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.     

Large Mass Enhancement in 3He Monolayer Fluids Adsorbed on Graphite Preplated with 4He

A fluid monolayer of ³He adsorbed on a graphite surface is an ideal twodimensional (2D) Fermi system. We have observed very large mass enhancement of the quasiparticles (m*/m=20±5) in preliminary heat-capacity measurements for high density ³He fluids adsorbed on graphite preplated with a ⁴He monolayer (³He/⁴He/gr). This could be divergence of m* toward the critical density for the 4/7 phase with a triangular lattice, which is a registered phase commensurate against the first layer. The observed m* is larger than those reported for other 2D ³He systems, e.g., m*/m=13 for ³He/HD/HD/gr system, presumably because of the higher critical density. The apparent divergence of m* toward the different Critical densities strongly suggests the Mott–Hubbard type transition between the 2D Fermi fluid phase and the registered phase under a strong onsite hard core repulsion between the 3He atoms.     

Universal Features of the Superfluid Transition of a Thin 4He Film on an Atomically Flat Substrate

We analyse recent observations, using a torsional oscillator, of the superfluid transition of a fluid ⁴ He bilayer, within the framework of a modified dynamic theory of the Kosterlitz-Thouless transition. A parametric plot of the real and imaginary parts of the complex superfluid density, determined from the measured period shift and dissipation, achieves a high degree of collapse of the experimental data at different coverages onto a universal curve. This is compared with the result of the recent theory.     

Superfluid 3He Thin Films with Specularly Reflecting Walls

We calculate the specific heat of superfluid ³ He film thinner than the coherence length. In such a thin film, quantization on the confined direction makes eigenstates discrete, thus there appears a set of Fermi disks which are just like subband. Because of both the discreteness and ABM-like energy gap, the specific heat exhibits Schottky type character, which becomes conspicuous in special thin region.     

Structural and Superfluid Properties of the 4He Monolayer on a C28 Molecule

We have performed path-integral Monte Carlo calculations to study the adsorption of N ⁴He atoms on a single C₂₈ fullerene molecule. Radial density distributions show a layer-by-layer growth of ⁴He with the first adlayer being located at a distance of 5.3 Å from the center of C₂₈. The monolayer is found to show a commensurate structure at N=16 with each of the 16 adsorption sites on the molecular surface being occupied by one ⁴He atom. As more helium atoms are adsorbed beyond N=16, the adlayer is in a mixed state consisting of 4 localized atoms at the hexagonal faces and the other atoms delocalized over the pentagonal faces. Another structurally-ordered state is observed at N=32, where the helium layer shows the same crystalline structure with an icosahedral symmetry as observed for 32 ⁴He atoms on a C₆₀ molecule. It is found that more ⁴He atoms can be squeezed into the first layer to disrupt this icosahedral structure when enough ⁴He atoms are added in the second layer. Finally we observe the reentrant superfluid response of the monolayer with superfluidity being quenched completely at the ordered states of N=16 and 32.     

Adsorption Structure of 3He Monolayer Solid Film on Grafoil

The adsorption structure of ³ He submonolayer solid film on a, Grafoil Surface is considered. The adsorption energies of some supposed structures are calculated by the path integral Monte Carlo simulations. According to these calculations the evolution of the adsorption structure is discussed. Two structural, phase transitions are predicted there. The phase boundaries coincide with the specific features in the results of heat-capacity measurements. The commensurate - incommensurate phase transition seems responsible for the sudden ferromagnetic peak of the exchange frequency.     

Thickness Dependence of Critical Current of Superfluid 3He Film

We have measured the critical current J _c , which is defined as the onset of dissipative flow, for a thickness range from 0.3 to 4 μm using inter-digitated capacitors. In the thickness dependence of J _c , two distinct dissipation regimes were observed. The crossover occurred at a thickness of ∼1 μm.     

Measurement of Superfluid 3He Film Flow by Inter-digitated Capacitors

We report the measurements of the critical current of superfluid ³He film for the thickness of 0.47 µm. By employing inter-digitated capacitors, 2.5×10^?2 mm^{3 3}He can be manipulated with 0.4% resolution. Critical current J ^f _c of superfluid ³He film were observed for the onset of dissipative flow. The magnitude of measured J ^f _c is considerably smaller than the theoretically predicted pair-breaking critical current. This resugt indicates that another mechanism must participate in the dissipation of superflow.     

