Landau parameters and thermodynamic properties of liquid helium II

The variation of the Landau parameters with density and hard-core diameter is calculated for liquid helium using interparticle potentials composed of a hard core followed by a combination of a repulsive and an attractive Gaussian potential, and a hard core followed by a Lennard-Jones potential. Calculations are done only for high-density boson systems. The calculations of specific heat and entropy show good agreement with the experimental results in the very low-temperature region (T<1.0 K) and for the Gaussian potential only.     

Dependence of the viscosity of plastisols on temperature and time

The method of rapid heating of the medium was used for investigating the dependence of the viscosity of polyvinyl chloride plastisols on the temperature and time in connection with isothermal molding in casting practice.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 51, No. 4, pp. 609–613, October, 1986.     

Dependence of hydrodynamic decrement on vibration frequency

Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 7, pp. 99–102, July, 1989.     

Thermodynamic properties of adsorbed helium

After a brief discussion of the principles of adsorption thermodynamics, the current status of knowledge concerning the properties of adsorbed helium is reviewed. Experimental measurements of heats of adsorption, adsorption isotherms and heat capacities of adsorbed films are considered and related to one another and to theory wherever possible.     

Particle dynamics in helium II

No Heading The recent implementation of the Particle Image Velocimetry (PIV) method in liquid helium has opened the possibility of directly visualising turbulence in helium II. To interpret the data, however, it is necessary to understand the motion of small inertial particles in the presence of normal fluid and superfluid. We present the governing equations of motion and preliminary numerical results of particle trajectory in the initial growth of a turbulent tangle.PACS numbers: 67.40.Vs; 47.27.–i.     

Trapped-ion motion in helium II

We present the results of a series of experiments on the mobility of negative ions trapped on quantized vortex lines in superfluid helium. Trapped-ion mobilities were measured as a function of temperature, pressure, and He³ concentration. Some of the experimental data have been previously reported.^1,2 The temperature and pressure dependence of trapped-ion mobilities is qualitatively different than that of free ions. The He³ data strongly suggest that the negative ion bubble does not become deformed on the vortex line. We present a model for a vortex line having a central core of normal fluid extending over a distance of several angstroms surrounded by a tail, a region of excess roton density whose momenta are predominantly aligned opposite to the direction of circulation of the superfluid. This model is used to calculate the drag on a negative ion trapped on a vortex line. The model appears to account for the experimental results satisfactorily.Submitted to the Department of Physics of the University of Chicago in partial fulfillment of the requirements for the PhD degree.     

Analysis of helium II film boiling

The heat flux removed in the boundary transition from a vapour film to helium II is studied. It was found that the properties of the heat flux can be essentially explained physically by non-equilibrium molecular-kinetic effects at the vaporization condensation interface. A method of calculating the value of the heat flux is proposed.     

Rotating Couette flow of helium II

An experimental study of He II Couette flow is reported. Using second-sound measurements (Doppler effect, attenuation), we show the existence of different regimes and study their dependence on temperature and gap between the cylinders. We propose a simple model which allows a qualitative understanding of our experimental results. According to this model, the transition from laminar to turbulent flow takes place in two stages by a series of instabilities. First, the laminar regime vortex array is disorganized. Then, normal fluid instabilities lead to turbulence of the whole He II.Laboratoire associé au Centre National de la Recherche Scientifique.     

Wetting properties of liquid helium on rubidium metal

Wetting properties of liquid helium on rubidium metal have been investigated in the temperature range 1.0–1.7 K using a heat conduction method. The rubidium surface is found to be wetted under saturated vapor conditions. Prewetting transitions have been observed for rather weak offsets from saturation, indicating that rubidium is close to the nonwetting limit at T = 0 K. Presumably because of substrate inhomogeneities, the prewetting transitions are experimentally found to be continuous and hysteretic. No evidence for the prewetting critical point has been found up to 1.7 K. The prewetting line verifies a simple linear relation between the liquid-vapour surface tension and the two third power of the chemical potential offset, but does not conform quantitatively to the simple model currently used.On leave of Rochester University.Laboratoire de L'Ecole Normale Supérieure et de l'Universté Pierre et Marie Curie, associé au CNRS (URA 18).     

