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

Additional data from our on-going experiments 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 1.4 bulk-density atomic lagers, on a ⁴ He film of thickness 4.33 bulk-density atomic lagers. This is a two-dimensional Fermi liquid system, in which we can change the ³ He coverage and thus tune the Fermi temperature.     

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.     

Heat Capacity Measurements of 3He-4He Mixture Films

We report heat capacity measurements from our on-going experiments on two-dimensional liquid ³He on superfluid ⁴He thin films absorbed on Nuclepore. The ³He coverage dependence of the heat capacity for 0.02–0.50 bulk-density layers of ³He with 3.10 bulk-density layers of ⁴He over the temperature range 40≤T≤200 mK is presented. We find the effective mass of ³He on 3.10 layers of ⁴He is greater than that on 4.33 layers of ⁴He, consistent with our previous magnetic susceptibility measurements.     

Spin Diffusion Measurements in Spin-Polarised Liquid 3He-4He Mixtures

We report on investigations of spin diffusion in isotopic helium liquid mixtures using a multi-echo technique in applied magnetic field gradients. Samples (up to 1 cm ³ in volume) are obtained by liquefaction of optically polarised gas. NMR measurements are performed at saturated vapour pressure in a low magnetic field (2.2 mT). Signal-to-noise ratios stay high down to ³ He concentrations of order a few parts per thousand at 1K for instance. We demonstrate that the experimental accuracy of spin diffusion measurements is substantially improved using optically polarised mixtures. We have observed that the dynamics of the precession of the transverse magnetisation can be altered due to large dipolar fields at high magnetisation densities. Effects on spin-echo signals and implications for the extension of spin diffusion measurements with our technique are discussed.     

Quartz Oscillator Study of Monolayer 4He and 3He - 4He Mixture Films Adsorbed on Solid H2

The superfluid transition in submonolayer and monolayer ⁴ He films and ³ He - ⁴ He mixture films on solid H ₂ has been studied using a quartz crystal microbalance technique. Kosterlitz-Thouless (KT) transitions were observed in submonolayer ⁴ He films with density greater than 0.062 ± 0.002 Å ^–2 . We determine a binding energy of ⁴ He to 0.241 Å ^–2 H ₂ of –15.7 K in the. presence of 1 monolayer of ⁴ He. At several ⁴ He coverages, a range of submonolayer ³ He coverages was studied (n ₃ 0.0567 Å ^–2 ). With each increase in the ³ He coverage, the KT transition temperature decreased. For the higher coverage mixture films studied (n ₄ 0.0726 Å ^–2 ) we observed an apparent second decoupling of the film from the quartz oscillator frequency in addition to the KT transition. We have studied the. coverage dependence of this new feature.     

Evaporative Cooling of 3He-4He Mixture Films

Heat transport by saturated ³ He- ⁴ He films has been studied at temperatures 50..350 mK and the bulk concentration of ³ He ranging from 0.1 ppm to 5%. The cooling of the film, when locally heated above 160 mK, is mainly via 2D flow of surface ³ He from colder area followed by evaporation of ³ He. At certain heating power the 2D flow becomes a bottleneck, the heated spot runs out of ³ He and its temperature abruptly increases. The critical power is nearly proportional to the surface density of ³ He. For higher ³ He concentrations another distinct step in temperature has been observed at a lower heating power. It is attributed to the existence of an excited ³ He surface state whose population starts at ³ He surface density of 3.5×10 ¹⁴ cm^–2 . The second state is located about 1.2 K higher in energy than the ground state and provides an additional channel for the 2D flow of ³ He.     

The Two-Dimensional World of 3He in 3He-4He Mixture Films

For thin films of ⁴ He at low temperatures the addition of sub-monolayer numbers of ³ He atoms results in the ³ He residing in a ground state at the free surface of the ⁴ He film. For low ³ He coverage the ³ He are a nearly ideal two-dimensional Fermi system. With increasing numbers of ³ He atoms the interactions among the ³ He increase. The results from a number of experiments that reveal the remarkable properties of this rich, interacting, two-dimensional ³ He system are described. These results include the magnetization, the energetics of the ³ He atoms in quantum states, the diffusion of the ³ He along the ⁴ He film, and the ³ He effective mass. The techniques employed include NMR, specific heat, quartz crystal microbalance and third sound. A clearer picture of the properties of the ³ He is now emerging, and preliminary numbers for the ³ He coverage dependence of the Fermi Liquid parameters are presented. Prospects for the observation of superfluidity of the ³ He in this system are discussed.     

