Dissipation in vibrating superleak second sound transducers

We have performed an experimental study of the generation and detection of second sound in⁴He using vibrating superleak second sound transducers. At temperatures well belowT and for low driving amplitudes, the magnitude of the generated second sound wave is proportional to the drive amplitude. However, nearT and for high drive amplitudes this is no longer the case—instead, the second sound amplitude saturates. In this regime we also find that overtones of the drive frequency are generated. Our results suggest that this behavior is due to critical velocity effects in the pores of the superleak in the generator transducer. This type of measurement may prove to be a useful way in which to study critical velocity effects in confined geometries.     

Anomalous hysteresis of the critical current in long proximity-effect bridges

The critical current hysteresis of long Ag-Sn proximity-effect bridges, observed in the current-voltage characteristics obtained at helium bath temperatures T above and below the point of helium T , shows a jump discontinuity at T = T . The relevance of the observed phenomenon to existing theories of superconductors driven into resistive states by direct currents is discussed.Research carried out at the Tata Institute of Fundamental Research.     

Cavitation in Normal Liquid Helium 3

We have studied cavitation, i.e. bubble nucleation, by focusing ultrasound hurts in normal liquid helium 3 at temperatures down to 40 mK. As in helium 4, cavitation is found to be stochastic, with a cavitation probability 0.5 at a given value of the sound amplitude, which we define as a cavitation threshold. This threshold is found significantly lower in helium 3 than in helium 4, a result which agrees with theoretical predictions of a spinodal limit at - 3.1 bar in helium 3 instead of- 9.5 bar in helium 4- We also measured the temperature variation of this cavitation threshold; it decreases with temperature as expected for a thermally activated nucleation process. We have not yet found any evidence for a crossover toward cavitation by quantum tunneling below 120 mK as predicted by several authors; if confirmed, it might indicate that the superfluid coherence plays a role in quantum cavitation.     

Precision second-sound velocity measurements in helium II

The velocity of second soundu ₂ in helium II under its saturated vapor pressure was measured in an acoustic resonator with a typical precision of 0.07% and was found to agree with existing measurements. Data were taken from 1.135 to 2.09 K, with special attention given to the local maximum near 1.65 K, which can be used as a temperature reference point. A least squares cubic spline fit is given to the present data and selected data of other authors givingu ₂ from 0.55 K to within 60 µK of T.     

Precision Experiment on the Study of Helium Isotopes Cryosorption in Terrestrial Conditions and at Lower Gravity

Characteristic features of gaseous helium adsorption near the -point at low pressure were discussed at the international seminar on low temperature physics in microgravity CWS-97. It has been shown in that paper that at ⁴He cryosorption on the argon cryolayer near the ⁴He -point, a jump-like pressure change above the cryolayer took place. At ³He cryosorption on the argon layer formed in identical conditions an abrupt pressure change above the cryolayer in the ⁴He -point area has not been observed. The present study deals with the isothermal behavior of the cryopanel in terrestrial conditions. Experiments were conducted to study the character of the temperature change with respect to the column height of liquid helium inside the helium cryostat in the case of vapor pumping and in the case of warming up of liquid helium and cryopanel due to heat coming.Temperature stratification of liquid helium due to natural convection has been calculated. Calculation program for integral characteristics of the liquid heating and cooling process has been compiled accounting for the stratification effect. These characteristics include the cold nucleus volume, nucleus temperature, height of the stratification layer, temperature profile in this layer, time of these stages depending on the given rate of the pressure growth in the system.Problems of the precision experiment to study helium isotopes cryosorption (⁴He and ³He) in terrestrial conditions and at lower gravity have been discussed.     

Nucleation of Bubbles by Electrons in Liquid Helium-4

We report on experiments in which we study cavitation resulting from electrons in liquid helium. Electrons are introduced into the liquid by a radioactive source. After an electron comes to rest in the liquid, it forces open a small cavity referred to as an electron bubble. To study cavitation, a sound pulse is generated by means of a hemispherical piezoelectric transducer producing a large-amplitude pressure oscillation at the acoustic focus. If an electron is in the vicinity of the focus and the negative-going pressure swing exceeds a critical value, a cavitation bubble is produced which can be detected by light scattering. Two distinct critical pressures \( P_{\text{el}} \) and \( P_{\text{rare}} \) have been measured. The first corresponds to cavitation resulting from the application of a reduced pressure to liquid containing an electron which has already formed an electron bubble. The second is the critical pressure needed to lead to cavitation when an electron enters the liquid at a time and place where there is already a reduced pressure. We have measured these two pressures as a function of temperature and consider possible explanations for the difference between them. In addition to these clearly seen cavitation thresholds, there are some cavitation events that have been detected with a threshold that is at an even smaller negative pressure than \( P_{\text{el}} \) and \( P_{\text{rare}} \).     

