinary classical liquids. In any study of a nucleation process this is an important advantage because impurities introduce the complication of heterogeneous nucleation. The second reason for interest in helium is that at low enough temperatures nucleation is expected to be dominated by quantum tunnelling rather than thermal activation

[en] From available information concerning the wetting behavior of pure ³He and pure⁴He on alkali metal substrates, as well as the known properties of bulk ³He-⁴He mixtures, the complete wetting phase diagrams for such mixtures, showing prewetting, isotopic separation and lambda transitions for the film phases have been derived. We predict new phenomena such as a triple-point induced dewetting transition, and the absence of a superfluid film wetting Cs, Rb and K walls under concentrated ³He solutions

[en] Mobile interacting quantum-mechanical kinks of very low mass explain the roton contribution to the growth resistance and the thermal phonon transmission coefficient of the rough surface of ⁴He crystals

[en] Since free electrons form small voids in liquid helium they are expected to be preferred sites for nucleating macroscopic bubbles when the liquid is exposed to sufficiently large negative pressures. We have performed a series of cavitation experiments using focussed ultrasound where free electrons were introduced into the liquid by a radioactive source. The electron bubbles are found to explode at negative pressures significantly lower than those required for homogeneous nucleation. We present measurements of the thresholds for cavitation at electrons in the temperature range 1 - 4.5 K. Reasonable agreement with a simple model for the stability limit of the electron bubble is obtained. (author)

[en] We present the results of experiments to study cavitation in liquid ⁴He at negative pressures. A focused sound wave is used to generate a negative pressure in a small region of the liquid, and the cavitation is detected by light scattering. We have studied the cavitation as a function of the temperature and static pressure applied to the liquid. We are able to observe the statistical nature of the cavitation process, and to determine the nucleation barrier and the attempt frequency. The results for the attempt frequency are lower than expected for homogeneous nucleation. We discuss the possibility that the nucleation is occurring on quantized vortices. The tensile strength at constant pressure decreases slowly with increasing temperature in the range 0.8 to 1.5 K, drops rapidly as the temperature approaches the λ point, and then decreases slowly above T_λ