Visualization of Bubble Nucleation in Boiling 3He

The heat transfer properties of ³He bubbles in the nucleate boiling state have been investigated in liquid ³He below 1.0 K by using the shadowgraph method. The temperature difference between the copper surface and liquid ³He temperature was also measured as a function of heat flux in steady state. The size and number of bubbles departing from the surface in a specific time were compared using photograph recorded by a high-speed video camera at various heat flux and liquid ³He temperature of 0.5, 0.7 and 1.0 K.     

Heat Capacity of 3He Solid Film on Graphite in Magnetic Fields

Heat capacities of a submonolayer ³He solid film adsorbed on a graphite surface are measured in magnetic fields up to 400 Oe. The measured heat capacity shifts to higher temperatures with an increasing magnetic field. The amplitudes of the shifts are twenty times larger than the magnitude of the Zeeman energy, which is anomalously large. With regard to the origin of these large shifts, the reduction of frustrations and the weakening of the competition between multiple spin exchange interactions are discussed.     

4He Films near Monolayer Completion

We study the role played by the substrate–helium interaction in determining whether a ⁴He film is fluid or solid near monolayer completion. In order to glean information concerning the possible fluid-solid transition we calculate the equation of state as well as the behavior of phonons and rotons for a high density monolayer fluid film. By analyzing the behavior of the long-wavelength excitations and the roton gap, we argue that we can infer instability in the fluid phase. We study a model Lennard-Jones two parameter potential for the substrate–helium interaction. We map out in the two-parameter space the separatrix between those weak substrates that can only support a fluid monolayer from those strong substrates that support a solid. Our approach utilizes a combination of information from both variational calculations and also correlated basis function theory to examine in detail the excitation structure in the monolayer liquid as a function of film coverage and substrate potential.     

Exactly Solved Model for 4He Adsorption on Carbon Nanotube Bundles

The thermodynamics of atomic deposit adsorbed in grooves, on outer surface, and into intertube channels of a closed-end carbon nanobundle has been investigated theoretically. The grand canonical partition function for the groove subsystem was found within the transfer-matrix method with regard for interparticle interactions in chains and between the chains. The adsorption isotherms, isosteric heat and heat capacity have been calculated. The proposed theory is in excellent quantitative agreement with the experimental data on thermodynamics of helium adsorbate.     

Heat Transfer Between 3He Solid Films and Graphite in Magnetic Fields

The thermal conductance between second-layer ³He solid films and a graphite substrate was measured in magnetic fields up to 300 Oe. The thermal conductance is strongly influenced by the magnetic fields. Previous measurements in zero magnetic fields have revealed that heat is transferred only by some local spots between the ³He films and graphite substrates. This suggests that the magnetic Kapitza conductance functions as the heat transfer mechanism. The results of the measurements confirm the magnetic Kapitza mechanism.     

QCM Measurements of Superfluid Response in 4He Films up to 180 MHz

At finite frequencies, a dynamic Kosterlitz–Thouless (KT) theory predicts a frequency dependence of the superfluid transition in ⁴He films on planar surfaces. We report results of QCM measurements to study the superfluid response on planar gold surfaces for very high frequencies up to 180 MHz in the temperature range of 0.6–1.0 K. As the frequency is increased, we observed the expected KT behavior that the superfluid transition shifts to a higher temperature from the static transition temperature T _KT and the transition temperature region broadens. The frequency dependence of the dissipation peak temperature at the transition agrees with a simple equation of the frequency dependence based on the dynamic KT theory. The microscopic parameter for the dynamic transition, the ratio of the diffusion constant to the square of the vortex core radius D/r ₀ ², is estimated to be on the order of 10¹⁰ s⁻¹.     

Heat Capacity of Adsorbed Helium-3 at Ultra-Low Temperatures

We report on direct measurements of the heat capacity of monolayers of ³He adsorbed on the surface of a copper cell filled with superfluid ³He. We found that at ultra low temperatures the surface ³He heat capacity dominates over the heat capacity of the bulk liquid ³He. The replacement of adsorbed ³He by ⁴He changes the heat capacity of the sample by an order of magnitude. These investigations were made in the framework of the “ULTIMA” project, a dark matter detector based on superfluid ³He in the limit of ultra low temperatures.     