Density correlation function in superfluid helium nearT λ

The previously calculated hydrodynamic form of the density correlation function of liquid helium is written out explicitly forT nearT . This function, which is proportional to the intensity of light scattered from the fluid, has a finite value at =0, which grows asTT from below. In evaluating the precise form of this contribution in terms of transport coefficients, it is important to keep terms of order (C _p /C _v )–1.     

The amplitude dependence of the velocity of nonlinear second sound in helium II nearT λ

The amplitude dependence of the velocity of nonlinear second sound in Helium II, as given by the Khalatnikov nonlinear coefficient, was measured nearT using a single-pulse travel-time technique. Using nonplanar pulses which were amplified and focused by a spherical reflector, the confirmation of Khalanikov's second order two fluid model prediction was extended to within 7 mK of T much closer to the critical temperature than previously reported. This study was restricted to single pulses of second sound in order to ensure that the measurement pertained only to an undisturbed, vortextangle-free sample of Helium II.     

State equation of liquid helium-4 from 0.8 to 2.5 K

A state equation for liquid helium is constructed in the range from 0.8 K to about 2.5 K, based on density and temperature as independent parameters. The equation has been fitted to experimental PVT, specific heat,(P/T) _v, sound velocity, and lambda line properties from 14 different authors to an accuracy comparable with reasonable experimental errors in the measured quantities. Inclusion of logarithmic terms leads to agreement with experimental data as close as 100 µK from the lambda line. It is found that the logarithmic amplitude ratio is not constant as a function of distance along the lambda line. A new determination of lambda line density is also presented. The equation is available in computer form.     

Dissipation by pinned vortex lines in the superfluid helium film

The status of theoretical and experimental work on dissipation in the helium film is reviewed, and it is concluded that there does not yet exist a satisfactory theoretical interpretation of dissipation in the film which can account for the complete range of observed phenomena below the superfluid transition temperature T. Although the most recent theory, which accounts for dissipation in terms of intrinsic fluctuations in the flow, has been successful in a temperature interval just below T, attempts to extend the theory to include all temperatures below T have not met with the same degree of success. A new model is proposed which accounts for dissipation in superfluid helium film transport in terms of the continuous generation of pinned vortex lines. In principle, this model is similar to one advanced by Vinen, involving the growth and decay of a tangled array of vortex lines. Qualitatively, it is shown that the present mechanism can account for many of the phenomena observed in helium film transport experiments at temperatures well below the transition. For example, sharp changes in the flow rate are associated with changes in the number of pinned vortex lines. In addition, the theory predicts that at superfluid stream velocitiesv _sthat just barely exceed the critical velocityv _c0for the appearance of dissipation, the rate of dissipation Q is given by Q=AN(v_s–v_c0)^3/2 whereN is the number of pinned vortex lines, andA is a constant determined by the vortex line parameters. The value of 3/2 for the exponent is a clear prediction of the theory, and it represents the first precise, numerical prediction by any theory of a physical quantity which is associated with dissipation in the helium film, and which can be measured experimentally.The research for this paper was supported by the Defence Research Board of Canada, Grant number 9550-57.     

Second sound very near Tλ

The results of an experimental investigation of the evolution of planar, non-linear, second-sound pulses in superfluid⁴He, to within 650 nK of the -transition, are presented. A new method for extracting the second-sound velocity and damping is demonstrated. As predicted from two-fluid hydrodynamics, the pulses are well modeled by the solutions of Burgers' equation. The second-sound velocity (u ₂₀) and damping (D ₂) are extracted from fits of the model to the data. Damping data are obtained in this fashion to 3×10^–7 in reduced temperature at saturated vapor pressure; nearly two decades closer to T then any previous measurements. The superfluid density is extracted from theu ₂₀ measurements and the critical exponent, , is determined. A study of very large amplitude pulses near T is also presented. These pulses extend well beyond the range of validity of Burgers' equation. The amplitude of the shock that forms at the trailing edge of the pulse is observed to saturate as a function of heater power and then decrease suddenly, as has been previously observed away from T. However, the pulse shapes are quite different from any previously observed.     