Universal Features of the Superfluid Transition of Thin 4He Films on Mylar

Data from a series of torsional oscillator experiments performed on ⁴ He films on Mylar have been available for some time. However, up until now it was thought that disorder in the substrate leads to a wide variation in the results. Here the data is analysed in a new way which captures for the first time the universal features of the results. Comparisons are made with recent theoretical work which predicts universal behaviour in the superfluid film transition and with the results of similar experiments performed on a Grafoil substrate.     

Low Temperature Heat Capacities of Submonolayer Solid 3He

Heat capacities (C) of ³ He submonolayer solids adsorbed on a graphite surface are measured down to 100 K, a factor of twenty lower temperatures than previous work. At a real densities near the commensurate solid (6.4 nm ^–2 ), an anomalous temperature dependence, C 1/T, is observed in a wide temperature range over two orders of magnitude (0.1 T 20mK). Similar behavior was observed for the commensurate solid in the second-layer with the same density, suggesting a common microscopic mechanism. It is, however, a puzzle that the solid is ferromagnetic according to recent magnetization measurements by ISSP group, while the solid is known to be antiferromagnetic. Possible explanations for this conflict, such as vacancy effects or multiple-spin exchanges modulated by a substrate potential corrugation, are discussed.     

Heat Capacity of the Second Layer of 3He Adsorbed on Graphite Pre-plated with a 4He Monolayer

The second solid layer of ³He on graphite is a highly frustrated magnetic system, as a resugt of its triangular symmetry and the importance of ring exchange (mugtiple spin exchange). This is predicted to lead to a quantum spin liquid ground state. Experiments in which the first paramagnetic solid layer is replaced by ⁴He have been undertaken by several groups, allowing more precise measurements of the second layer magnetisation in an attempt to infer the spin gap of the putative spin liquid phase. We report the first measurements of the influence of such pre-plating on the second layer heat capacity, in the vicinity of the commensurate solid phase. We find the differences in heat capacity of the two systems to be significant. The effective exchange constant is reduced by pre-plating by a factor of order 2/3. In addition the value of the heat capacity at its maximum is significantly reduced by approximately 20%. Can these differences be accounted for within a model of intra-layer mugtiple spin exchange on a triangular lattice?     

Effective Heat Conductivity and Relaxation Processes in Superfluid Mixtures of 3He-4He

The effective thermal conductivity coefficient'ceff in superfluid ³He-⁴He mixtures with concentration of 9.8% ³He has been studied experimentally between 100 and 500 mK, where the main contribution to the kinetic processes is made only by phonons and ³He impurity excitations. In this case the effective thermal conductivity is a combination of diffusivity, thermal conductivity and thermal diffusion. The κ _eff value was found from stationary measurements of the temperature gradients caused by the thermal flow and from the temperature relaxation kinetics. Both the methods provide consistent resugts which also agree with those on effective thermal conductivity calculated in terms of the kinetic theory of phonon-impuriton system.     

Self-Organized Heat Transport Near the Superfluid Transition in 4He

Heat transport through a column of superfluid ⁴ He has been observed experimentally to self-organize, resulting in a thermal gradient that exactly matches the gradient in the superfluid transition temperature across the column, leaving the entire sample at a constant distance from the superfluid critical point¹. We describe a new experiment that is designed to accomplish three objectives: 1) search for the upper critical heat flux above which self-organized heat transport can no longer occur, 2) measure the heat capacity of the self-organized heat transport state, and 3) test recent theoretical predictions^2,3 of the microscopic mechanism that is responsible for this self-organization.     