Novel Aspects of Wedge Filling by Liquid Helium

We discuss several features of the condensation of helium in linear wedges with Cs and Na walls, such as the existence of pre-filling jumps, and the relationship to prior calculations of the growth of 1D helium stripes in the outer grooves of carbon nanotube bundles. We also present preliminary results of condensation between parallel Cs plates.     

Pulsed Third Sound in the Presence of a Superfluid Flow

We report measurements of pulsed third sound in the presence of a uniform superfluid flow at T=0.89 K. The superfluid flow is created using a heater technique. For increasingly large values of power dissipated in the heater used to create the flow, the shapes of the received third sound pulses evolve. The pulses propagating parallel to the flow grow while the pulses propagating against the flow decrease. When the heater power is increased further such that film thinning effects occur, the pulses propagating antiparallel to the flow qualitatively resemble the derivative of the drive pulse. Similar effects were also observed in thin films at T=1.35 K. A possible explanation for the growth and decrease of the pulses will be discussed. The cause of the pulse shapes that resemble the derivative of the drive pulse is not understood.     

Quantum Reflection, Evaporation, and Transport Currents in 4He

We study transport currents induced by excited states in the surface and in bulk ⁴He. This requires the calculation of particle currents to second order in the excitation amplitudes. For that purpose, we extend a well-tested microscopic theory of inhomogeneous quantum liquids to find the currents created when either a quasiparticle propagates through the liquid, when atoms scatter off its surface, or when excitations evaporate atoms. Specifically, we look at the phonon lifetime and quantum reflection. We show that quantum reflection is sensitively affected both by details of the liquid surface profile and by many-body effects.     

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.     

A Temperature-Controlled Device for Volumetric Measurements of Helium Adsorption in Porous Media

We describe a set-up for studying adsorption of helium in silica aerogels, where the adsorbed amount is easily and precisely controlled by varying the temperature of a gas reservoir between 80 K and 180 K. We present validation experiments and a first application to aerogels. This device is well adapted to study hysteresis, relaxation, and metastable states in the adsorption and desorption of fluids in porous media.     

Preliminary Heat-Capacity Measurements of 2D Solid 3He Adsorbed on Graphite Preplated with 4He

We describe preliminary resugts of heat capacity measurements of the 4/7 phase in two dimensional ³He adsorbed on graphite preplated with a ⁴He monolayer in a temperature range between 0.17 and 20 mK. In zero magnetic field, we observed a double-peak structure similar to that reported previously for the same phase in a different system (³He/³He/gr) in which the gapless quantum spin-liquid ground state is proposed. The exchange interaction deduced from the high temperature data is by a factor of 30% smaller than the previous one, presumably because of the smaller lattice constant for the ³He/⁴He/gr system. The present resugt provides strong evidence that such a peculiar temperature dependence of heat capacity is characteristic of the 4/7 phase. Application of magnetic fields up to 0.65 T on this phase does not seem to change appreciably the higher temperature broad peak at around 1.4 mK.     

Measurements of Third Sound Attenuation at T=1.35 K

The results of measurements of ordinary third sound attenuation at T=1.35 K are reported. The attenuation is measured by using a continuous wave technique for frequencies up to 40 kHz. We find that for helium film thicknesses in the regime 5.09≤d≤9.90 layers the attenuation of third sound increases both with frequency and film thickness. We discuss our results in the context of previous work in this area.     

The solubility of spin-polarized3He in4He and the superfluidity of3He in3He-4He mixtures

We investigate the possibility of a large enhancement of the T = 0 finite solubility of³He in⁴He due to spin-polarization. The size of the effect depends on the fraction of³He atoms in the system. We present two different approaches for the limits of a small and a large number of³He atoms compared to the number of⁴He atoms. Since the possible³He superfluid phase transition depends on³He density, we calculate the consequences of this change in the solubility for its superfluid transition temperature. It is shown that for small fractions of³He, the transition temperature is enhanced mostly due to the enlargement of the up-spin Fermi sphere. In the opposite limit the transition temperature is enhanced as a result of the increased³He solubility.     

