Characterization of spin polarized dilute3He-4He solutions below 500 mK

We have prepared a large sample of spin polarized liquid ³ He- ⁴ He at 500 mK. Polarization is obtained by laser optical pumping, and studied by pulsed NMR. We have observed a nuclear magnetization lifetime T ₁ longer than 1 hour, and an anomalously long, non linear decay of tranverse magnetization.Unité de recherche de l'Ecole Normale Supérieure et de l'Université Pierre et Marie Curie, associé au CNRS (URA 18).     

NMR studies of highly polarized liquid3He-4He mixtures

An efficient technique for maintaining large³He nuclear spin polarizations in liquid³He-⁴He mixtures has been developed and used to polarize samples contained in narrow tubes. Results obtained in a U-shaped cell and a coil-shaped cell are compared. A systematic study of the effects of dipolar fields within the sample on the characteristics of the observed long-lived magnetostatic modes is in progress. NMR signals obtained in the coil-shaped cell reveal phase separation and the existence of a highly polarized³He-rich region in the sample.Laboratoire de L'Ecole Normale Supérieure et de L'Université Pierre et Marie Curie, associé au CNRS (URA 18)     

3He-3He quasiparticle interaction in3He-4He mixtures under pressure

Based on our recent phase separation curve of³He-⁴He solution at elevated pressures, we propose new³He-³He quasiparticle interaction potentials, which reproduce the existing experimental results pretty well except for³He effective mass under pressure.     

3He impurity excitations in solid4He

The problem of³He mobility in dilute³He-⁴He solid mixtures has been treated both theoretically and experimentally. The Hamiltonian of the system can be reduced to the Hamiltonian of strongly interacting impurity quasiparticles corresponding to some time-averaged states. The experiments carried out on solid mixtures with³He concentration 2.17%, 0.75%, 0.25%, or 0.092% by the NMR method show that at sufficiently low (T<1.2 K) temperatures the diffusion coefficient becomes temperature independent and inversely proportional to³He concentration. These results substantiate qualitative conclusions of the theory. Analysis of the experimental data makes it possible to take into account the phonon part of the³He diffusion coefficient and to obtain, up to a constant, the³He-⁴He exchange energyJ10^–7 K. All the facts mentioned above testify to the substantially quantum nature of the³He diffusion process in dilute³He-⁴He mixtures.     

Liquid4He as a cryogenic coating for gaseous spin polarized3He

Nuclear spin polarization can be produced by optical pumping of³He gas in sealed samples at a temperature below 1 K. The wall relaxation can be minimized by covering the container walls with a⁴He film, and relaxation timesT ₁ longer than 1000 sec have been observed. A study of the residual relaxation is reported here. Spin relaxation appears to take place near the helium-substrate boundary. This situation is characterized by an adsorption energy very different from the known binding energies of³He quasiparticles on and in bulk liquid⁴He. Models are presented which reproduce the observedT ₁ variations on temperature and on other experimental parameters.Laboratoire associé au Centre National de la Recherche Scientifique et à l'Université Paris VI.     

NMR characterization of phase separation in highly polarized3He-4He solutions above 450 mK

Laser optical pumping techniques have been successfully applied to concentrated³He-⁴He gaseous mixtures. Using a³He cryostat, these mixtures were subsequently liquefied and studied down to 450 mK, using pulsed NMR techniques.Observation of highly nuclear polarized solutions and of phase separation (demixion) in the mixtures is reported. Measurements of magnetic relaxation times and the dynamics of coexisting phases in this system are discussed.Unité de Recherche de l'Ecole Normale Supérieure et de l'Université Pierre et Marie Curie, associée au C.N.R.S. (URA18)     

NMR Studies on 3He-3He and 3He-4He tunneling motions in solid 3He-4He mixtures

Helium-3 nuclear spin relaxation times T ₁, T ₂, and T ₁have been measured for ³He-⁴He solid mixtures at the exchange plateau region (~0.5K). The ³He concentrations X ₃of the samples were 7.2, 2.9, 1.8, 1.4, 0.67, 0.65, and 0.22%, and their molar volumes varied between 19.9 and 20.9cm³/mole in hcp phase. The spectral density function J() for dipolar field fluctuations was determined in the low-frequency branch from T ₁measurements and in the high-frequency branch from conventional T ₁measurements. It was found that J() is given by J() = cJ()|_3–4 + (1–c)J()|_3–3, where J()|_3–4 is the spectral density function due to the ³He-⁴He tunneling motions, and J()|_3–3 is that due to the ³He-³He tunneling motions. Using the Torrey theory, the correlation frequency of the ³He-⁴He tunneling motions was evaluated from T ₁data, and was found to be in good agreement with Landesman 's theory.Supported in part by the Japan Society for the Promotion of Science through a grant to Y.H.     

