Ultra-Low Temperature Magnetic Properties of Liquid 3He Films

We report measurements of the nuclear magnetization of submonolayer liquid ³ He films adsorbed on a graphite substrate (Papyex) preplated by a monolayer of ⁴ He. In the submilliKelvin temperature range we observe a substantial enhancement of the nuclear magnetization with respect to the degenerate Fermi Liquid value. The unusual temperature dependence of this new contribution to the liquid ³ He film magnetization agrees well with that expected from the theory of weak disorder in two-dimensional (2D) correlated Fermion systems. The effects of disorder and reduced dimensionality suppress the superfluid transition at least to below 180 K.     

Thermal resistance between liquid3He and copper potassium tutton salt

The thermal resistance between liquid³He and copper potassium tutton salt (CPS) has been measured through its magnetic ordering temperature (T _c=29.6 mK). The thermal resistanceR for pure³He has a broad minimum near 60 mK and increases continuously throughT _cwith decreasing temperature, except for a dip atT _c. BelowT _c,R is proportional toT ^–1.5. Effects of⁴He coating have been studied by stepwise addition of⁴He into liquid³He. The thermal resistance increased drastically for the liquid containing 150 ppm⁴He and more for 95%⁴He. By sudden depressurization of the liquid³He containing 480 ppm⁴He, a considerable decrease ofR was observed. SinceR for pure³He was much smaller than the calculated Kapitza resistance, the present experimental results indicate the existence of surface magnetic coupling between liquid³He and CPS.     

The solubility of4He in3He below 0.1 K

The Zharkov-Silin Fermi Liquid theory of solutions of⁴He in non-superfluid liquid³He has been applied to the recent phase separation data of Nakamura et al. At zero pressure, the difference in binding between a⁴He atom in liquid⁴He and in liquid³He is smaller than previous estimates, and the⁴He effective mass is close to the bare mass. The volume measurements of Laheurte show that the difference in binding has a minimum near 11 atm. This implies an enhanced solubility of⁴He in³He below 0.1 K at this pressure, although there is experimental evidence that the solubility at 0 K remains zero.     

The extraordinary phenomena of weak adsorption

A summary is provided of a variety of intriguing phenomena involving particles weakly bound to surfaces. One kind of problem involves wetting and prewetting transitions (for H₂, He and Ne on alkali metals) and thus far mysterious behavior of He on an H₂ substrate. A second kind involves the adsorption of atoms on the surfaces of liquid⁴He or³He.     

Relaxation in Powdered Metallic Antimony in Contact with Liquid 3He

The nuclear spin-lattice relaxation in finely powdered antimony metal immersed in liquid ³ He has been studied at low temperatures. For T < 75 mK, a new surface relaxation mechanism becomes significant. This surface relaxation is due to the modulation by the quantum zero-point motion 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. Replacement of the liquid ³ He by ⁴ He restores the relaxation rates to the bulk value. This work helps explain anomalies found in previous relaxation studies on powdered Pt immersed in liquid ³ He.     

Thermal resistance between powdered cerium magnesium nitrate and liquid helium at very low temperatures

Experiments have been performed from magnetic temperatures of 2–20 mK on the effect of minute⁴He impurities and a magnetic field on the thermal resistance between powdered cerium magnesium nitrate (CMN) and liquid³He. The thermal resistance decreases with decreasingT and increasing small field but is increased dramatically at a givenT by the addition of roughly a monolayer of⁴He. The resistance is interpreted as resulting from a surface magnetic coupling between CMN and liquid³He.This work has been supported by the U.S. Atomic Energy Commission under Contract No. AT(04-3)-34, P.A. 143, and by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant No. AF-AFOSR-631-67A.     

Investigation of the lifetime of longitudinal phonons at GHz frequencies in liquid and solid4He

The attenuation of longitudinal 1-GHz phonons was measured as a function of temperature in liquid ⁴ He at svp and at 23 bar as well as in hcp ⁴ He at 36 bar. The lifetime of the phonons which were generated by stimulated Brillouin scattering was determined optically by a probing light pulse. The results in liquid ⁴ He are discussed in terms of relaxation processes in the phonon and roton gas and are in good agreement with existing work. The attenuation in hcp ⁴ He, which approximately shows aT ⁴ temperature dependence, is attributed to three-phonon processes with longitudinal thermal phonons. A quantitative comparison with Landau-Rumer theory gives satisfactory agreement with the data. No effect of phonon dispersion on sound attenuation is found down to 0.8 K.     

