Effective interactions and transport properties of degenerate spin-polarized3He-4He mixtures

We present calculations of the thermal conductivity, viscosity, and longitudinal spin diffusion of degenerate³He-⁴He mixtures as a function of spin polarization. Physical arguments and sum rules are used to describe qualitatively the behavior of the effective interactions between two³He quasiparticles. A recently proposed potential model is consistent with these expectations and provides the momentum- and polarization-dependent transition probabilities that enter into transport calculations. In the limit of small³He concentrations, our results agree with previous calculations based on the hard sphere model. For concentrations nearing saturation, however, we find structure in the thermal conductivity as a function of polarization with the³He density kept fixed. While the behavior of the viscosity remainsqualitatively similar to that at low concentration, the spin diffusion coefficient is qualitatively different. This behavior of the transport coefficients is largely a consequence of the strongly polarization dependentm _z =1 triplet scattering rate which becomes comparable to that in the singlet channel at large polarizations.     

Recent Experiments on Spin Polarized Liquid 3He

The importance of spin fluctuations on the physical properties of liquid ³ He can be probed by studying the polarization dependence of its magnetic susceptibility and viscosity. We discuss the present state of two such experiments. Using the rapid melting technique, to strongly polarize liquid ³ He, we have measured the viscosity enhancement induced by the polarization up to unprecedented large polarizations at a low temperature (80 mK). At a polarization of 60 %, the enhancement reaches a factor 2.5, comparable to that of the inverse susceptibility at the same polarization. Now at Oxford Instruments Research Instruments     

Engineering Co2MnAlxSi1−x Heusler Compounds as a Model System to Correlate Spin Polarization,Intrinsic Gilbert Damping,and Ultrafast Demagnetization

Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping, responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond timescale. Here, engineered Co₂MnAl_xSi_1-x Heusler compounds are used to adjust the degree of spin polarization at the Fermi energy, P, from 60% to 100% and to investigate how they correlate with the damping. It is experimentally demonstrated that the damping decreases when increasing the spin polarization from 1.1 × 10⁻³ for Co₂MnAl with 63% spin polarization to an ultralow value of 4.6 × 10⁻⁴ for the half-metallic ferromagnet Co₂MnSi. This allows the investigation of the relation between these two parameters and the ultrafast demagnetization time characterizing the loss of magnetization occurring after femtosecond laser pulse excitation. The demagnetization time is observed to be inversely proportional to 1 – P and, as a consequence, to the magnetic damping, which can be attributed to the similarity of the spin angular momentum dissipation processes responsible for these two effects. Altogether, the high-quality Heusler compounds allow control over the band structure and therefore the channel for spin angular momentum dissipation.     

Zero-temperature attenuation and transverse spin dynamics in fermi liquids. III. Low spin polarizations

This is the third in a series of papers on microscopic theory of transverse dynamics in spin-polarized Fermi liquids. In the lowest order in polarization our exact general theory of Ref. 1 reproduces the conventional Landau-Silin-Leggett theory of Fermi liquids. The next term in polarization expansion contains a zero-temperature attenuation with the magnitude that does not depend on polarization. This attenuation results in a finite relaxation time in transverse spin dynamics at zero temperature and is responsible for anomalous temperature behavior of spin diffusion. The zero-temperature attenuation is determined by two angular harmonics of the derivatives of the mass operators and the irreducible vertex in off-shell directions, and cannot be expressed via standard Fermi liquid harmonics. At high polarizations, the parameters of transverse spin dynamics are calculated as an expansion in polarization. The expansion involves complex values of interaction function and energy on a set of isoenergetic surfaces with the radii between the Fermi momenta for up and down spins,p andp. The results explain recent experimental data on spin diffusion in spin-polarized liquid³He and³He-⁴He mixtures. The comparison with experimental data indicates that the superfluid transition temperature for³He in³He-⁴He mixtures may be much lower than the current estimates.     

Polarization Induced Spin Wave Damping in Spin Polarized Liquid 3He–4He

We have measured the temperature and polarization dependence of Silin spin wave spectra in a saturated ³He–⁴He mixture with a concentration of 9.4% at a pressure of 8 bars. The mixture has been cooled and polarized by a Leiden dilution refrigerator to temperatures in the range 10–15 mK and polarizations as high as 9.2% corresponding to 3.4 times the equilibrium polarization of 2.7% in the external magnetic field of 11.36 T. The analysis takes into account the dipolar interactions and results in the relaxation time τ _⊥ and spin diffusion constant D _⊥ . We find that τ _⊥ and D _⊥ are proportional to where T is the temperature, is the polarization enhancement factor and T _a0 is the anisotropy temperature for the mixture at equilibrium in the external field. Our result T _a0=3.66±0.14 mK is 30% higher than the theoretical prediction for very dilute mixtures and is evidence for the existence of polarization induced relaxation of transverse spin currents.     