Spin Diffusion Anisotropy in Liquid 3He

We report new experiments on the transverse and longitudinal spin dynamics of liquid ³ He polarized by an 11.3T field. At the lowest temperatures probed, the transverse spin diffusion coefficient saturates with an apparent anisotropy temperature of T_a=12±2 mK. This temperature dependence is in accord with the general idea of spin-diffusion anisotropy but the value of T_a is half that expected from extrapolating earlier experiments at a lower field. Strong magnetostatic effects in the form of long-lived spin modes are also observed for small gradients and small tipping angles.     

Spin-Wave Modes and Spin Diffusion Anisotropy in a 3He-4He Mixture at High B/T

We present nmr measurements in a 6.2% ³ He- ⁴ He mixture carried out at 11.3 T with temperatures down to 3.5 mK, yielding a very high B/T ratio. We have measured the transverse spin diffusion with the spin-echo technique. We observe a saturation of the spin diffusion coefficient at low temperature corresponding to an anisotropy temperature T_a13mK. Using the same setup, we were able to excite Silin spin-wave modes in the experimental cell.     

Fast Diffusion Process in Quenched hcp Dilute Solid 3He–4He Mixture

The study of phase structure of dilute ³He–⁴He solid mixtures of differing quality is performed by the spin echo NMR technique. The diffusion coefficient is determined for each coexistent phase. Two diffusion processes are observed in rapidly quenched (non-equilibrium) hcp samples: the first process has a diffusion coefficient corresponding to hcp phase and the second one has huge diffusion coefficient corresponding to liquid phase, which is evidence of liquid-like inclusions formation during fast crystal growing. It is established that these inclusions disappear in equilibrium crystals after careful annealing.     

Transverse Spin Diffusion in 3He-4He Mixtures—Part II

This is the second article in a series of two dealing with anisotropic spin diffusion. We describe a model for calculating spin-wave modes and spin-echo signals in ³He-⁴He mixtures. The model takes into account finite-size effects for a cylindrical sample. Using this model, we analyze the data presented in the first article, and conclude that the spin-rotation parameter saturates at low temperature in spin-polarized 6.2% ³He-⁴He mixture.     

Transverse Spin Diffusion in 3He-4He Mixtures—Part I

There is currently a debate on whether or not the transverse relaxation time is finite at low temperature in spin-polarized Fermi liquids. The experimental results so far have been contradictory. In this paper, the first in a series of two articles, we present NMR measurements carried out in 6.2% ³He-⁴He mixture at 11.3 T down to 3.5 mK. A careful analysis of our data yields an anisotropy temperature T _a = 6.1±1 mK, which is smaller than the values reported previously. The difference between this value and the earlier results is ascribed to restricted diffusion effects which were neglected in the previous analyses. The details of the analysis are given in the companion paper.     

Recombination of Spin Polarized Atomic Hydrogen Adsorbed on a Dilute 3He-4He Mixture

We have been searching for two-dimensional (2-D) superfluidity of spin-polarized atomic hydrogen (H) adsorbed on liquid helium surface. We have investigated H adsorbed on ³ He- ⁴ He mixtures instead of pure ⁴ He because the thermal coupling of H to the mixtures is stronger than ⁴ He. ESR at 129 GHz was carried out to detect H at 4.5 T by using the Fabri-Perot confocal cavity with the cold spot located at the confocal point and thus the 2-D H signal was effectively detected. We investigated the two-body bulk and surface recombination processes of H on a 3% ³ He ³ He- ⁴ He mixture film. From the analysis of the surface recombination process, we found the adsorption energy of H was a 0.6 K for the mixture film in the temperature range between 120 mK and 250 mK, greater than the 0.3–0.4 K found previously for pure ³ He and a 67% ³ He ³ He- ⁴ He mixture.     

