NMR Time Reversal Experiments in Highly Polarised Liquid 3He-4He Mixtures

Long-range magnetic interactions in highly magnetised liquids (laser-polarised ³He-⁴He dilute mixtures at 1 K in our experiment) introduce a significant non-linear and non-local contribution to the evolution of nuclear magnetisation that leads to instabilities during free precession. We recently demonstrated that a multi-echo NMR sequence, based on the magic sandwich pulse scheme developed for solid-state NMR, can be used to stabilise the magnetisation against the effect of distant dipolar fields. Here, we report investigations of echo attenuation in an applied field gradient that show the potential of this NMR sequence for spin diffusion measurements at high magnetisation densities.      

3He Impurities in 4He Systems Adsorbed on Curved Substrates

It is shown that ³He impurities in sufficiently large ⁴He systems adsorbed onto substrates with curved geometries form surface bound states, analogous to the Andreev state on a planar liquid--vapor interface. We report the analysis performed for superfluid ⁴He adsorbed on the external surface of the nano-fullerene C₆₀ and on cylindrical nano-wires of Au. It is found that a single ³He impurity diluted into such adsorbed structures behaves as on films on planar substrates and as on pure ⁴He clusters.     

Development of a low-temperature He compressor for superfluid He-He mixtures

The development, testing and modeling of a “compressor” that is capable of increasing the concentration of the ³He component of a liquid superfluid ³He-⁴He mixture is discussed. This compressor was developed to drive refrigeration cycles for cooling below 1 K. The compressor design and performance testing is described in detail. The compressor was operated at 1.2 K and ³He molar flow rates of 130 μmol/s were achieved. Compression ratios in excess of 6 were also demonstrated. The theoretical models presented are used to estimate the expected efficiencies of the compressor as well as the effect of the ⁴He component on the power required to drive the compressor.     

Measurements of the Superfluid Joule–Thomson Refrigerator Using High Concentration 3He–4He Mixtures

The superfluid Joule–Thomson refrigerator (SJTR) uses a liquid superfluid ³He–⁴He mixture to provide cooling below 1 K. Performance measurements of the SJTR using 5% and 11% ³He concentration mixtures are reported. High concentration operation shows higher cooling powers at high temperature. Ultimate temperatures are seen to increase with increasing concentration due to a pinching of the temperature defect in the recuperative heat exchanger. This pinching effect is due to the variation of the heat capacity of the ³He–⁴He mixture with temperature and concentration and is discussed in detail and design changes are suggested to mitigate it.     

Temperature and Concentration Relaxation in Phase-separated Superfluid 3He–4He Mixtures

The kinetics of the temperature and concentration variations in the superfluid ³He–⁴He mixtures with initial concentration of 9.8% ³He, and heated from below, was studied experimentally under the pressure of 0.38 bar over a temperature range of 150–400 mK. It is found that in contrast to homogeneous liquids, the temperature and concentration relaxation in phase-separated mixtures can be described by a superposition of two exponential processes in which the time constants of temperature and concentration variations coincide. If the initial mixture was homogeneous and phase separation was triggered by a heat flow, the temperature and concentration vary non-monotonically and exhibit anomalous features at the moment of phase separation. In this case the phase transition starts in the metastable superfluid, formed out of a quite supersaturated mixture where the nucleation of the new phase may be caused by quantized vortices. The results are analyzed in terms of two possible mechanisms of relaxation–the acoustic mechanism with the second sound velocity and the diffusive one connected with dissipative flows of impurity and thermal excitations. It is shown that the measured relaxation times agree with a prediction of the theory.      

Quantum Monte Carlo

Progress has been made in the development and applications of quantum Monte Carlo methods for calculations of many-body systems over the past year. Significant advances in methodology, such as new forms of trial wave functions, improved calculations of periodical systems and algorithm developments, have been commented upon, as have applications which include jellium models, H, He, molecular, cluster and solid systems. These results demonstrate that the influence and usefulness of quantum Monte Carlo methods are increasing rapidly.     

