Quantum Cavitation: A Comparison Between Superfluid Helium-4 and Normal Liquid Helium-3

Cavitation has now been studied in superfluid helium-4 and in normal liquid helium-3, both theoretically and experimentally. We compare the two cases and discuss the existence of a crossover from quantum cavitation, where bubbles are nucleated by tunneling, to classical cavitation where their nu-deation is thermally activated. In helium-3, where evidence for quantum nucleation is lacking, the interpretation of the experimental results leads to two related questions. The first one concerns the extrapolation of the properties of this Fermi liquid at negative pressure. The second one concerns the validity of present theories of quantum cavitation in a Fermi liquid.     

Quantum cavitation in superfluid helium 4 ?

We have observed the nucleation of bubbles in superfluid helium-4 at negative pressure and at temperatures down to 65 mK. Cavitation is found to be a random process. Above 1 K, its probability varies with temperature in a manner consistent with classical activation by thermal fluctuations. Below 0.4 K, Cavitation is still random but temperature independent, in agreement with a recent theory of quantum tunneling. We also consider another possible interpretation involving turbulence in the focal region.     

Thermodynamics and nucleation of bubbles in normal and superfluid liquid helium-4 at negative pressures

We report on a theoretical investigation of liquid helium-4 at negative pressures. For normal liquid helium we estimate the thermodynamic functions for negative pressure via extrapolation of measurements made for positive pressures. We determine the free energy as a function of density and temperature and find the location of the liquid-vapor spinodal. The results of these calculations are used to construct a temperature-dependent density-functional scheme to describe the inhomogeneous liquid. This density functional is then applied to calculate the rate at which bubbles nucleate in the liquid at negative pressures. We include a discussion of the properties of the superfluid phase based on the use of Landau's quasi-particle model.     

Transport in fluid helium-3 under pressure

The thermal conductivity of liquid helium-3 has been measured at temperatures between 1.3° K and 4.2° K and at pressures up to 55 atm. The viscosity data for liquid helium-3, reported briefly in Ref. 1 are here presented in full for temperatures 1.20° K, 2.00° K, and 3.02° K and at pressures up to 20 atm. Thermal conductivity and viscosity data at 2.00° K are combined with estimated values ofC _vto giveMK/ C _v(whereM is molecular weight,K is thermal conductivity, is viscosity andC _vis the molar specific heat), a dimensionless parameter that is 5/2 in gases at limiting low pressures. Finally, the thermal conductivity of gaseous helium-3 at 1-cm Hg pressure has been measured between 1.3° K and 4° K and compared with available theoretical estimates.     

Study of cavitation in superfluid helium-4 at low temperatures

We report studies of the negative pressure required to lead to the nucleation of bubbles in superfluid helium-4 in the temperature range down to 0.8 K. At the lowest temperatures studied, the tensile strength is found to be 3 bar. The results are compared with nucleation theory and the possible importance of nucleation on vortices is discussed.     

Flow impedance construction for liquid helium continuous refrigerators

Continuous refrigeration below 1 K implies the circulation of a cooling fluid. This requires some kind of constriction to limit the flow and allow the build-up of the pressure difference which gives rise to the enthalpy difference on which the cooling process depends. This situation is met by cooling with helium-4 to 1 K,^1,2 with helium-3, to 0.3 K, and also in dilution refrigerators; these flow impedances have to be small, rugged, and hard to plug with dust and oil mist.     

A three-stage Stirling pulse tube cryocooler operating below the critical point of helium-4

Precooled phase shifters can significantly enhance the phase shift effect and further improve the performance of pulse tube cryocoolers. A separate three-stage Stirling pulse tube cryocooler (SPTC) with a cold inertance tube was designed and fabricated. Helium-4 instead of the rare helium-3 was used as the working fluid. The cryocooler reached a bottom temperature of 4.97 K with a net cooling power of 25 mW at 6.0 K. The operating frequency was 29.9 Hz and the charging pressure was 0.91 MPa. It is the first time a refrigeration temperature below the critical point of helium-4 was obtained in a three-stage Stirling pulse tube cryocooler.     