3He Anomalously High Spin Diffusion in Solid Para-Hydrogen

NMR measurements of ³He spin diffusion coefficient in solid para-H₂ are carried out at the temperatures 0.45–1.5 K. The crystals have been grown under constant pressure 20–130 bar. The ³He concentrations in the initial para-H₂–³He gas mixtures were 0.1% and 0.3%. It is found out that the decay of echo amplitude vs both magnetic field gradient G and time interval τ between RF pulses is of non-exponential character, typical of one-dimensional diffusion in restricted geometry. The values of true spin-diffusion coefficient D _S measured are found to be ∼10⁻⁴ cm²/s at 20 bar. At 108 bar D _S value is one order of magnitude less. D _S does not depend on temperature. Such spin diffusion coefficient values seem to be anomalously high in comparison with well-known values of D _S =10⁻⁵ cm²/s for bulk liquid ³He at 27 bar and D _S =10⁻⁸ cm²/s for bulk solid ³He at 108 bar. The special experiments with the crystal annealing make it clear that the high spin diffusion here is connected with fast diffusion along dislocation lines.     

Superfluid Phase Transition of 3He-4He Mixture Films Adsorbed on Alumina Powder

The superfluid phase transition of ³ He- ⁴ He mixture films adsorbed on 500 Å alumina powder has been studied for mixture films whose superfluid thickness is less than a monolayer. The transitions are found to be controlled by the Kosterlitz-Thouless critical line, but a strong broadening of the transition is observed as the ³ He concentration is increased. Analyzing the broadening in terms of a KT vortex-pair theory modified for the the finite powder size yields a vortex core parameter which increases nearly linearly with added ³ He. Also observed in these measurements is a temperature-dependent and ³ He-dependent depletion of the superfluid density at low temperatures, which is thought to arise from the high-frequency ripplon/third sound excitations of the film.     

Compressibility of 4He Liquid Confined in Nuclepore

Adding and removing ⁴ He from an experimental cell which contains the porous material Nuclepore results in capillary condensation, hysteresis and avalanches. The volume of liquid in the nearly cylindrical pores is monitored with a capacitance technique. For the situation when all of the pores are full, the initial removal of ⁴ He from the cell results in a slight non-hysteretic decrease in the capacitance, which we attribute to the isothermal compressibility of ⁴ He. As ⁴ He atoms are removed, the surface tension at the meniscus applies a negative pressure to the liquid in the filled pores, which results in a reduction of the density and a decrease in the capacitance. We present results for ⁴ He capillary condensed in 2000 Å pore diameter Nuclepore.     

Cooling Mechanism of Spin Polarized Atomic Hydrogen Adsorbed on Dilute 3He-4He Mixture Film

We cooled down the two-dimensional (2-D) spin-polarized atomic hydrogen (H) adsorbed on a small surface called cold spot, whose temperature was kept much colder than the rest of the sample cell. The 2-D density of H was controlled by an input flux of H into the sample cell. We investigated the surface recombination of H adsorbed on 3% ³ He ³ He- ⁴ He mixture. ESR at 129GHz was carried out to detect H by mirror cavity with the cold spot located at the confocal point. We calculated the thermal boundary resistance between the absorbed H and the mixture. From the temperature dependence of the surface recombination together with calculated boundary resistance, we obtained the temperature dependence of _a. The result agrees with the previous measurement. Two-D H on the mixture film was cooled well down to 40mK.     

3He and 4He Coverage Dependence of FA 0 and FS 1 in Thin Mixture Films

No Heading We report additional heat capacity measurements and analysis from our experiments on two-dimensional liquid. ³He on superfluid ⁴He thin films absorbed on Nuclepore. The ³He heat capacity for sub-monolayer coverages of ³He with 3.10 bulk-density layers of ⁴He for T 40 mK is discussed. We incorporate these measurements with earlier NMR work in similar coverage ranges and deduce preliminary values for the two-dimensional Fermi Liquid parameters for the ³He for 3.10 layers of ⁴He.PACS numbers: 67.60.Fp, 67.65.+z.     

Simulations of Non-linear Spin Dynamics in Spin Polarized Dilute 3He–4He Mixtures

Studies of spin dynamics of highly polarized dilute mixtures of ³He in superfluid ⁴He have been performed by various researchers over the past three decades. One series of experiments performed at Cornell University in the early 1990’s revealed a novel long timescale excitation. We present the numerical solution of the non-linear Leggett spin dynamics equation in one spatial dimension subject to boundary conditions consistent with the Cornell experiments. Experimentally observed phenomena are composed of trains of bursts in the transverse magnetization lasting several seconds. The simulations capture the time evolution of the individual bursts localized in time. Preliminary results of two dimensional simulations are also presented.