Pulsed Nuclear Magnetic Resonance on 3He Adsorbed on Bare and 4He Preplated MCM-41 Using DC SQUID Detection

We report low field DC SQUID NMR measurements down to 1.5 K of ³He adsorbed in the pores of the mesoporous molecular sieve MCM-41. In the first experiment measurements were made on ³He adsorbed onto the bare pore walls of MCM-41 with coverages ranging from $n_{^{3}\mathrm{He}}=0.86n_{1}$ to full pores at $n_{^{3}\mathrm{He}}=1.79n_{1}$ , where n ₁ is the coverage for monolayer completion. A second experiment was performed with low ³He coverages ( $n_{^{3}\mathrm{He}}\sim0.01n_{1}$ ) on ⁴He preplated pores, where a crossover to a quasi-1D state is expected to occur at temperatures sufficiently below 700 mK. In both experiments relaxation times T ₁ and T ₂ ^* were measured as a function of temperature and coverage at frequencies from 80 to 240 kHz. The frequency dependence of the linewidth in the pure ³He experiment is extremely weak therefore T ₂ ^* ≈T ₂. The 1.5 K isotherm shows a small minimum in T ₂ ^* at a coverage corresponding to monolayer completion. In the experiment with ⁴He preplating there was no significant change in T ₁ or T ₂ ^* when the ³He coverage was doubled from $n_{^{3}\mathrm{He}}=0.01n_{1}$ to 0.02n ₁ at a ⁴He preplating of $n_{^{4}\mathrm{He}}=1.05n_{1}$ . This suggests that the relaxation times are dominated by single particle effects in the low density regime.     

Experiments on the Self-Organized Critical State of 4He

When a heat flux Q is applied downward through a sample of ⁴He near the lambda transition, the helium self organizes such that the gradient in temperature matches the gravity-induced gradient in T _λ . All the helium in the sample is then at the same reduced temperature and the helium is said to be in the Self-Organized Critical (SOC) state. We have made the first measurements of the ⁴He SOC state specific heat, C _{∇ T} (T(Q)). There is no measurable difference between C _{∇ T} and the static zero-gravity ⁴He specific heat for temperatures between 650 and 250 nK below T _λ . Closer to T _λ , the specific heat is depressed and reaches a maximum value at 50 nK below T _λ . This depression is similar to that predicted theoretically as reported by R. Haussmann (Phys. Rev. B 60, 12349, 1999). Contrary to the expectations of theory, however, we see another depression far below T _λ . In addition, over the heat flux range of 30 nW/cm² to 13 μW/cm², we have made improved measurements of the speed of a recently discovered propagating thermal mode, which travels only upstream against the nominal heat flux of the SOC state. We are able to accurately predict the speed of this wave by treating the helium of SOC state as a traditional fluid with a temperature dependent thermal conductivity.     

Interfacial Friction of 3He Films Adsorbed on Grafoil

We measured the interfacial friction of ³He films adsorbed on Grafoil (exfoliated graphite) using the quartz-crystal microbalance (QCM) technique, and compare present results with those of ⁴He films. In the same manner as ⁴He films, the friction decreases below a certain temperature T _S , and T _S depends on the areal density of ³He film. In two-atom and three-atom thick films, however, T _S of ³He films is lower than that of ⁴He films. Furthermore, the decrease in the friction still remains above five-atom thick films of ³He while it disappears in four-atom thick films of ⁴He and the superfluid onset is manifest.     

Thermal Effects on the Microscopic Properties of 4He Drops

We present path integral Monte Carlo results of small ⁴He drops at very low temperatures around the transition temperature to the normal phase. The results obtained are compared with the ones obtained at zero temperature using the diffusion Monte Carlo method. The analysis is carried out for drops of different size and it is focused on the determination of their energetic and structural properties. Particular emphasis is devoted to the temperature dependence of the density profile, mainly to determine if any sign of the λ-transition could be observed on them.     

Influence of 4He Preplating on 3He Fluid Formed in One-Dimensional 28 Å-Pores

No Heading A degenerate Fermi fluid has been realized recently for ³He films adsorbed in one-dimensional (1D) pores 28 Å in diameter, preplated with ⁴He layers. In order to study the influence of the ⁴He preplating on the ³He Fermi fluids, heat capacity was measured down to 4 mK for two preplated ⁴He films of different thickness. At low temperatures, no substantial difference was found between the two cases. On the other hand, a large difference appeared above about 150 mK. The large ³He heat capacity for the thick ⁴He preplating suggests thermal excitation of the ³He atoms to the excited energy levels of the Andreev states in the ⁴He fluid layers.PACS numbers: 67.70 +n. 67.55 Cx.