Investigation of adsorption and wetting of3He on cesium and cesiated glass

Experiments have been carried out to investigate the binding of ³ He on cesium substrates, using optical pumping to spin-polarize the atoms. The behaviour of ³ He on the walls at low temperature can be analyzed through the evolution of the nuclear magnetization of the sample. Preliminary results are presented, including: (i) adsorption studies of gaseous ³ He on cesiated glass; (ii) magnetic relaxation time of polarized liquid ³ He on cesium and cesiated glass; (iii) evidence for wetting of liquid ³ He on cesium.     

The interatomic potential in liquid4He

In this paper we examine the validity of and consider alternatives to the traditional Lennard-Jones (6, 12) potential as applied to liquid⁴He. We find the Lennard-Jones potential to be subject to a number of serious defects, the worst among them that it does not satisfy the virial theorem. In its place we propose to use the Morse-V _DD potential function, which has none of the flaws in the Lennard-Jones potential and has the convenient feature of possessing a Fourier transform. The parameters appropriate to liquid⁴He are computed for the new potential as are a number of quantities derivable from it which are of current theoretical interest. A comparison is made between the values of these quantities as they are obtained using both potential functions.     

Thermal resistance between a paramagnetic salt (CMN) and liquid helium at millikelvin temperatures

We report new measurements of the thermal resistance between cerium magnesium nitrate (CMN) and liquid helium. AtT=100 mK, the thermal resistanceR for powdered CMN in contact with liquid³He-⁴He or liquid³He is about 2000 times the Kapitza boundary resistance. From the field and temperature dependences ofR we conclude that it is dominated by the phonon-bottlenecked spin-lattice resistance. However, if high-purity liquid³He is in contact with CMN, we observe, below 20 mK, a magnetic boundary resistance in parallel to the spin-lattice resistance. This resistance is two orders of magnitude smaller than the spin-lattice resistance atT=10 mK, and is described byR T ^1.4. The absolute values of the magnetic boundary resistance, as well as the spatially limited phonon-bottlenecked resistance, are extremely sensitive to the surface treatment of the CMN crystals.     

Elastic reflection of4He atoms from the surface of liquid4He

The reflection of incident⁴He atoms from the surface of liquid⁴He can yield information on the microscopic features of the liquid at the surface. Edwards et al. have measured this reflectivity and have developed a theoretical model which successfully described the measurements. However, the density profile of the liquid⁴He surface has recently been determined experimentally, and this can now be used to model the reflectivity without any freely adjustable parameters. We use this density profile and the experimentally-determined interatomic pair potential to derive an effective potential for the incoming atom, and then solve the single-particle Schroedinger equation to find the reflectivity as a function of wavevector perpendicular to the surface. Within the uncertainty in the density profile and interatomic potential, we have chosen values that give good agreement between the measured and calculated reflectivities.     

Magnetic heat conductance between CMN single crystals and liquid3He

Measurements of the magnetic Kapitza resistance between single crystals of CMN and lanthanum-diluted CMN in contact with ultrapure liquid³He are presented. The investigations were performed in the temperature range 2–100 mK and in magnetic fields up to 600 Oe. For the first time, a magnetic thermal coupling between bulk crystals with dimensions of up to 3 mm and liquid³He was observed. In the case of pure CMN and liquid³He below 7 mK, the relationR T ⁿ withn=4±1 is seen. A strong dependence ofR with magnetic field and Ce³⁺ content is found.     

Specific heat of amorphous3He films and confined liquid3He

We have measured the heat capacities of³He films and liquid³He in porous Vycor glass at 10 to 600 mK. With increasing the film thickness from 1 to 3 atomic layers, the specific heat evolves gradually from that typical to solid to that of liquid³He. At about 2 atomic layers, however, its low-temperature part is nearly temperature-independent; we interpret this as a result of gradual freezing of spins in an amorphous solid³He film with decreasing the temperature. The contribution of liquid³He in the center of the Vycor pores can be described as the specific heat of bulk liquid³He at corresponding pressures in the range 0 to 28 bar. The thickness of amorphous solid on the pore walls increases with external pressure roughly linearly. Preplating the walls with⁴He allows to determine the positions of³He atoms contributing to the surface specific heat at 10 to 50 mK. In addition, the contribution from the specific heat of³ He -⁴He mixing at 100 to 600 mK is discussed as a function of pressure and amount of⁴He.0n leave from ISSP Acad. Sci. of Russia, Chernogolovka, Russia     

