NMR in pure3He films on a Nuclepore substrate

The properties of³He films on a Nuclepore substrate have been measured by pulsed NMR at a Larmor frequency of 10 MHz between 1.3 and 4.2 K. The³He film thickness was varied from 0.14 to 2 layers. The spin-spin relaxation timeT ₂ agrees well with previous measurements of³He films on Mylar and Vycor glass at low temperatures. The spin-lattice relaxation timeT ₁ for submonolayer films shows a strong temperature dependence consistent with a thermally activated process. This behavior has not previously been observed on amorphous substrates. The spin diffusion coefficient was measured for the thickest films at 4.2 and 2.6 K and found to be consistent with free atom motion of the³He in the vapor. In thin films or at low temperatures, the diffusion was too small to be observed. The magnetic coupling between the³He nuclei in a film and the protons in the Nuclepore substrate was determined from the effect of the³He on the proton-lattice relaxation time. It is about 100 times weaker than the interaction between³He and the fluorine nuclei in a Teflon substrate.     

Superfluid Transition of a 4He Thin Film Pressurized by Bulk Liquid 3He

We measured transverse acoustic impedance Z of normal fluid ³He at 46.6 MHz on a surface coated with a thin ⁴He film. The real component of the impedance, Z′, in the coated samples deviates from Z′ in the pure ³He in the low temperature region. Z′ on the coated samples is almost identical with Z′ in the pure sample at high temperature and gradually deviates below a particular temperature T _onset . T _onset  is possibly the superfluid onset temperature of the ⁴He film pressurized by the bulk liquid ³He. The gradual decrease in Z′ means that the superfluid component in ⁴He film increases gradually, which is expected from the dynamic KT transition at high frequency. The thicker is the film, the higher is the T _onset . The range of T _onset we observed was between 40 and 160 mK. This is much lower than that at the saturated vapor pressure. Suppression of T _onset achieved by the applied pressure from bulk liquid ³He was presumably caused by the dissolved ³He in the film, thickening of the inert layers and/or by the strong correlation effect. The result shows that the specularity of ³He quasiparticle scattering is strongly affected by superfluidity of the ⁴He film.     

Compression of3He by refluxing4He: A model for computing HEVAC effects in3He-4He mixtures

This article comprises the first part of a study concerning the effect that a flow of gaseous⁴He has on the concentration distribution of³He atoms in the presence of a super fluid film. We present a simple model in which hydrodynamic effects in the gas phase (diffusion and viscosity), and local thermodynamic equilibrium with a superfluid film are considered. Results are derived and discussed for the simple case in which a heat flow is sustained along a cylindrical tube lined with a helium film. This heat flux drives a superfluid flow in the film, and a corresponding counterflow of⁴He in the vapour. The pressure, temperature,³He concentration, and film thickness profiles along the tube are computed. Over a wide range of conditions, a dramatic reduction of the³He concentration, a large temperature increase, and a spectacular film thinning towards hotter regions are predicted to result from a heat flow. The results of a series of experiments supporting this model will be presented in a forthcoming article.Unité de recherche de l'Ecole Normale Supérieure et de l'Université Pierre et Marie Curie, associée au CNRS (URA 18).     

4He Fluid in Extremely Narrow 1D Channels 1.5?nm in?Diameter

To examine whether one-dimensional (1D) helium quantum fluid is realized in narrower channels than those studied previously, we have measured heat capacities of ⁴He adsorbed in nanoporous material FSM with straight 1D channels 1.5 nm in diameter. From the heat of desorption for adsorbed ⁴He, the coverage n _f, up to which ⁴He film grows in the channels, is determined to be 15.4 ??mol/m² using the Frenkel-Halsey-Hill model. At coverages sufficiently below n _f, the temperature dependence of the ⁴He heat capacity has a shoulder, above which adsorbed ⁴He is delocalized from the substrate. On the other hand, the depression of the heat capacity indicating quantum effects has not been observed up to n _f, which suggests that ⁴He film in the channels remains amorphous-like normal fluid. Just above n _f, the quantum effect is observed in ⁴He adatoms on the grain surface of FSM powder, which indicates that 1.5 nm channels are slightly below the limit required to realize quantum effect in the inside ⁴He fluid.     