Energy Loss of Fourth Sound in Superfluid 3He within Small Pores

We performed a high-sensitivity fourth sound resonance experiment for pure superfluid ³He at 29.0 bar in sintered silver powder cell to clarify the hydrodynamic property of ³He in the aerogel-sintered silver system. We discuss the energy loss of fourth sound. An anomaly in the energy loss at the AB phase transition as found in the aerogel system was not observed. Our analysis shows that the energy loss Q ⁻¹, which monotonically decreases with decreasing temperature, can be understood by the hydrodynamic theory in the B phase qualitatively and quantitatively. We estimated the effective pore radius R in the powder cell.      

Path-Integral Monte Carlo Study of Phonons in the bcc Phase of 3He

Using Path Integral Monte Carlo and the Maximum Entropy method, we calculate the dynamic structure factor of solid ³He in the bcc phase at a finite temperature of T = 1.6 K and a molar volume of 21.5 cm³. From the single phonon dynamic structure factor, we obtain both the longitudinal and transverse phonon branches along the main crystalline directions, [001], [011] and [111]. Our results are compared with other theoretical predictions and available experimental data.     

Density Equation for Saturated <Superscript>3</Superscript>He

A density equation for saturated vapor and liquid ³He is presented based on 205 experimental measurements for temperatures greater than 0.2 K collected after a careful survey of the literature. The average deviation of the densities predicted by the equation against the experimental values is 0.39%. There are only 16 points with deviations larger than 1%. This equation is valid for both liquid and vapor densities of ³He up to the critical temperature of 3.3157 K. The form of the equation satisfies known scaling laws approaching the critical point, with β=0.3653. In the low-density limit, the vapor curve of our equation matches smoothly to the published virial equation density at a temperature of 1.62 K and at the saturation pressure. The rectilinear density deviates from the critical density by less than 0.28% down to 0.48 K.     

Scaling Results for Superfluid 3He in 98% Open Aerogel

We present experimental observations of the suppressed superfluid transition temperature, T _ca , superfluid fraction, ρ _s /ρ and Leggett frequency of ³He-B in aerogel, Ω _Ba . We determine T _ca from mass decoupling and the vanishing of the frequency shift away from the Larmor frequency in our different samples and different laboratories. We find that the suppressed transition temperature for ³He in aerogel occurs at a sample dependent, but approximately pressure independent, length, , where T _c and ξ ₀(P), are the transition temperature and the pressure dependent zero temperature coherence length for bulk ³He. T _ca also occurs at a pressure independent value of the Leggett frequency of bulk ³He-B. Further, we find that when the superfluid fraction and square of the Leggett frequency are plotted against T _ca −T (and not (T _ca −T)/T _ca ), the results of each measurement nearly collapse on to a pressure independent but sample dependent plot, with no further scaling. When plotted on a log–log scale, both measurements exhibit power laws in the range 1.33–1.45.      

The Effect of 3He Impurities on Anomalous Oscillations in the Expansion of Solid 4He

The regular periodic intensity bursts recently observed in the expansion of solid ⁴He into vacuum have anomalies that have been attributed to a transition to some new solid phase induced by excess vacancies. Here it is shown that a small concentration of ³He, from 1% down to 0.1%, added to the ⁴He solid is sufficient to remove all the anomalies. The origin of the observed anomalies is discussed in the light of these new experiments.      

Second Sound Contribution to Temperature Gradient Evolution in Superfluid Mixtures

Starting from the full system of hydrodynamic equations for helium isotopic mixtures, the problem of temperature and concentration relaxation is solved. The limiting case of ultralow temperatures, when the contribution of thermal excitations can be neglected, is considered. A comparison with the experimental data is carried out.      

Quantum Fingering of the Inverted Liquid-Crystal 4He Interface in the Field of Gravity

The simplest example of fingering occurs when a heavier liquid occupies the half-space above a lighter one. The fingers of the heavier liquid driven by gravity grow down, displacing the lighter liquid upward. The fingering of the superfluid phase into the ⁴He crystal facet below its roughening transition temperature can provide us an example of macroscopic quantum nucleation phenomena. Due to cusp-like singularity in the surface tension as a function of the facet orientation the crystal facet has a relative stability and its fingering is accompanied by overcoming some critical nucleation barrier. The barrier height is proportional to the square of the facet step energy and depends on a ratio of the facet size to the capillary length. At sufficiently low temperatures the thermal activation mechanism for the interface fingering is replaced with the quantum one associated with the penetration through the nucleation barrier.      