The Effective Mass and Binding Energy of 4He in 3He in the Fermi Liquid Region

We have measured the solubility of ⁴ He in liquid ³ He down to about 40 mK and at pressures from zero up to 24 atm. The solubility was obtained from the thickness of the superfluid film in contact with unsaturated solutions of ⁴ He in ³ He as a function of temperature. By fitting the solubility data to Fermi liquid theory, we obtained the parameters m ₄ ^* and as a function of pressure. Here, m ₄ ^* is the effective mass of ⁴ He in liquid ³ He and the difference in binding energy between ⁴ He in pure ⁴ He and ⁴ He in liquid ³He. This difference has a minimum near 10 atm. The average of the results for m ₄ ^* , at different pressures, is (1.3 ± 0.2)m ₄. This agrees with the many body calculations of de Saavedra et al., and with the Stokes hydrodynamic mass using the partial volume of ⁴ He in ³ He, v ₄ ^* . The partial volume was obtained by taking the derivative of with respect to pressure.     

The magnetic susceptibility of dilute mixtures of3He in liquid4He

The relative changes in the low-temperature susceptibility of a number of dilute mixtures of³He in liquid⁴He at various pressures are reported and analyzed. Under elevated pressures, the solubility of³He in⁴He increases and the effects of the interaction between³He quasiparticles clearly become larger. However, no evidence of a BCS pairing transition was found in direct measurements of susceptibility and nuclear resonance frequency down to the temperature 1.5 mK in saturated solutions with a pressure of 21 bar.Guggenheim Fellow on leave from Cornell University, Ithaca, New York.     

The Kapitza resistance to a variety of metallic surfaces below 0.3 K

The Kapitza resistance has been measured between liquid or solid ³ He or ⁴ He and surfaces of Mg, Cu, W, and Au in the temperature range 0.03–0.3 K. The experimental data are in relatively good agreement with the acoustic mismatch theory modified so as to include phonon attenuation in the metal. Below ～0.1 K the Kapitza resistance of liquid ³ He is often reduced relative to that of liquid ⁴ He, suggesting the presence of an enhanced thermal conduction between liquid ³ He and metals as has been observed in other laboratories.     

Investigating surface wave resonances with electrons on helium

Waves on liquid⁴He were excited and detected by capacitively coupling to an electron sheet trapped at the free surface. The standing wave resonances, resulting from the finite size of the sample cell, were measured and their frequencies are in good agreement with theory. The resonant linewidth displays a nonmonotonic dependence on temperature which indicates that it is determined by viscous friction at the watts of the sample cell.     

The collective excitations of superfluid4He in aerogel glass

When liquid⁴He is condensed in porous aerogel glass (typical pore size ≈ 500 Å), many of the superfluid properties are significantly altered. Neutron inelastic scattering has been used to measure the collective phonon-roton excitations of liquid⁴He in this restricted geometry. Although at low temperature (1.3 K) the observed dispersion relation is identical to that of bulk⁴He, its temperature dependence is shown to be different. Previously unexplained superfluid fraction data are shown to be derivable from these microscopic measurements. An intrinsic broadening of the excitations associated with the restricted geometry is also present. Possible microscopic explanations for the unusual temperature variation of the collective excitation energies are discussed.     

Fluctuations above the superfluid transition in liquid3He

It is shown that fluctuations above the superfluid transition in liquid³He depend strongly upon the relative angular momentum1 for which Cooper pairing occurs. The effects may be calculated for any value of1 and they should be observable in the static magnetization, viscosity, and spin diffusion coefficient, giving a means of determining1. Conclusions to be drawn from existing experiments are discussed.Work supported by Energy Research and Development Administration.     

Nanokelvin thermometry at temperatures near 2 K

We present a method of temperature measurement based on a liquid³He vapor pressure thermometer with a resolution of one part in 10⁹ of the absolute temperature over the range from 1.6 to 2.2 K. The thermometer, as well as apparatus suitable for the assessment of the resolution and stability of the device, are described in detail. A method for the determination of a fixed point on the temperature scale with a resolution of 2×10^?9 is presented. Two different procedures for monitoring the long-term stability of the thermometer are discussed. The present resolution and stability of the thermometer are an improvement by two orders of magnitude over conventional germanium resistance thermometry. Although this performance level is adequate for presently planned phase transition experiments using liquid⁴He, future improvements by yet another order of magnitude seem feasible and will bring the device within an order of magnitude of the thermal noise limit.