Thermal Relaxation of 3He Solid Films Adsorbed on Graphite

No Heading The thermal conductivity between the ³He solid films and the graphite substrate was measured by the relaxation method between 100 K and 1 mK. The areal-density depedence of the thermal conductivity shows behavior similar to that of the exchange frequency J both in the submonolayer and in the second layer. These facts indicate that heat is transferred by magnetic mechanisms in the ³He solid film itself. They also imply that the ³He solid film is thermally connected with the graphite substrate only at some local spots.PACS numbers: 67.70.+n, 67.80.Gb     

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     

Thermal Relaxation in 3He Solid Films on Graphite

The thermal conductance (κ) between ³He solid films and graphite substrates is measured in weak magnetic fields of 0, 150, 300, and 600 Oe by a relaxation method. κ is markedly reduced in a magnetic field of 150 Oe and shows complicated magnetic-field-dependent variations. Previous measurements have revealed that heat flows along ³He solid films over a long distance and then flows into the graphite substrate at some local spots. The reduction in κ in magnetic field indicates an increase in the length of the thermal flow path and a decrease in the number of local spots through which heat is transferred to the graphite substrate. These suggest the important role of magnetic impurities present in the graphite substrate. The temperature-dependent variation in the ratio of κ and heat capacity (C), κ/C, is nearly unchanged when the magnetic field increases from 150 to 600 Oe, while κ shows complicated variations. κ/C shows a clear minimum at a temperature around 1 mK, which depends on the areal density of ³He. Above and below this temperature, heat transfer is thought to take place along the solid ³He film, respectively by phonons and spin excitations.     

Thin3He films on a disordered substrate: A Metal-Insulator transition

The heat capacity of thin³He films and of liquid³He underpressure up to 28 bar in porous Vycor glass has been measured at temperatures 10–600 mK. An evolution from solid (Insulator) to liquid (Metal) behaviour has been observed with increasing the film thickness from one to three atomic layers. At low temperatures the second layer has a temperature independent specific heat. Using a model of glassy second layer of³He on a rough substrate, that has a broad uniform distribution of the logarithm of nuclear exchange interaction, we calculate its specific heat and susceptibility and compare them with the available experimental data.On leave from the Institute of Solid State Physics, Russ. Acad. Sci., Chernogolovka 142432 Moscow distr., Russia     

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.      

Observation of the Fermi Fluid in 3He-4He Mixture Films Formed in One-Dimensional 28 Å Pores

A ³He film formed in quite narrow pores is one of the possible model systems for a one-dimensional Fermi fluid. Here we report our new heat capacity resugts of ³He and ⁴He film adsorbed in a straight pore 28 Å in diameter down to 5 mK. The coverage and temperature dependence of the heat capacity indicates that a fluid phase appears between the second layer promotion and the complete filling of the pores. Since the heat capacity of ³He adsorbed on the bare substrate shows a large upturn due to the nuclear heat capacity of the localized ³He, it is difficugt to observe the true heat capacity of the fluid phase. By replacing the localized ³He, we can successfully suppress the upturn and observe the true heat capacity of the fluid phase.     

Heat Capacity of Dilute 3He–4He Films on Graphite

The heat capacity of dilute ³He–⁴He films is measured to clarify whether the second adsorbed layer of ⁴He films on graphite solidify into the so-called “4/7 phase.” The ³He areal density is fixed at 0.2 nm^?2, whereas the ⁴He areal density is gradually increased. The measured heat capacities suddenly decrease with an increasing areal density approaching that of the 4/7 phase. Above the areal density of the 4/7 phase, the heat capacities do not reduce completely to zero and have finite values. The behavior of the heat capacity does not change over a rather wide areal density regime, although it suddenly increases or recovers at around the areal density of the third-layer promotion. These behaviors can be interpreted as the separation of ³He–⁴He mixture films into a ³He-rich phase and a ⁴He-rich phase, with the ³He-rich phase solidifying into the 4/7 phase and the ⁴He-rich phase remaining fluid below the areal density of the third-layer promotion. These observations strongly suggest that a ⁴He film adsorbed on a graphite surface does not solidify into the 4/7 phase.     

Surface Effects of 4He Coating on the Viscosity and the Slip Length of Normal and Superfluid 3He

We have measured the viscosity, , and the slip length, , of normal and superfluid ³ He using a torsional oscillator with a thick sample space. We coated the oscillating surface with 2.5 layers of ⁴ He film to study how the ⁴ He thin film changes the scattering mechanism of ³ He quasiparticles at the cell wall at 5 bar and 21 bar. In the normal phase, the temperature dependence of the viscosity was changed a little by the ⁴ He film at 21 bar but no change was observed at 5 bar. The slip length was enhanced by ⁴ He coating at 5 bar. This enhancement indicates the increase of specularity of ³ He quasiparticles scattering at the oscillating surface. On the other hand, a reduction of the slip length was observed at 21 bar. In the superfluid phase, the temperature dependence of supports the existence of Andreev reflection even with ⁴ He film on the surface at 5 bar and 21 bar.     

Self-Diffusion in 3He crystals

Measurements of self-diffusion in bcc ³ He 24,20-24,80 cm ³/mole were carried out by a new method in the temperature range 0.4 - 0.8 K. The vacancy diffusion coefficient was obtained by comparison of the self-diffusion data and the vacancy specific heat. It is found out that the vacancy diffusion is independent of temperature, because of spin disorder in this region. The data obtained shows that vacancies in bcc ³ He are wide-gap quasiparticles.     