Dependence of frequency and amplitude of the ac current on the GMI properties of Co based amorphous wires

The frequency and the amplitude of the driving ac current dependence of the Giant Magneto-Impedance (GMI) behaviour in (Fe₆Co₉₄)_72.5Si_12.5B₁₅ amorphous wire have been studied. The single and two-peak behaviour in the GMI characteristics are observed which depends on the amplitude and frequency of the ac driving current flowing through the sample. The GMI ratio initially increased and then decreased monotonically with the increase of frequency, f, and the amplitude of the ac current, I_ac. With the increase of amplitude of ac current, the response of the GMI voltage is found non-linear with the generation of higher harmonics. With the increase of higher harmonics, the first harmonic component of the GMI voltage started decreasing. With the increase of the ac current, the field sensitivity of the first harmonic components of the GMI voltage decreases and in higher harmonics it shows an increasing trend. The maximum field sensitivity was observed 7.5% per A/m for I_ac = 2 mA and at frequency 100 kHz. The paper investigates the response of the second and third harmonics with the frequencies and amplitude of the driving current. A voltage equation derived from the rotational model to study the response of the GMI voltage.     

微型回热器等效直径与振荡频率的理论研究

通过对微通道中影响振荡流体的粘性损失的两个准则数的论述,得出实际的微型热声制冷机微通道回热器的等效直径、驱动膜片的振幅以及对应的辐射声功等参数的对应关系,即是:回热器通道直径或通道等效水力直径必须满足高频下的沃姆斯莱数小于5的限制;同时还必须满足在高频下的马赫数小于0.1;频率的上限也必须满足马赫数、沃姆斯莱数、回热器微孔直径以及膜振动平均位移计算前提(ka＜0.5).     

Dependence of ceramic fracture properties on porosity

A connected-grain model developed earlier to study the modulus of elasticity as a power-law of density was extended to study the dependence of the flexural strength of polycrystalline ceramics on porosity. Relations were derived for specific surface fracture energy, fracture toughness and flexural strength as power laws of (1 ?p), wherep is porosity. Model validity was confirmed with data on α-alumina, UO₂, Si₃N₄, and the YBa₂Cu₃O_7?δ superconductor.     

Atomic order in liquid helium II

We study the gauge theory of superfluidity and find analytically the shape of the liquid structure factor for low values of the momentum transfer; in terms of our model we understand the diffraction data; new predictions are made.A preliminary version of this work was presented at the Sanibel Symposium of Quantum Fluids and Solids, held in Florida, January 1977.     

Heat transfer properties in a vertical channel filled with saturated and pressurized helium II

Various heat flow regimes were investigated in a vertical slit opened at both ends of a helium II bath, as a means of simulating the properties of a typical heat transfer channel in a superconducting device. In pressurized He II, the heat fluxes before the transition of He II of — He I are in agreement with the Gorter-Mellink model. In saturated He II, a heat transfer regime characterized by temperature oscillations superimposed on a Gorter-Mellink regime has been found.     

Transport properties of helium near the liquid-vapor critical point. V. The shear viscosity of3He-4He mixtures

A torsional oscillator operating at 158 Hz has been used to measure the shear viscosity for two mixtures,³He_0.65-⁴He_0.35 and³He_0.80–⁴He_0.20. Data for each mixture are reported along near-critical isochores 0.85</ _c <1.18, where _c is the critical density. Just as in a previous paper on pure³He and⁴He, the observed viscosity includes the sum of background and critical contributions and the effect from the earth's gravity nearT _c . The analysis of the data provides (1) the background viscosity versus density and temperatureT, (2) the viscosity ratio / along the various isochores and also along selected isotherms obtained by interpolation, showing the critical contribution, and (3) the shape of the dew-bubble curve for each mixture, as obtained from the maxima in or from the discontinuities in the sloped/dT at the transition from the two-phase to the single-phase regime. A comparison between the data of these mixtures and those for pure fluids is presented. The background viscosity data along isotherms and along isochores are found to vary smoothly from³He to⁴He. The critical viscosity ratio [ / ]( _c ) for mixtures along the critical isochore is comparable with that for the pure fluids, i.e., it diverges weakly asT _c is approached. Hence, just as for previously investigated critical transport properties in³He-⁴He mixtures, there is no significant difference in behavior from that in the pure fluids.     