Compressing Spin-Polarized 3He With a Modified Diaphragm Pump

Nuclear spin-polarized ³He gas at pressures on the order of 100 kPa (1 bar) are required for several applications, such as neutron spin filters and magnetic resonance imaging. The metastability-exchange optical pumping (MEOP) method for polarizing ³He gas can rapidly produce highly polarized gas, but the best results are obtained at much lower pressure (~0.1 kPa). We describe a compact compression apparatus for polarized gas that is based on a modified commercial diaphragm pump. The gas is polarized by MEOP at a typical pressure of 0.25 kPa (2.5 mbar), and compressed into a storage cell at a typical pressure of 100 kPa. In the storage cell, we have obtained 20 % to 35 % ³He polarization using pure ³He gas and 35 % to 50 % ³He polarization using ³He-⁴He mixtures. By maintaining the storage cell at liquid nitrogen temperature during compression, the density has been increased by a factor of four.     

Bulk nuclear polarization of solid3He

Measurements are given on the bulk nuclear spin polarization in a liquid-solid³He mixture cooled by compressional cooling to below 5 mK in a magnetic field of 54.5 kG. Owing to the low Pauli spin susceptibility of liquid³He, the observed polarization is primarily due to solid³He. A maximum average nuclear polarization of 47% was observed, although the corresponding solid³He polarization is believed to be higher. Our novel detection system, using a dual directional coupler for cw NMR, is a simple and versatile means of working in the awkward frequency range around 180 MHz. We also report transient heating measurements in the³He system which indicate that the internal thermal equilibrium time in bulk solid³He on the³He melting curve appears to be quite short (less than 5 min) at these temperatures. One type of transient measurement is complicated by the dramatic effect of the contribution of the³He nuclear magnetization to the total local magnetic field. This contribution is considered via a simple model.This work has been supported by the U.S. Atomic Energy Commission under Contract No. AT(04-3)-34, P.A. 143.     

Magnetic susceptibility and collisionless spin waves in liquid3He and3He-4He mixtures

The transverse spin susceptibility for a Fermi liquid in a uniform magnetic field is calculated in the collisionless regime using the Landau theory of Fermi liquids. The singularities corresponding to the l=0 and l=1 spin waves are investigated in pure and dilute³He as a function of the wave vector q up to the threshold of the Landau damping.     

The effect of the demagnetizing field on coherent spin precession in a normal fermi liquid

We consider an effect of the demagnetizing field, produced by the magnetic moments of nuclei of³He, on coherent precession of spin in liquid spin polarized³He or in spin polarized solutions of³He in⁴He. Formulae for the shape of the precessing domain wall and for the relaxation rate of the coherently precessing structure are rederived taking into account the demagnetizing field. For pure³He and concentrated solutions of³He in⁴He the width of the domain wall increases and the relaxation rare decreases with polarization. Measurement of the relaxation rate of the precessing structure can be used as an alternative method for the determination of the spin diffusion coefficient in Fermi liquids.     

Polarized liquid3He in a4He circulating dilution refrigerator

We have used a⁴He circulating dilution refrigerator to produce cold liquid³He with a steady state out-of-equilibrium nuclear spin polarization. Polarizations on the order of 15% (more than 7 times higher than the equilibrium polarization in the external field of 6.6 T) have been obtained in the mixing chamber of the refrigerator at temperatures between 10 and 15 mK. The polarization is enhanced at high pressure because the molar susceptibility of concentrated³He is larger than that of the dilute phase. The polarization exchange between the dilute and concentrated phases (in direct contact in the heat exchanger of the refrigerator) amplifies the enhancement. The polarization diminishes below a pressure of 2.6 bar. This allows us to scale and reinterpret susceptibility data of the dilute phase¹ in combination with the effective mass deduced from osmotic pressure measurements². We find 1+F ₀ ^a = 0.89±1% on the phase separation line in the pressure range 0–20 bar.We would like to thank Profs. D.M. Lee and M.S. Tagirov for the many discussions during their visits.     

Brute Force Polarization of 129Xe

Hyperpolarized ¹²⁹Xe has numerous applications in Nuclear magnetic resonance (NMR) and magnetic resonance imaging(MRI). As an alternative to the laser optical pumping method of production of hyperpolarized ¹²⁹Xe gas, we have investigated the brute force technique which uses a very high magnetic field and millikelvin temperatures. One obstacle to this technique is the extremely long spin lattice relaxation times of nuclei in solids at very low temperature. We exploit the fact that liquid ³He can be used as an effective relaxant to enable ¹²⁹Xe to be spin polarized on a high surface area substrate in a few hours. We are able to identify the contributions of different atomic layers to the magnetization by analyzing the ¹²⁹Xe NMR spectrum. Furthermore, the addition of ⁴He allows us to turn off the relaxation mechanism, thereby preserving the polarization. The technique could be applied to nuclei other than ¹²⁹Xe.     