Vortex-Loops and Solid Nucleation in Superfluid 4He and 3He

We propose a new model for the nature of the nucleation of solid from the superfluid phases of ⁴He and ³He. Unique to the superfluid phases the solid nucleation involves an extremely fast solidification front. We depart from the usual quasi-static treatment of solid nucleation by proposing that the nucleation of a solid seed is helped by the simultaneous nucleation of vortex-loops in the superfluid around it. It is the composite entity which is nucleated out of the over-pressurized liquid. This occurs when the local release of pressure creates a velocity field in the superfluid which in turn facilitates the nucleation of vortex-loops. The kinetic energy gain of this process balances the surface tension, as the solid surface is quickly covered by many vortex-loops (hairy snow-ball). We show that this scenario gives good agreement with many experiments on heterogeneous nucleation, where the energy barrier is found to differ with the classical theory of homogeneous nucleation by 8 orders of magnitude. We propose several experiments that could show the involvement of vortices with solid nucleation.     

Pulsed Nonlinear Sound in Superfluid 4He and 4He-3He Mixtures

Sound modes in ⁴ He and ⁴ He- ³ He mixtures which arise out of the two-fluid equations are made up up a vector convective flow and scalar temperature changes. A method for modeling nonlinear pulses of sound with geometric approximations to the vector and scalar components has recently been applied towards understanding nonlinear second sound near the lambda point. ⁶ The same method may be used, in general, for modeling linear and nonlinear sounds in Helium II. We demonstrate with a model for nonlinear second sound pulses in ³ He- ⁴ He mixtures and compare the results to experimental observations.     

Vortex-Loops and Phase Nucleation in Superfluid 4He and 3He

We propose a new model for the nature of the nucleation of solid from the superfluid phases of ⁴ He and ³ He. A fast solidification event in the superfluid results in a local release of pressure and a velocity field in the superfluid. This in turn facilitates the nucleation of vortex-loops. The kinetic energy gain of this process balances the surface tension, as the solid surface is quickly covered by many vortex-loops (hairy snow-ball). We show that this scenario gives good agreement with experiments on heterogeneous nucleation.     

Quartz Microbalance Study of Thick 4He Film Near the Superfluid Transition

We present the resugts of a quartz microbalance measurement of a ～440 Å thick ⁴He film adsorbed on Au. This measurement confirms the resugt of an earlier capacitance measurement that showed a dip in the film thickness just below Ta due to the critical Casimir force. The magnitude of the dip in the two measurements agrees within 15% . In addition to the dip, we see a signal due to the onset of superfluidity in the film. An apparent peak in the film thickness, not observed previously, is also found coinciding closely with the bulk T _λ. This extra feature may be due to a sound resonance in the cell associated with the bulk fluid.     

A First Order Normal to Superfluid Transition: Prewetting in the 4He-Rich Film below Liquid 3He

We summarize our earlier work on the prewetting transition in the ⁴He-rich film in contact with liquid ³He at low temperatures. We make a new fit to the experimental data using a simpler assumption than previously. The strong pressure dependence of the jump in film thickness at the transition D^s _min(P) is quantitatively explained. It is due to the depletion of the ³He concentration in the superfluid film. This is caused by the van der Waals field of the wall. As a result of the pressure dependence, the prewetting line begins at a quantum (T=0) critical point at 13.2 atm. The T=0 dewetting pressure is predicted to be 32.5 atm. The low temperature phase diagrams for the system are drawn in the D^s vs pressure plane and in the plane (₄–₃) vs ₃, where ₄ and ₃ are the chemical potentials. We predict that the prewetting line is metastable above an applied pressure of 25.75 atm. Above this pressure, bulk hcp ⁴He crystals should appear.     

Critical Supersaturation of Superfluid 3He-4He Mixtures

We report the results of our systematic study of the phase separation in supersaturated ³He-⁴He liquid mixtures of the ³He-dilute phase. The amount of the critical supersaturation is determined within the wide ranges of the temperature (0.4–645mK) and the pressure (1–8.5kgf/cm²). Using the data we construct the surface of critical supersaturation, x _3,cr (T,P), which enables us to recognize almost the overall behavior of the critical supersaturation in superfluid ³He-⁴He mixtures. The main specific features observed are (i) below 10mKx _{3, cr} is almost temperature-independent, (ii) above 10mK up to 500mKx _{3, cr} increases with the temperature, and (iii) above 500mKx _{3, cr} decreases smoothly with the increase of temperature down to zero at the tricritical point. We give various discussions from the viewpoint of the quantum nucleation, the classical thermal nucleation and the crossover between them.