Relativistic resonance traveling-wave tube with tunable oscillation frequency

A high-power microwave oscillator is realized experimentally, employing a relativistic traveling-wave tube with a feedback circuit closed by tunable Bragg waveguide reflectors. A gigawatt-pulse power level is attained in the oscillator in the 3-cm wavelength range, which is maintained when the carrier frequency is tuned in a range of about 5%.     

A comment on rotons in two-dimensional liquid helium II

The spectrum of rotons in two-dimensional liquid helium II is estimated within the Bijl-Feynman theory. Due to stronger short-range order and more effective back flow, the roton gap is found to be about half as big as the bulk roton energy.Supported by grants of the Deutsche Forschungsgemeinschaft and, in part, by the U.S. National Science Foundation through Grants DMR73-02363 and DMR72-02977 A03.     

Transport properties of helium near the liquid-vapor critical point. IV. The shear viscosity of3He and4He

Shear viscosity measurements with a precision of 0.05% are reported for³He and⁴He along near-critical isochores 0.85 c <1.12, where _c is the critical density. The temperature range was –10^–4<<1, where =(T – T _c)/T _c is the reduced temperature. The experiments were carried out with a torsional oscillator operating at 158 Hz, driven at resonance in a phase-locked loop. The absolute value of the viscosity was obtained by calibration at the superfluid transition of⁴He, based on published values and from direct calculations using the free decay time constant of the oscillations. The data are analyzed in terms of a model using the recent mode-coupling (MC) expressions by Olchowy and Sengers, and where account is taken of the earth's gravity effects. The theory could be fitted very well to the experiment with a single free parameter, the cutoff wave numberq _D, which was found to be 3.0×10⁶ and 7.0×10⁶ cm^–1 for³He and⁴He, respectively. We have used for the critical exponent the MC predicted value of z=0.054, which permits a fit superior to that using z=0.064 predicted by dynamic renormalization group (DRG) theories. Detailed comparisons are made between the model calculations and data for various isochores and isotherms and good agreement is obtained. The effects of gravity are described in some detail. The predicted frequency effect in viscosity measurements is calculated for³He and is shown to be obscured by gravity effects. Using the Olchowy-Sengers formulas, we have also fitted the MC theory to the critical thermal conductivity data of³He, again withq _D as the only free parameter. This fit gaveq _D=6 × 10⁷ cm^–1, which in the ideal situation should have been the same asq _D from viscosity. We also discuss a representation of the³He viscosity data along the critical isochore by a power law and first correction-to-scaling erm. Using the viscosity and the critical conductivity data for³He, we have calculated the dynamic amplitude ratio and obtained =1.05±0.10, in agreement with predictions from MC and DRG theories. Also, agrees with data of classical fluids. Finally, a comparison is made of recent shear viscosity data for CO₂ by Bruschi and Torzo with those on He. The CO₂ data are also analyzed in terms of the MC theory, and the discrepancies are discussed. In the Appendices, we present the results of new compressibility measurements on³He along the critical isochore, as used in the MC analysis. We also present a brief analysis of the fluid hydrodynamics in the torsional oscillator leading to relations for the viscosity as a function of the measured quantities. Finally, we give a short outline of the vertical density profile calculations from the earth's gravity field for the calculations of the viscosity nearT _c.     

Radio frequency transistors based on ultra-high purity semiconducting carbon nanotubes with superior extrinsic maximum oscillation frequency

In this paper, we report polyfluorene-separated ultra-high purity semiconducting carbon nanotube radio frequency transistors with a self-aligned T-shape gate structure. Because of the ultra-high semiconducting tube purity and self-aligned T-shape gate structure, these transistors showed an excellent direct current and radio frequency performance. In regard to the direct current characteristics, these transistors showed a transconductance up to 40 μS/μm and an excellent current saturation behavior with an output resistance greater than 200 kΩ·μm. In terms of the radio frequency characteristics, an extrinsic maximum oscillation frequency (f _max) of 19 GHz was achieved, which is a record among all kinds of carbon nanotube transistors, and an extrinsic current gain cut-off frequency (f _T) of 22 GHz was achieved, which is the highest among transistors based on carbon nanotube networks. Our results take the radio frequency performance of carbon nanotube transistors to a new level and can further accelerate the application of carbon nanotubes for future radio frequency electronics.

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