Dilute 3He-4He Mixtures Research Articles

Simulations of Non-linear Spin Dynamics in Spin Polarized Dilute 3He–4He Mixtures

Studies of spin dynamics of highly polarized dilute mixtures of 3He in superfluid 4He have been performed by various researchers over the past three decades. One series of experiments performed at Cornell University in the early 1990’s revealed a novel long timescale excitation. We present the numerical solution of the non-linear Leggett spin dynamics equation in one spatial dimension subject to boundary conditions consistent with the Cornell experiments. Experimentally observed phenomena are composed of trains of bursts in the transverse magnetization lasting several seconds. The simulations capture the time evolution of the individual bursts localized in time. Preliminary results of two dimensional simulations are also presented.

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 $$\tau_{\mathrm{osc}}$$ 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 $$\tau_{\mathrm{osc}}$$ varies as T −1. The variation of $$\tau _{\mathrm{osc}}$$ 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.

Slip in Vibrating Wire Experiments on 3He-4He Mixtures

We present vibrating wire viscometer experiments in concentrated and dilute 3He-4He mixtures showing that the slip length may become orders of magnitude larger than the mean free path due to increased specular scattering of the 3He quasiparticles with the superfluid 4He film on the wire. The experimental results are in good agreement with a recent theory for slip which accounts for the cylindrical geometry of the wire and for velocity slip in directions normal as well as tangential to the surface of the wire.

Spin transport in very dilute 3He-4He at very high B/T

We report preliminary results for pulsed-NMR measurements on a very dilute 3He-4He mixture (x3=900 ppm) in a 14.74 T field at temperatures down to 3 mK. The aim of the project is to investigate low temperature transverse spin relaxation and possible spin diffusion anisotropy. Spin-echo and T1-type measurements were employed to measure the transverse and longitudinal spin diffusion coefficients. At high temperatures both spin diffusion coefficients are well explained assuming a quasiparticle interaction consistent with previous experiments. Conversely, at low temperatures the measured value of |λωτ⊥| is apparently limited to about ten by a poorly-understood process that accelerates spin-echo decay. This hampers an accurate determination of the transverse diffusion coefficient at the lowest temperatures reached.

Vibrating Wire Measurements in 3He-4He Mixtures in the Ballistic Quasiparticle Regime

We present our first high resolution vibrating wire resonator data on dilute 3He-4He mixtures at ultra low temperatures. Measurements were performed at saturated vapor pressure with 3He concentrations below the phase separation limit. The behavior of the damping at very low temperatures does not follow the modified slip correction analysis previously applied to extend the validity of the hydrodynamic treatment.

Cooling Mechanism of Spin Polarized Atomic Hydrogen Adsorbed on Dilute 3He-4He Mixture Film

We cooled down the two-dimensional (2-D) spin-polarized atomic hydrogen (H↓) adsorbed on a small surface called cold spot, whose temperature was kept much colder than the rest of the sample cell. The 2-D density of H↓ was controlled by an input flux of H↓ into the sample cell. We investigated the surface recombination of H↓ adsorbed on 3% 3He 3He-4He mixture. ESR at 129GHz was carried out to detect H↓ by mirror cavity with the cold spot located at the confocal point. We calculated the thermal boundary resistance between the absorbed H↓ and the mixture. From the temperature dependence of the surface recombination together with calculated boundary resistance, we obtained the temperature dependence of ea. The result agrees with the previous measurement. Two-D H↓ on the mixture film was cooled well down to 40mK.

Acoustical Experiments on Superfluid 3He-4He Mixtures in Aerogel

This report discusses our results on the superfluidity of 3He-4He mixtures in a 98% porosity silica aerogel. We have used low frequency sound to probe helium mixtures confined to aerogel, and have observed both the slow mode of superfluid 3He in aerogel, which is manifested only below Tc, and an additional sound mode present only in the mixture. We attribute this novel sound mode to the slow-mode in the 4He rich phase of the dilute 3He-4He mixture. This mode exhibits positive frequency shifts below Tc in aerogel, while above Tc the mode is observed at a temperature independent frequency until close to Tλ where it shifts to zero frequency.

NMR Instabilities in Spin-Polarised Liquids: 3He, 3He-4He Mixtures and 129Xe

We report on pulsed NMR experiments performed in low magnetic field (1–2 mT) on various spin-polarised liquid samples obtained from optically pumped gas. Thin films of pure 3 He and bulk samples of dilute 3 He- 4 He mixtures are prepared with polarisations up to 40%. Xenon is liquefied (at 166 K) within a narrow U-shaped tube with polarisations of order 6%. In all these highly magnetised anisotropic samples, dipolar magnetic couplings have a strong influence on spin dynamics. Spectral clustering is obtained for small tipping angles and usually results in precession lifetimes of several seconds. Much shorter precession lifetimes (down to 10 ms) are obtained for larger tipping angles in highly polarised samples. In all systems, 90° tipping pulses are followed by precession signals which abruptly decay after a variable delay, as long as 1 s for moderate polarisations in xenon. Use of different liquids permits us to vary the effects of dipolar couplings, quantum exchange and spin diffusion, and to probe their influence on the observed phenomena.

