3He Effect on 2D Superfluidity in 3He–4He Mixture Films on Planar Gold

There have been a number of experiments exploring the nature of 2D superfluidity and the configuration of ³He–⁴He mixture films on various substrates. To date, a possible film-structure at T=0 is that of a simple layer model, ³He/superfluid ⁴He/solid-like ⁴He/substrate, in which the submonolayer superfluidity is strongly affected by the coverage of the ³He overlayer. Yet the mechanism is not been fully understood. In this paper, we report a QCM study at 60 MHz for the ³He effect on the superfluidity of mixture films on flat gold, mainly focusing on the anomalous depletion of the temperature dependence of the superfluid density σ _s. In the measurements, we kept the ³He coverage constant (n ₃= 0, 3.6, 7.2, 19.0, 57.2, or 92.8 μmol/m²) and then incrementally added ⁴He. We observed the evolution of the ³He effect on σ _s(T) with increasing ³He coverage; this depletion of σ _s(T) rapidly increases and then saturates near n ₃～1 layer. From the analysis of the linear-temperature region in the plot of the dissipation peak temperature T _p as a function of the superfluid ⁴He coverage n _4s and comparison with previous studies on Mylar and porous gold, we found a universal function for the strength of the ³He effect for all substrates.     

Simultaneous Measurement of Torsional Oscillator and?NMR in Solid 4He with 10?ppm of 3He

We have performed the simultaneous measurement of torsional oscillator and NMR in solid ⁴He with 10 ppm of ³He at 3.6 MPa. In this solid, NCRI response appears below about 400 mK. NMR measurement shows that there is the same kind of phase-separated ³He cluster which is found in our previous measurement in solid ⁴He with over a hundred ppm of ³He. When we warm the solid above the phase separation temperature, the cluster disappears gradually. Below and above the phase separation temperature, the distribution of ³He atoms changes significantly with long time constant, which is as long as a day. However, even in such a long time span, we do not observe any systematic changes in the torsional oscillator response. This result suggests that the phase separation and related changes of the distribution of ³He is not directly related to the impurity effect of the NCRI response.     

The Electron–Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering

Atom scattering is becoming recognized as a sensitive probe of the electron–phonon interaction parameter λ at metal and metal-overlayer surfaces. Here, the theory is developed, linking λ to the thermal attenuation of atom scattering spectra (in particular, the Debye–Waller factor), to conducting materials of different dimensions, from quasi-1D systems such as W(110):H(1 × 1) and Bi(114), to quasi-2D layered chalcogenides, and high-dimensional surfaces such as quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of λ obtained using He atoms compare favorably with known values for the bulk materials. The corresponding analysis indicates in addition, the number of layers contributing to the electron–phonon interaction, which is measured in an atom surface collision.     

Path Integral Calculation of 4He in?Quasi-one-dimensional Channels

We consider ⁴He particles confined in a quasi-one-dimensional nanoscale channel in the range of liquid density, and numerically calculate the energy per particle by the imaginary time path integral Monte Carlo method. When the channel diameter becomes less than 1.6 nm, T _onset, at which the energy drops due to the particle exchange effect, shifts to lower temperature, and disappears below 0.48 nm. The diameter dependence of T _onset is similar to the behavior of the superfluid onset temperature, T _o, for mesoporous materials FSM16 observed by the torsional oscillator.     

Theoretical Study of Quantum Gel Formation in?Superfluid 4He

Bosonic density functional theory calculations were carried out for neon, argon, and fluorine based systems in superfluid ⁴He with an emphasis on the formation of dimeric species in the liquid. These atomic species display relatively strong binding with helium and hence their solvation structures in the liquid exhibit highly localized liquid helium layers around them. These solvent layers modify the gas phase dimer potentials by inclusion of a recombination barrier, which provides stabilization for the solvated atoms. Of closed shell species neon is shown to exhibit a recombination barrier of 3 K for the dimer and up to 5.8 K for specific cluster geometries. For argon, the liquid induced potential barrier is only 0.7 K and it has a rather large amount of excess energy available along the recombination coordinate indicating that it is not possible to stabilize argon atoms in superfluid helium. Atomic fluorine shows the most pronounced effect with the recombination barrier of 26.8 K for producing ground state F₂. It is concluded that neon and fluorine atoms are good candidates to form impurity based quantum gels in bulk superfluid helium.     

??On the Superfluidity of Liquid He3??. Author??s Recollections

The history of the author??s paper (Pitaevskii in J. Exp. Theor. Phys. 37:1794, 1959; Sov. Phys. JETP 10:1267, 1960) on the superfluidity of liquid He³ is presented.     