Phase Diagram of 4He Film in 3D Nanopores of ZTC

In the study to examine effects of three-dimensional (3D) film connectivity on superfluid ⁴He film in the pore network, we have measured heat capacities of ⁴He film adsorbed in zeolite templated carbon (ZTC), which is an assembly of graphene fragments and has network of 1.2 nm pore with the 3D period of 1.4 nm, shorter than those of nanoporous materials studied so far. From the heat of desorption, the ⁴He film adsorbed in ZTC was observed to be formed up to about 1.4 atomic layers. Heat capacities of the ⁴He film are rather similar to those on SiO₂-based porous materials than those on graphite, except for large heat capacities of monolayer ⁴He film. At low coverages, the heat capacity rapidly decreases below a temperature T _L, suggesting a localized state of ⁴He at T<T _L. The T _L almost monotonically lowers with increasing the coverage. Heat capacity isotherms show maxima around 1.1 layers, which suggests quantum degenerate fluid (Bose fluid) above the coverage. From these results, the phase diagram of ⁴He film adsorbed in ZTC has been determined.     

Spreading of superfluid4He on MgF2

We have investigated the spreading of superfluid⁴He on top of a polished fused silica plate which is coated by a thin layer of MgF₂. Our interferometric experiments indicate that a superfluid⁴He film of about 100 nm thickness is not able to spread uniformly over a nearly horizontal substrate, and hence these films cannot be governed by the Van der Waals forces only. Even in the stationary state, we observe nonzero (1 – 20 mrad) contact angles which display hysteresis, spatial variation, and strong history dependence.     

Properties of 3He in thin 4He films

Substantial progress has been made in our understanding of the properties of ³He atoms in thin films of ⁴He. For such films the ³He occupies discrete quantum states in the film and the system is both rich and complex. Here we discuss progress in this field from several points of view; we briefly discuss early heat capacity and third sound results and concentrate on more recent NMR measurements of the magnetization and relaxation times T ₁ and T ₂. Further experimental work and theory for systems of finite ³He coverage is needed to fully understand this fascinating system.Presenter at the Elba Summer School; authors are listed alphabetically.     

Heat Capacities of Monolayer 3He Fluids Floated on A Superfluid 4He Thin Film

We have measured heat capacities of monolayer ³ He floated on a superfluid ⁴ He thin film (three atomic layers) adsorbed on graphite at low temperatures. The ³ He films behave as degenerate 2D Fermi fluids with m* 1.3m ₃, where m* is the quasiparticle effective mass and m ₃ is the bare mass of ³ He, in the whole temperature range we studied (1 T 80 mK). No anomalous behavior suggesting puddling nor other phase transitions is observed. In contrast to our previous measurements for pure ³ He films without the underlying ⁴ He film, the temperature independent heat-capacity contribution is not observed. This can be explained by the second-layer localized spins trapped on substrate heterogeneities being replaced by nonmagnetic ⁴ He.     

Specific Heat Measurements of 3He in 3He-4He Mixture Films

Preliminary data for the heat capacity of ³ He in ³ He- ⁴ He mixture films on a Nuclepore substrate are reported over the temperature range 90T165 mK, for ³ He coverages between 0.05 and 1.7 bulk-density atomic layers, and a ⁴ He film thickness of 4.33 bulk-density atomic layers. In this two-dimensional Fermi liquid system, a step structure appears in the specific heat as a function of ³ He coverage, similar to the step previously observed in the magnetization.     

Influence of4He impurities on nuclear spin relaxation in solid3He

The NMR properties of solid³He, mainly the ratio of the heat capacities of exchange and Zeeman energies and the exchange-lattice relaxation times are very sensitive to the presence of⁴He impurities, while the transverse relaxation timeT ₂ does not depend on the impurity concentration when the latter remains small. These properties can be explained in two different ways. (1) We assume an enhancement of exchange interactions around⁴He impurities. We derive the consequences of such an assumption and compare them with experimental results. For two molar volumes in the bcc phase, the locally enhanced exchange is equal to approximately7J, withJ being the exchange in pure³He. (2) Guyer and Zane introduce a mass fluctuation wave to explain the excess of heat capacity and the dependence of the longitudinal relaxation time with concentration. Both these models give rise to a four-energy bath system. As in the bcc phase, the exchange-lattice relaxation time in the hcp phase decreases when × increases at low enough⁴He concentrations. ForV=18.48 cm³ we deduce the coefficient for translational diffusion from high-temperature experiments with the help of Torrey's theory for spin-lattice relaxation.This paper is based on a thesis to be submitted in partial fulfillment of the requirements for the degree ofDocteur ès Sciences in Physics at theFaculté des Sciences d'Orsay in 1970. This thesis will be registered at the CNRS under the number AO 3704.