Influence of the liquid helium meniscus on neutron reflectometry data

Neutron reflectometry offers a unique opportunity for the direct observation of nanostratification in ³He–⁴He mixtures in the ultra-low temperature limit. Unfortunately the results of recent experiments could not be wellmodelled on account of a seemingly anomalous variation of reflectivity with momentum transfer. We now hypothesize that this effect is attributable to an optical distortion caused by the liquid’s meniscus near the container wall. The validity of this idea is tested and confirmed through a subsidiary experiment on a D₂O sample, showing that the meniscus can significantly distort results if the beam size in the horizontal plane is comparable with, or bigger than, the diameter of the container. The meniscus problem can be eliminated if the beam size is substantially smaller than the diameter of the container, such that reflection takes place only from the flat region of the liquid surface thus excluding the meniscus tails. Practical measures for minimizing the meniscus distortion effect are discussed

Mutual interactions between objects oscillating in isotopically pure superfluid ⁴He in the T → 0 limit

We report the results of experiments to explore interactions between physically separated oscillating objects in isotopically pure superfluid ⁴He at T ∼ 10 mK. The investigations focused mainly on 32 kHz quartz tuning forks, but also consider a nearby 1 kHz oscillating grid. The low-drive linewidth (LDL) and resonant frequency fd of a detector fork were monitored while the maximum velocity of a transmitter fork, separated from the detector by a few mm, was varied over a wide range. Clear evidence was found for mutual interactions between the two forks, and for the influence of the grid on the forks. Monitoring the detector's LDL and fd provides evidence for a generator critical velocity in the range 0.3 < vc₁ <1.0 cm/s for onset of the detector responses, in addition to a second critical velocity vc₂ ∼ 13 cm/s probably corresponding to the production of quantum turbulence at the generator. The results are discussed, but are not yet fully understood

Neutron reflection from a liquid helium surface

The reflection of neutrons from a helium surface has been observed for the first time. The ⁴He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state we also observe a surface layer ~200 Å thick which has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose–Einstein condensate fraction close to the helium surface. The application of neutron reflectometry described in this paper creates new and exciting opportunities for the surface and interfacial study of quantum fluids

Observation of acoustic turbulence in a system of nonlinear second sound waves in superfluid ⁴He

We discuss the results of recent studies of acoustic turbulence in a system of nonlinear second sound waves in a high-quality resonator filled with superfluid ⁴He. It was found that, when the driving amplitude was sufficiently increased, a steady-state direct wave cascade is formed involving a flux of energy towards high frequencies. The wave amplitude distribution follows a power law over a wide range of frequencies. Development of a decay instability at high driving amplitudes results in the formation of subharmonics of the driving frequency, and to a backflow of energy towards the low-frequency spectral domain, in addition to the direct cascade

Nonlinear and shock waves in superfluid He II

We review studies of the generation and propagation of nonlinear and shock sound waves in He II (the superfluid phase of ⁴He), both under the saturated vapor pressure (SVP) and at elevated pressures. The evolution in shape of second and first sound waves excited by a pulsed heater has been investigated for increasing power W of the heat pulse. It has been found that, by increasing the pressure P from SVP up to 25 atm, the temperature Tα, at which the nonlinearity coefficient of second sound reverse its sign, is decreased from 1.88 to 1.58 K. Thus at all pressures there exists a wide temperature range below Tλ where α is negative, so that the temperature discontinuity (shock front) should be formed at the center of a propagating bipolar pulse of second sound. Numerical estimates show that, with rising pressure, the amplitude ratio of linear first and second sound waves generated by the heater at small W should increase significantly. This effect has allowed us to observe at P 133. atm a linear wave of heating (rarefaction) in first sound, and its transformation to a shock wave of cooling (compression). Measurements made at high W for pressures above and below the critical pressure in He II, Pcr 22. atm, suggest that the main reason for initiation of the first sound compression wave is strong thermal expansion of a layer of He I (the normal phase) created at the heater-He II interface when W exceeds a critical value. Experiments with nonlinear second sound waves in a high-quality resonator show that, when the driving amplitude of the second sound is sufficiently high, multiple harmonics of second sound waves are generated over a wide range of frequencies due to nonlinearity. At sufficiently high frequencies the nonlinear transfer of the wave energy to sequentially higher wave numbers is terminated by the viscous damping of the waves

Dynamics of ions in the selectivity filter of the KcsA channel

The statistical and dynamical properties of ions in the selectivity filter of the KcsA ion channel are considered on the basis of molecular dynamics (MD) simulations of the KcsA protein embedded in a lipid membrane surrounded by an ionic solution. A new approach to the derivation of a Brownian dynamics (BD) model of ion permeation through the filter is discussed, based on unbiased MD simulations. It is shown that depending on additional assumptions, ion’s dynamics can be described either by under-damped Langevin equation with constant damping and white noise or by Langevin equation with a fractional memory kernel. A comparison of the potential of the mean force derived from unbiased MD simulations with the potential produced by the umbrella sampling method demonstrates significant differences in these potentials. The origin of these differences is an open question that requires further clarifications

RINGS AMONG THE ROTONS : MEASUREMENTS OF THE VORTEX NUCLEATION RATE FOR NEGATIVE IONS WHOSE DRIFT VELOCITIES ARE LIMITED BY ROTON EMISSION

Nous avons mesuré les fréquences de nucléation ν d'un anneau vortex pour des ions négatifs se déplaçant dans l'He II sous l'influence de champs électriques 0,8 5 kV cm-l, les résultats expérimentaux sont bien décrits par une théorie due à Bowley. Une dépendance anormale de ν en température trouvée pour de petites valeurs de E est attribuée à l'influence d'impuretés isotopiques.We have measuted vortex ring nucleation rates ν for negative ions moving in He II under the influence of electric fields 0.8 5 kV cm-l, the experimental data are well described by a theory due to Bowley. An anomalous dependence of ν on T found at lower lower E is attributed to the influence of isotopic impurities