The nonwetting behavior of dilute 3He-4He mixtures on Cs

We present evidence for3He interfacial bound states at the Cs-liquid He interface and show their influence on the interfacial free energies and the nonwetting behaviour of dilute3He-4He mixtures on Cs. The bound state energy and effective mass for the3He atoms at this interface are determined and found to be in good agreement with theoretical predictions. From the temperature dependence of the contact angle, direct evidence for a first order wetting transition at the lower wetting temperature is found. A detailed model is presented for the Cs-He mixture interface free energy and the contact angle and this model is shown to be in excellent agreement with the measurements. There is a short discussion on hysteresis and the thin film state. Finally it is emphasised that the large measured contact angles are only consistent with3He bound states together with ripplons at the Cs-He mixture interface

Thermal Conductivity and Viscosity in Dilute 3He-4He Mixtures Below 0.6 K

Thermal Conductivity and Viscosity in Dilute 3He-4He Mixtures Below 0.6 K

Transport properties of dilute superfluid mixtures of3He in4He

Measurements of the heat transport are reported for dilute superfluid mixtures of3He in4He between 1.6 and 2.17 K. The temperature transients leading to relaxation times also are presented. The3He molar concentration X ranged from 1 × 10−6 to 1 × 10−2. Cells with different fluid layer thicknesses h were used with 0.5<h<4.8 mm. The fluid mixture thermal resistance was obtained from the steady state data, after correction for the boundary (Kapitza) resistance Rb measured with pure4He. The bulk fluid results were found to be in agreement with the predictions by F. London and I. M. Khalatnikov. However there is an additional thermal resistance Ro that dominates for the most dilute mixtures. Ro is compared with Rb and both are found to have nearly the same temperature dependence and a comparable magnitude. A description and discussion of the onset of non-linear behavior at a critical heat current is presented. Analysis of the transient experiments shows that the observed relaxation times are larger than expected for the bulk fluid. This additional bottleneck is ascribed to Ro and possible origins of these effects are proposed. In the Appendix, analysis of observed relaxation times in the normal phase of4He near Tλ and correction for instrumental contributions are described. The intrinsic fluid relaxation times scale with h2 and agree quantitatively with predictions. This analysis, including the corrections, is then extended to dilute3He-4He mixtures in the superfluid phase.

Bound states of3He in3He-4He mixture films

3He atoms dissolved in super fluid4He may form aimers (3He)2 in twodimensional (2D) geometries. We study dimer formation in films of dilute3He-4He mixture. After designing a schematic3He-4He interaction potential we calculate the dimer binding energy for various substrates. It is shown that3He impurity states localized near the substrate give rise to the largest magnitudes of the binding energies.

First viscosity of dilute3He-4He mixtures below 0.6 K

Starting with the Boltzmann transport equation, the first viscosity of dilute3He-4He mixtures for various3He concentrations x is evaluated up to around T ≅ 0.6 K by including the contribution from three-phonon processes (3PP) in the anomalous elementary excitation spectrum of liquid4He. Due to 3PP, the characteristic time τη for3He viscosity at high temperatures, i.e., T⩾2TF where TF is the3He Fermi temperature, is evaluated as 5 × 10−12/xT, which is smaller than the value estimated by Rosenbaum et al. This is interpolated with τη in the degenerate (quantum) region, T≪TF. The obtained viscosities are in better agreement with experimental results than those of Baym and Saam, whose theory does not include 3PP. However, at very low concentrations there exists a discrepancy between the present theory and experiments, so that an alternate treatment should be considered.

Transport experiments on dilute, spin-polarized Fermi fluids

A series of experiments has been carried out on very dilute3He-4He mixtures and on pure3He liquid at very high field/temperature ratios (H≤8 T, T≥4.3mK). In this regime these systems display a strong Leggett-Rice effect, weakly damped paramagnetic spin waves, and significant polarization effects on spin and momentum transport. The dilute mixture experiments used NMR to observe standing spin waves, and vibrating-wire viscometry to measure momentum transport. A very satisfactory agreement is found with the recent kinetic-equation calculations of Jeon and Mullin. The pure3He experiments may provide the first evidence for field-induced relaxation of transverse spin currents.