Mobility of 2D Electrons on Pure 4He and on 3He–4He Dilute Solution

The mobility of 2D electrons on pure ⁴He and on 0.5 % solution of ³He in ⁴He was investigated for different electron densities in the temperature range 0.12 to 1.3 K. The electrons in the same electron density show the same transition temperature from liquid state to Wigner crystal state in both pure ⁴He and in the solution. In the high temperature range where the gas-scattering is dominant, the electrons show a smaller mobility in the solution than in the pure ⁴He due to the electron collision with ³He gas atoms which have a higher vapor pressure. In the middle temperature range where the ripplon-scattering is dominant, the mobility in the solution is smaller than in ⁴He. This is explained by a smaller surface tension caused by ³He atoms collected at the surface. In the low temperature range where electrons are in the Wigner crystal state, the mobility gradually increases with decreasing temperature in the solution, while it stays almost constant in the pure ⁴He. The mobility increase is more pronounced in the low electron density. The results are qualitatively in agreement with the existing theory which includes the bulk ³He quasiparticle reflection from surface dimples and the effect of the surface layer of ³He atoms.     

Particles-Vortex Interactions and Flow Visualization in 4He

Recent experiments have demonstrated a remarkable progress in implementing and use of the Particle Image Velocimetry (PIV) and particle tracking techniques for the study of turbulence in ⁴He. However, an interpretation of the experimental data in the superfluid phase requires understanding how the motion of tracer particles is affected by the two components, the viscous normal fluid and the inviscid superfluid. Of a particular importance is the problem of particle interactions with quantized vortex lines which may not only strongly affect the particle motion, but, under certain conditions, may even trap particles on quantized vortex cores. The article reviews recent theoretical, numerical, and experimental results in this rapidly developing area of research, putting critically together recent results, and solving apparent inconsistencies. Also discussed is a closely related technique of detection of quantized vortices by negative ion bubbles in ⁴He.     

Experimental Studies of the Influence of a High‐Frequency Electromagnetic Field on the Convection of Fluids in a Vertical Tube

Results of experimental studies of the influence of a highfrequency electromagnetic field on a change in the levels of fluids in a vertical tube are reported.     

Mass Coupling and Q ?1 of Impurity-Limited Normal 3He in?a?Torsion Pendulum

We present results of the Q ^?1 and period shift, ??P, for ³He confined in a 98% nominal open aerogel on a torsion pendulum. The aerogel is compressed uniaxially by 10% along a direction aligned to the torsion pendulum axis and was grown within a 400???m tall pancake (after compression) similar to an Andronikashvili geometry. The result is a high Q pendulum able to resolve Q ^?1 and mass coupling of the impurity-limited ³He over the whole temperature range. After measuring the empty cell background, we filled the cell above the critical point and observe a temperature dependent period shift, ??P, between 100?mK and 3?mK that is 2.9% of the period shift (after filling) at 100?mK. The Q ^?1 due to the ³He decreases by an order of magnitude between 100?mK and 3?mK at a pressure of 0.14??0.03?bar. We compare the observable quantities to the corresponding calculated Q ^?1 and period shift for bulk ³He.     

Transport in 3He–4He Mixtures in Restricted Geometry

The effect of wall scattering on transport in dilute degenerate ³He–⁴He mixtures in quasi-2D flow channels or films is discussed. The calculation of the quasiparticle mean free path combines particle–wall and particle–particle collisions including the interference between them. The spin polarization affects the wall-driven contribution by changing bulk mean free path and particle wavelength. The expressions for the wall contribution to transport coefficients are especially transparent in the limiting cases of large and small bulk mean free paths. The calculated temperature, concentration, and polarization dependences of the transport coefficients allow one to extract parameters of surface roughness from experimental data on transport.     

Effective Masses and Characteristic Velocities of Charge Carriers in Low-Dimensional ANB8 –N Structures

Technical Physics Letters - Simple structural models and a previously proposed theory have been used for estimating bandgap widths, effective masses, and effective velocities of charge carriers in...     

On the possible superfluidity of 6He—Its phase diagram and those of 6He-4He and 6He-3He mixtures

The properties of pure ⁶He and ⁶He-⁴He and ⁶He-³He mixtures are studied within the context of the quantum theorem of corresponding states. For pure ⁶He, it is found that there is a small, but definite, region where it is expected to be superfluid. For the ⁶He-⁴He mixture, it is found that there is a -eutectic temperature, approximately 1.2 K, below which it is expected that both ⁶He and ⁴He will be part of the superfluid at all concentrations. Experiments to observe ⁶He superfluidity are discussed, and it is concluded that such experiments are feasible, although they may be quite difficult to carry out. Experiments are discussed to use a ⁶He generator (should it be possible to construct it) as a probe to study various properties of liquid and solid helium.Work supported in part grant from the National Science Foundation.     