Energy gap suppression and instability in superconducting tin films under strong quasiparticle injection

Double tunnel junctions have been used to investigate the properties of superconducting Sn films under quasiparticle injection. At high injection level, the gap is discontinuously suppressed to zero from a finite value. This instability for ambient T_a>T, the temperature of liquid helium, results from the film current exceeding its critical value. Below the instability the general behavior of the gap suppression is shown to be consistent with the modified heating model.This work was supported by National Science Foundation Grant No. DMR 7817813.     

Thermal conductivity and heat capacity of solid silver bromide (AgBr) under pressure

The thermal conductivity, , and the heat capacity per unit volume, c _p , have been measured for solid silver bromide (AgBr) using the transient hot-wire method. Measurements were made at temperatures in the range 100–400 K and at pressures up to 2 GPa. c _p was found to be independent of temperature and pressure over these ranges. of AgBr was found to be similar to that of AgCl, which was measured previously. For AgBr, only acoustic phonons needed to be taken into account up to 340 K, but optic phonons probably carried some heat at higher temperatures. The Leibfried-Schlömann (LS) formula could describe the ratio (AgCl)/(AgBr), but not the ratio (1 GPa)/(0) for either substance. An empirical modification of the LS formula could describe the latter ratios but not the former. Further theoretical developments are required for understanding of (P) for even such relatively simple substances as AgCl and AgBr.     

Calorimetric study of the molecular mobility in a filled plasticized epoxide resin

Results are presented on the observed dependence of the thermophysical parameters C_p, , anda on temperature, filler content, and plasticizer concentration for ÉD-5 epoxide resin.Notation C_p specific heat capacity at constant pressure - thermal conductivity - a thermal diffusivity - C_p specific heat discontinuity - T_v vitrification point - density - u_so velocity of sound - l phonon mean free path - f frequency Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 5, pp. 853–858, May, 1971.     

Thermal conductivity and heat capacity of liquid toluene at temperatures between 255 and 400 K and at pressures up to 1000 MPa

The thermal conductivity and heat capacity c _p of liquid toluene have been measured by the ac-heated wire method up to 1000 MPa in the temperature range from 255 to 400 K. The total error of thermal conductivity measurements is estimated to be about 1 %, and the precision 0.3 %. The heat capacity per unit volume, pc _p, obtained directly from the experiment is uncertain within 2 or 3%. The vs p isotherms are found to cross one another at approximately 700 MPa. The minima in the pressure (or volume) dependence of c_p of toluene are evident at all temperatures investigated.     

Mutual friction and the thermal conductivity of superfluid helium near Tλ

Temperature gradients in superfluid helium carrying a heat current in a 13.8-mm-wide tube or in a 0.1-mm-wide slit have been measured at 0.09 m°K–T<16 m°K. The results are interpreted in terms of a mutual friction force between the normal component of He II and vortices in its counterflowing superfluid component. This force is found to diverge near T with a mutual friction constant proportional to (T–T)^T 0.35±0.06.     

Acoustical Experiments on Superfluid 3He-4He Mixtures in Aerogel

This report discusses our results on the superfluidity of ³ He- ⁴ He mixtures in a 98% porosity silica aerogel. We have used low frequency sound to probe helium mixtures confined to aerogel, and have observed both the slow mode of superfluid ³ He in aerogel, which is manifested only below T_c, and an additional sound mode present only in the mixture. We attribute this novel sound mode to the slow-mode in the ⁴ He rich phase of the dilute ³ He- ⁴ He mixture. This mode exhibits positive frequency shifts below T_c in aerogel, while above T_c the mode is observed at a temperature independent frequency until close to T where it shifts to zero frequency.     

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.     

Size effects in He II as measured by fourth sound

Fourth-sound measurements are used to determine the depletion in the superfluid density _s which occurs in the helium filled small pores of packed powders. The measurements also give the reduction in superfluid onset temperature. The pore size distribution of several samples was determined from desorption isotherms. Using this distribution and an independent pore model, calculations based on the Ginzburg-Pitaevskii ansatz with a healing length first proposed by Mamaladze are found to be in good agreement with experimental results in several cases. The superfluid onset temperature seems to be determined by the average pore size. Fourth-sound measurements as a function of temperatureT in large pore geometries accurately give _s (T –T)^2/3 in the neighborhood of the lambda temperatureT .     

On size effects in helium II below 1 K

The normal fluid density _n and the specific heatc _v are calculated for helium II flowing through plane parallel slits and capillaries with a square profile at temperatures below1 K. Both functions increase over their bulk values if the thermal phonon wavelength becomes comparable to or larger than the smallest dimensiond of the container. The asymptotic expansions which are obtained in this paper have an overlapping range of validity, and therefore describe _n andc _v rather accurately for all values of /d.