The proximity effect between superfluid3He and a3He-4He mixture

The quasiclassical theory is used to study the proximity effect between superfluid ³ He and a mixture of ³ He- ⁴ He. The order parameter and the surface tension are calculated, and their dependence on temperature and on the hypothetical transition temperature of the ³ He- ⁴ He mixture is discussed.     

Universal behavior of3He-4He mixture films near the Kosterlitz-Thouless transition

We report measurements of the convective thermal conductance of³He-⁴He mixture films near the Kosterlitz-Thouless transition. The thickness of our⁴He films is 14.7 and 19.1 Å above the inert layer and the³He concentration ranged from 0.033 to almost 2%. The thermal response is tested for the critical behavior as observed in pure films, and it is found to be preserved in the mixture films case. However, the parametersb, D/a ² andT _c exhibit a strong dependence on the concentration. The mixture film conductance at fixedT-T _c is found to decrease upon addition of³He implying a decrease in the 2D correlation length. Mixture films thus exhibit 2D behavior over a narrower temperature range than pure films. Further, for temperaturesT _c , the largest measurable conductance decreases sharply with the addition of³He. We attribute this behavior to a³He-⁴He scattering mechanism and a³He induced free-vortex density.     

Spin-Spin Relaxation in Phase-Separated 3He-4He Solid Mixtures

The spin-spin relaxation time T ₂ in a ³ He- ⁴ He solid mixture with an initial concentration of 3.18% ³ He is measured during its phase separation in a temperature range of 1-250 mK. Cooling down to the region of separation was carried out by small steps (10 mK) followed by temperature stabilization for many hours. The time T ₂ was measured by using the pulsed NMR technique at a frequency of 250 kHz. The spin echo method makes it possible to distinguish the contributions to magnetic relaxation from both the concentrated and the dilute separated phases. The relaxation time T ₂ in the concentrated phase is found to be practically independent of temperature down to 50 mK and is determined by the same ³ He- ³ He exchange interaction as in pure bulk solid ³ He. It is found that the behavior of the spin echo signal at ultralow temperatures exhibits an anomaly, which may be connected with quasi-one-dimensional spin diffusion. In the dilute daughter phase the spin-spin relaxation time is inversely proportional to concentration and is described correctly by the Torrey model taping into account ³ He- ⁴ He tunnel exchange. The values of T ₂ in this phase coincide with those for a homogeneous (non-separated) mixture of the same concentration.     

The preferential adsorption of 4He from 3He-4He mixtures onto Vycor glass

The preferential adsorption of ⁴He from liquid ³He-⁴He mixtures onto Vycor glass has been measured over a wide concentration range between 1 and 3 K. Over most of this temperature range the preferential adsorption extends beyond the second atomic layer, with a preferential binding energy of about 3.5 K/mole. Near 1 K and below, the thickness of the ⁴He layer increases rapidly as a precursor to the bulk liquid phase separation.     

Thermal equilibration of fluids near the liquid-vapor critical point:3He and3He-4He mixtures

We study the temperature-equilibration process of fluids at constant volume in a thermal conductivity cell, where an initial temperature gradient relaxes to zero. The calculation is performed in the linear approximation for a pure fluid and a binary mixture. Near the critical point of the pure fluid, the adiabatic heating process, which takes place at constant volumeV, causes equilibration to proceed four times faster whenC _P /C _V 1 than for the process at constant pressureP. For the mixtures, the relaxation rate enhancement at constantV compared with constantP is restricted to a temperature region where the coupling between temperature and mass diffusion is small. The predictions are compared with experimental results for³He and for two³He-⁴He mixtures along their critical isochores. Finally, we discuss the thermal relaxation in the two-phase (liquid-gas) and one-phase (gas) regimes at the critical density, as measured with a conductivity and a calorimetry cell. The contrasting behavior for³He and a³He-⁴He mixture in these two regimes and under these different constraints is pointed out and discussed.     

3He impurities on the bulk surface of liquid4He: Possible existence of excited states

We study, using a density-functional approach, the properties of the two-dimensional system formed by³He atoms on the surface of liquid⁴He, as a function of³He coverageN _s , at zero temperature. We find several types of surface states accessible to the³He atoms. For small values ofN _s , the surface tension is, as expected, linear inN _s ² . For a coverage of about half a monolayer, a new type of surface state starts being occupied, and this produces a change in the slope of as a function ofN _s ² and, more clearly, a step in the surface specific heat, which increases by a factor of almost two. Another step is predicted to occur for a coverage of 1.3 monolayer. Existing data are compatible with this structure of surface states but are not numerous enough to prove or disprove the existence of steps.Unité de Recherche des Universités Paris 11 et Paris 6 Associée au CNRS.     