Adsorption and specific heat studies of4He in 0.2-µm Nuclepore filters

We have made adsorption studies and specific heat measurements for helium adsorbed on Nuclepore filters with a pore size of 0.2 µm. We have been able to identify the first and second layer completion on this substrate by a Langmuir and BET analysis of the data. For coverages near the second layer completion the data obey a Frenkel-Halsey-Hill isotherm, and we have obtained the van der Waals constant =(2.4±0.4)×10^–37 erg-cm³. We find this result in excellent agreement with a theoretical estimate of 2.17 × 10^–37 erg-cm³. The interpretation of the isotherm data is confirmed by measurements of the specific heat. Data near a monolayer completion is found proportional toT ² with a characteristic two-dimensional Debye temperature of _D=23±1 K. Measurements of specific heat with helium samples equivalent to about 17 layers and higher show two transitions shifted in temperature, as is characteristic of helium confined to two different small dimensions. These data are in agreement with the thermodynamic instability for capillary condensation in a cylindrical geometry as calculated by Saam and Cole.Work supported in part by the SUNY Research Foundation, National Science Foundation, under grant DMR 75-09499 A01, and Research Corporation.     

Majorana Edge Modes of Superfluid 3He A-?and?B-Phases

Motivated by a recent experiment on the superfluid ³He confined in a thin slab, we discuss the Majorana edge modes under the experimental situation. We solve the quasi-classical Eilenberger equation, which is quantitatively reliable, to evaluate several observables, such as local density of states, mass current for the A-phase, and spin current for the B-phase. On the basis of the quantitative calculation, we propose several experiments to check the existence of the Majorana modes.     

Convective Turbulence in Superfluid Solutions 3He–4He

In present work, we continue our experimental investigations of heat instability in superfluid ³He–⁴He solutions heated from below. We research two solutions with ³He concentrations 5.0% and 9.5% for temperature of 270 mK. It is found that for 5% solution the dependence is linear in temperature range studied whereas for the solution of 9.5% we observed the deviation from linear dependence above some critical value . This effect manifests the thermal instability which appears under start of phase separation in 9.5% solution if heat flow is switched on. For 5.0% solution where one does not observe the phase separation at the values of applied, the instability was not observed. To identify the possible mechanism of a thermal instability in stratified solution, we estimated the dependence of the Nusselt number on relative Raileigh number Ra/Ra _c . One observes that the dependence can be fitted as Nu=(Ra/Ra _c ) ^b where b=0.31±0.04. Note that the dependence obtained agrees rather good with the empiric expression of (Busse in Rep. Prog. Phys. 41:1929, 1978) and connecting the numbers Nu and Ra for turbulent convection. This gives grounds to conclude the heat transfer in a stratified solution is realized by transition to the regime of turbulent convection.      

Experiments on 3He–4He Mixtures in Aerogel

We describe experiments on ³ He- ⁴ He mixtures in 98% open aerogel grown by cold-deposition (T80mK) of ⁴ He followed by the deposition of ³ He or alternatively by cooling down a homogenous mixture. The two approaches led to different ⁴ He film morphologies. We will also describe the observation of migration of ⁴ He from or toward the cell below 100 mK. This migration can lead to the exclusion of ⁴ He (other than the localized surface layers) for ⁴ He concentrations between 4% and 11% in the cell.     

Critical properties of4He in aerogel near the λ-line

We measured the superfluid fraction _s/ and the isobaric thermal expansion coefficient _P of⁴He confined in an aerogel. Data were obtained near the -line along several isobars. Powerlaw fits to the results for _s/ as a function of tT/T_c–1 (T_c is the transition temperature) give a pressure—independent exponent =0.755 when a confluent singular term is included. Fits to the _P data of power laws yield the specific-heat exponents –0.6 and –1.0 above and below T_c respectively. When an analytic background term a×t is included in the fit, the pressure-independent value =–0.59 is permitted, but the amplitude ratio A/A is found to be near zero and the coefficient of the analytic term is large. The measured values for and or are inconsistent with hyperscaling in three dimensions.     

Influence of Grain Size on the Superconductivity of?La1.85Sr0.15CuO4

We study the influence of varying grain size on superconductivity of bulk La_1.85Sr_0.15CuO₄ superconductor. The samples are synthesized by a sol-gel method. The grain size is varied by sintering the samples at various temperatures between 700?°C to 1050?°C. The samples are characterized by X-Ray Diffraction (XRD), Rietveld refinement, Scanning Electron Microscopy (SEM), resistivity and magnetization measurements. The electrical resistivity measurements revealed considerable lowering of the superconducting transition temperature ( $T_{\mathrm{c}}^{R=0}$ ) and broadening of the transition width (??T _c) with decreasing grain size though the onset of transition temperature ( $T_{\mathrm{c}}^{\mathrm{onset}}$ ) changes only marginally. The magnetic measurements carried out are consistent with each other and scale well with the grain size. Critical current density has been calculated from the magnetization hysteresis, assuming that supercurrents flow throughout the sample as a whole and within the individual grains as well. The observed results have been discussed on the basis of inter- and intra-granular boundary characteristics of high-temperature superconductors (HTSc). It is found that lowering of grain size deteriorates the superconducting properties in general.