Thermal transport properties in the normal phase of dilute3He-4He mixtures

We present steady-state measurements of the thermal diffusion ratiok T and of the heat conductivity κ for three dilute mixtures of3He in4He with concentrations 9×10−3≤X(3He)≤5×10−2 at saturated vapor pressure in the normal phase close to the superfluid transition. The data are compared with predictions by Dohm and Folk from the renormalization group (RG) theory. From auxiliary determinations of thermodynamic derivatives for these mixtures, we obtain the separation factor Ψ=−(k T /T)×(∂ρ/∂X) T,P /(∂ρ/∂T) X,P above Tλ over the range wherek T is positive. Here ρ is the mass density. From the transients of δX(t) as a function of time, we obtain an estimation for the mass diffusion coefficientD and compare the results with predictions by Dohm and Folk and with results from other experiments.

Experimental measurements of the attenuation of second sound in ultra dilute 3He4He mixtures

Abstract We report measurements of the damping of second sound for concentrations of 0.1, 0.05, 0.01 and 0.001% 3 He in 4 He.These measurements were performed in the temperature range 0.1K to 1K and in the frequency range 1kHz to 6kHz.

Experimental measurements of the attenuation of second sound in ultra dilute 3He-4He mixtures

Experimental measurements of the attenuation of second sound in ultra dilute 3He-4He mixtures

Transport properties in the superfluid phase of dilute3He-4He mixtures nearT ?

A cell for the simultaneous measurements of temperature and3He concentration gradients, induced by a heat currentQ across a fluid helium layer is described. This cell is operated over the temperature range 1.7≤T<2.5 K. Measurement of the anomalous boundary resistance ΔRb in superfluid4He(X(3He)∼2 ppb) near Tλ for heat currents between 8 and 47 µW/cm2 are described. The results for both the weakly divergent and the heat-dependent, more strongly divergent contributions to ΔR b are presented. The observed amplitude of the linear part is found to be larger than predictions by Onuki, by Ginzburg and Sobaynin and by Frank and Dohm, and also larger than recent data by Duncan and Ahlers (DA). These discrepancies are discussed in the light of the effective boundary area in the cell. The nonlinear part of ΔR b is consistent with the data by DA. It is found that the maximum observed ΔR b at the superfluid transition is independent ofQ. Analysis of the conductivity data of mixtures leads to the conjecture that ΔR b will decrease will the addition of3He. Our measurement of both the effective thermal diffusion ratiok * and of the thermal conductivity κeff over the range 10−6<X<5×10−2 show departures from the predictions by Khalatnikov. In particular forX<10−3,k * is found to be a function of X, and κeff deviates from the predicted κeff∝X −1. This last result confirms measurements with a previous cell. In the appendices, determinations and tabulations of auxiliary thermodynamic derivatives, such as (∂V/∂X)T,P and (∂S/∂X)T,P are presented.

The effect of a strong magnetic field on the damping of second sound in dilute3He-4He mixtures below 100 mK

We have measured the increase in damping of second sound in 0.1% and 0.05%3He-4He solutions upon application of a magnetic field of 10 T. The increase is caused by changes in the thermal conductivity and viscosity of the mixture as the system becomes spin polarized. We show how to fit the data for the viscosity in zero-field to obtain an estimate of the interaction between the3He quasiparticles. It is then possible to predict the viscosity in a field of 10 T using the calculation of Hampson et al. The data agree with the theory to within 2%.

Thermal transport properties in helium near the superfluid transition. III. Dilute3He-4He mixtures in the normal phase

An experimental study is presented for the thermal conductivity κ and the thermal relaxation for dilute mixtures of3He in4He with concentration 9×10−4⩽X(3He)⩽5×10−2 at saturated vapor pressure and in the normal phase near the superfluid transition. The conductivity results for κ are compared with predictions by Dohm and Folk from field-theoretic renormalization group(RG) theory. The conductivity κ s =[κ−1(T)−κ−1(Tλ)]−1, is compared with Ahlers' phenomenological arguments, and also with predictions by Dohm and Folk and by Onuki. The temperature difference transient ΔT(t) across the fluid, measured as a function of timet after switching on and off the heat current, is analyzed. The thermal diffusion ratiokT and the mass diffusion coefficientD are obtained by fitting the calculated transient to the experimental one. The results are compared with the predictions that follow from the RG approach by Dohm and Folk. Very good agreement is obtained forkT. The transient is not very sensitive toD, and hence the determination is not accurate. Yet within the uncertainty, the deducedD also agrees with predictions. Appendices give (1) the corrections to κ from finite heat effects, (2) the calculation of the concentration susceptibility (∂X/∂Δ) T,P , and (3) the calculation procedure for κ,kT, andD using the RG approach of Dohm and Folk.