The spin diffusion coefficient of3He in3He-4He solutions

The transport properties of³He in³He-⁴He solutions with molar concentrations of 5, 9, 14, and 24% have been studied for 0.9 KT2.5 K. The spin diffusion coefficientD _s and the longitudinal relaxation timeT ₁ were measured by the spin-echo method for temperatures both above and below the solution lambda temperatureT . The spin-echo method measures the diffusion coefficient for magnetizationD _s, which differs from the usual diffusion coefficient for particlesD belowT .D _s depends on the³He-³He scattering cross section _FF and the³He-roton/phonon cross section _FB, whileD depends only on _FB. The distinction betweenD _s andD is elaborated in terms of a simple mutual-friction model for diffusion. The two scattering mechanisms are clearly evident in the behavior ofD _s as a function of concentrationx and temperature. The contribution due to the³He-³He scattering is inversely proportional tox, indicating that the³He can be treated in first approximation as a classical gas (the Pomeranchuk model). The predictions of various theoretical models are compared with the results, where possible, but most of the previous theoretical work is not applicable to the concentration range and temperatures of these measurements.Supported in part by the National Science Foundation and the U.S. Office of Naval Research.     

Generalized Jastrow variational method for liquid3He-4He mixtures atT=0 K

The ground state energy of a dilute solution of mass-3 fermions in liquid⁴He is analyzed by a variational procedure based on the Jastrow many body theory. The antisymmetry of the wave function for fermions is incorporated following the procedure given by Lado, Inguva, and Smith. A set of coupled integrodifferential equations is solved in the hypernetted chain approximation yielding expressions for the binding energy of³He-⁴He mixtures; the radial distribution function is given together with the total energy for various values of density and the interparticle separationr _s.     

Spin diffusion in 3He-4He solutions under pressure

We have measured the spin diffusion coefficient D for ³He-⁴He solutions of nominal ³He concentration x = 1.3% and 5.0% at pressures of ~0, 10 and 20 atm, and an x=8.5% solution at 4, 10 and 20 atm. The results are interpreted in terms of the Bardeen-Baym-Pines theory for ³He-⁴He solutions, and curves for the effective ³He-³He interaction V(k) are presented. We find no evidence for a minimum in V(k) at nonzero k. Weak coupling BCS pairing transition temperatures calculated from the V(k) lie in the range 10^–6–10^–7 K.Research supported in part by Research Corporation.     

NMR experiments on pure3He and3He-4He mixtures in silica aerogel

We have carried out a series of NMR experiments on³He systems filling the pores of silica aerogel with 95% porosity. The systems studied included³He-⁴He mixtures with a wide range of³He concentrations x₃ as well as pure³He. All experiments were conducted in an 8 T field and for temperatures T 6 mK. This resulted in strong spin polarization at the lowest temperatures (up to approximately 80%) for the localized layer observed for a pure³He sample. For pure³He, the magnetic behavior is dominated by the localized spins, which are found to constitute 6% of the sample. The coupled system of localized plus liquid spins displays rapid transverse magnetization relaxation with an anomalous temperature dependence. For intermediate x₃ magnetization measurements provide a preliminary indication of the phase diagram of mixtures filling this porous material. The magnetization is hysteretic over some ranges of x₃.     

Distillation of Nuclear Polarization of 3He-4He Mixtures in Very High Magnetic Fields

We have used the method of fractional distillation to produce an enhanced nuclear polarization of a saturated ³ He- ⁴ He mixture. For the experimental study reported in this paper, we have developed an all plastic distillation setup, which allows high ³ He circulation rates at low temperatures (18 mol/s at 150 mK) up to the highest magnetic fields (22.5 T). The magnetization has been measured with a torque magnetometer. The nuclear magnetic relaxation time T₁ of the ³ He- ⁴ He mixture was found to increase with the square of the magnetic field, indicating that T₁ is dominated by surface relaxation. As a consequence, the enhancement of the magnetization due to the distillation process, which is amongst others proportional toT₁ , increased also with the square of the magnetic field. At the highest field at 140 mK, a polarization of 8% of the ³ He- ⁴ He mixture has been obtained, 1.6 times the equilibrium value. Various ways to improve the set-up are discussed.     

Viscosity of saturated3He-4He mixture below 200 mK

The shear viscosity of saturated³He-⁴He mixture has been measured at temperatures between 7 mK and 200 mK using a vibrating-wire viscometer and a calibrated pressure cell. The reliability of the vibrating-wire technique was tested by measuring the viscosity of pure⁴He. The results are internally consistent. A phenomenological expression is given for the viscosity of saturated³He-⁴He mixture between 7 mK and 80 mK.     

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

This report discusses our results on the superfluidity of ³ He- ⁴ He 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 ³ He in aerogel, which is manifested only below T_c, and an additional sound mode present only in the mixture. We attribute this novel sound mode to the slow-mode in the ⁴ He rich phase of the dilute ³ He- ⁴ He mixture. This mode exhibits positive frequency shifts below T_c in aerogel, while above T_c the mode is observed at a temperature independent frequency until close to T where it shifts to zero frequency.