Spin-dependent correlations in normal liquid 3He

Using a generalized Fermi hypernetted chain method on a Jastrow trial ground-state wave function, which includes a dependence on the z component of the nuclear spin, it is shown that spin correlations make a significant contribution to the ground-state energy of liquid ³He, accounting for much of the energy necessary to stabilize unpolarized liquid ³He relative to completely polarized ³He.Work supported in part by NSF grant DMR 7926447 and by the Deutsche Forschungsgemeinschaft.     

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.     

Neutron polarization with polarized 3He

An effective polarizer for thermal and epithermal neutrons is possible with polarized, high density, gaseous ³He. We describe the technique of ³He polarization by spin exchange with laser optically pumped Rb vapor which can provide 6 cm³of > 70% polarized ³He at a density of 3×10²⁰ atoms cm⁻³ (10 atm at STP). The ³He polarization can be rapidly reversed, a requirement for sensitive symmetry tests.     

Application of Surface Acoustic Wave Sensors for Liquid Helium-4 and Helium-3

A surface acoustic wave (SAW) sensor can be used to determine the acoustic properties of liquid He. We applied this sensor for liquid ³He and liquid ⁴He. Two kinds of SAW were used in this experiment, Rayleigh-SAW and shear horizontal SAW. The damping of Rayleigh-SAW was measured in liquid ⁴He. The results agreed well with the reported values of the acoustic impedance, and the superfluid transition was clearly seen. The damping and the phase shift of shear horizontal SAW were measured in the normal liquid ³He. Temperature dependence of the damping and the phase shift was analyzed by the viscoelastic model, and the results were compared with the transverse acoustic impedance in the normal ³He.     

Packed Powder as Superleak for Spin Pump Experiments in Superfluid 3He A1

Experimental exploration of highly spin-polarized states of liquid ³He by applying external magnetic field is limited by the availability of static magnetic field. In the “ferromagnetic” superfluid A₁ phase of liquid ³He there is an alternate method for boosting spin-polarization by the process of spin pumping without requiring such high magnetic field. The spin pumping in the A₁ phase takes advantage of a superleak (SL) acting simultaneously as a filter for both entropy and spin. The spin pump technique that uses the SL-spin filter and a mechanical actuator enables us to directly boost polarization of ³He. The amount of enhancement of spin polarization has been limited so far. We are now developing a new type of SL filter made of packed aluminum oxide powder (referred as PAP-SL), in order to achieve greater enhancement of spin polarization. Several kinds of the PAP-SL filter were constructed by pressing aluminum oxide powders into a cylinder holder. The packed structures were carefully characterized by a flow-rate-measurement, X-ray tomography, and mercury intrusion porosimetry. The preliminary result shows that the PAP-SL works as SL filter for the superfluid ³He.     

The attenuation of second sound in dilute3He-4He solutions below 1 K

The attenuation of second sound in dilute solutions of³He in⁴He has been measured from the width of standing wave resonances. Results are presented here for³He molar concentrations of 1.3% and 5% in the temperature range 50 mK to 1 K at pressures of 0 and 10 atm. The results are compared with the theory of Saam, Baym, and Ebner and suggest that theoretical values of the phonon thermal conductivity lifetime are too great. Comparison is also made with recent measurements of the second-sound damping constant in very dilute solutions (X0.5%); our result shows no sign of the anomalous peak in the damping constant observed for such concentrations. The shear viscosity of the dilute solutions is also computed and found to be in agreement with the capillary flow measurements of Kuenhold, Crum, and Sarwinski.This work was supported in part by a grant from the Research Corporation.     

Phase equilibrium in a polarized saturated3He—4He mixture

We present experimental results on the phase equilibrium of a saturated³He−⁴He mixture, which has been cooled to a temperature of 10–15 mK and polarized in a⁴He circulating dilution refrigerator to a stationary polarization of 15%, 7 times higher than the equilibrium polarization in the external field of 7 T. The pressure dependence of the polarization enhancement in the refrigerator shows that the molar susceptibilities of the concentrated and dilute phase of a saturared³He-⁴He mixture are equal atp=2.60±0.04 bar. This result affects the Fermi liquid parameters of the dilute phase. The osmotic pressure in the dilute phase has been measured as a function of the polarization of the coexisting concentrated phase up to 15%. We find that the osmotic pressure at low polarization (<7%) agrees well with thermodynamics using the new Fermi liquid parameters of the dilute phase.     

Nuclear Spin Relaxation of Powdered Metallic Antimony in Liquid 3He

Nuclear spin relaxation studies have been carried out to determine the surface relaxation of powdered Sb immersed in liquid ³ He. The surface relaxation mechanism was observed to be significantly more effective than the bulk Korringa relaxation rate for temperatures T < 75 mK. This surface relaxation is attributed to the modulation of the dipole-dipole interaction between the Sb spins at the metal surface and the ³ He spins in the solid-like layer on the surface by the quantum zero-point motion of these ³ He atoms. The total relaxation has been analyzed in terms of the surface relaxation and the spin diffusion through the bulk of the Sb particles. The experimental results are in excellent agreement with the theory developed for this system. Replacement of the ³ He by ⁴ He removes the surface relaxation component and restores the relaxation rates to the bulk value.