Neutron Diffraction Study of He Solidified in a Mesoporous Glass

He embedded in a porous glass has been studied by neutron diffraction. Along the isochoric path studied, about 30% of the pore filling solidify in the bcc structure at 1.7 K and remain in this state down to 60 mK, the minimum temperature of the experiment. The other part of the pore filling is either liquid or an amorphous solid. The behaviour is discussed in terms of a thermodynamic model.     

The solubility of spin-polarized3He in4He and the superfluidity of3He in3He-4He mixtures

We investigate the possibility of a large enhancement of the T = 0 finite solubility of³He in⁴He due to spin-polarization. The size of the effect depends on the fraction of³He atoms in the system. We present two different approaches for the limits of a small and a large number of³He atoms compared to the number of⁴He atoms. Since the possible³He superfluid phase transition depends on³He density, we calculate the consequences of this change in the solubility for its superfluid transition temperature. It is shown that for small fractions of³He, the transition temperature is enhanced mostly due to the enlargement of the up-spin Fermi sphere. In the opposite limit the transition temperature is enhanced as a result of the increased³He solubility.     

Long-Range Order in the A-like Phase of Superfluid 3He in Aerogel

A mutual action of the random anisotropy brought in the superfluid ³He by aerogel and of the global anisotropy caused by its deformation is considered. Strong global anisotropy tends to suppress fluctuations of orientation of the order parameter and stabilizes ABM order parameter. In a limit of vanishing anisotropy fluctuations of ABM order parameter became critical. It is argued that still in a region of small fluctuations the order parameter changes its form to be less sensitive to the random anisotropy. For a favorable landscape of the free energy of superfluid ³He the fluctuations remain small even in a limit of vanishing global anisotropy and the long-range order is maintained.      

Magnetic Distortion of the B-like Phase of Superfluid 3He Confined in Aerogel

We present measurements of the response of the B-like phase of superfluid ³He in aerogel to an applied flow. The measurements are made using a cylindrical piece of 98% silica aerogel attached to a vibrating wire resonator. The resonator is immersed in superfluid ³He at 16 bar pressure and at low temperatures. A variable magnetic field is applied such that the aerogel-confined superfluid may exist in the A-like or B-like phase, while the surrounding fluid is always in the bulk B-phase. The resonator response reveals a velocity dependence of the inferred aerogel-confined superfluid fraction. We discuss measurements of the temperature and magnetic field dependence of the response in the B-like phase. We find a significant field dependence indicating a strong magnetic distortion of the B-like phase order parameter.      

Physical Mechanisms for Effective Mass Enhancement in 3He

We use structural information from simulations and from variational ground state calculations for calculating the effective mass of ³He at zero temperature. It is found that the relatively large effective mass is due to a combination of several physical effects: Density fluctuations cause an effective mass enhancement due to predominantly hydrodynamic backflow. This effect is, around the Fermi momentum, a smooth function of the single particle wave number; its magnitude is consistent with the effective mass of ⁴He impurities in ³He. Spin-fluctuations, on the other hand, cause a pronounced peak of the effective mass around the Fermi wave number. We also find, consistent with earlier work, an instability of the single particle spectrum at about 2.5 k _F, this is due to the coupling to density fluctuations in the maxon region.     

Frictional Effects in Thin 3He Films

The recent high-precision torsional oscillator experiments of Casey et al. involving thin films of normal liquid ³He showed that the film decouples from the substrate with a time constant which is proportional to T ⁻¹ where T is the absolute temperature. We interpret this experiment by adapting a theory due to Meyerovich which was developed for dilute ³He-⁴He mixtures flowing between two relatively smooth plates. The analysis of the experiment confirms the central idea that varies as T ⁻¹. The variation of with film thickness, d, is affected by the change in the shape of the free surface of the film, due to van der Waals forces, as the film becomes thinner.