New Calculation of the Polarization Contribution to the Energy of a Negative Ion in Liquid Helium

An electron injected into liquid helium-4 forms a bubble of radius approximately 19 Å. The size of the bubble is determined by a balance between the zero-point energy of the electron, the surface energy of the bubble wall, and the polarization energy of the helium in the electric field of the electron. We derive a modified result for the polarization energy of the bubble. We show that previous calculations in which the electron is treated as a point charge localized at the center of the bubble are inaccurate, and that it is essential to allow for the quantum fluctuations in the position of the electron.     

Nucleation of Bubbles on quantized vortices in helium-4

We consider the properties of vortices in superfluid helium-4 at negative pressure. Based on a simple density-functional model, we first calculate the density profile in the vicinity of a vortex. We then determine the negative pressure at which the vortex becomes unstable against a uniform radial expansion all along its length. This instability occurs at –6.9 bars. We then calculate the pressure-dependence of the energy E_nuc required to nucleate a bubble on the vortex. The results of this calculation are used to estimate the effect of quantized vortices on the rate at which bubbles are nucleated in superfluid helium at negative pressure.     

Cryogenic neutron detector by InSb semiconductor detector with high-density helium-3 gas converter

The neutron-detection characteristics of a cryogenic neutron detector comprising an InSb semiconductor detector and a helium-3 gas converter were evaluated at a gas pressure up to 12.4 atm at 4.2 K. The detector successfully detected stable neutrons under these conditions, where the density of the helium-3 gas is a few-hundred times higher than that at room temperature. It was found that the neutron detection efficiency was correlated with the gas pressure—even in a backward-detection configuration—in low-temperature, high-pressure helium-3.     

0.2 K～300 K温区氦-3的p-h和T-s图

基于最新的德拜模型氦-3状态方程、氦-3饱和曲线特征方程和熔化曲线特征方程编写了氦-3热物性计算程序.在大量热物性的计算数据的基础上绘制了氦-3在0.2 K～300 K,0.000 1MPa～30 MPa范围内的p-h图和T-s图.与先前基于实验数据绘制的0.2 K～20 K温区氦-3的p-h图和T-s图相比,该图的绘制是建立在热力学理论计算的基础之上,适用温区得到了拓展,随机误差一般在2%以内.     

Three-Nucleon Forces and Triplet Pairing in Neutron Matter

We report an experimental study of the electrical properties of liquid helium-4 in the temperature range 1.2–3 K. The experiment is carried out in the millimeter wave range using a whispering gallery mode dielectric resonator, and the complex permittivity of liquid helium is extracted from the data using the resonant perturbation method. The results for the temperature dependence of the dielectric constant are consistent with the previous studies. In addition, we find strong enhancement of the loss tangent around the superfluid transition temperature.     

Study on Surface Tension of Fluid Helium Three

As a standard property of helium-3, the surface tension is not only an important aspect to study the normal characteristics of this special cryogenic fluid, but also helpful to understand the Fermi quantum effects at low temperatures. After completion of studies on the thermodynamic and two typical transport properties (thermal conductivity and viscosity) of helium-3, all of the published experimental data of the surface tension of ³He have been collected. Different measurement techniques are compared and analyzed. The peculiar behavior of ³He surface tension, perhaps dominated by Fermi-Dirac quantum statistics, is analyzed and discussed at temperatures starting from zero to its critical point (3.3157 K). Based on a regular theoretical model for surface tension, a semi-empirical correlation is proposed for ³He covering the whole temperature range. The surface tension extrapolated to zero temperature by this equation is 1.5579×10⁻⁴ N · m⁻¹. In the vicinity of the critical point, the equation could be smoothly switched to the known scaling law, which takes the critical index 1.289. Comparison for the surface-tension behavior is performed between ³He and its isotope ⁴He, which obeys Bose-Einstein statistics.     

Liquid helium at negative pressures: Nucleation of bubbles and anomalous phonon dispersion

We calculate the rate at which bubbles nucleate in⁴He when the liquid is at negative pressure. Previous calculations have predicted that at low temperatures (T0.3 K) the nucleation rate remains low until a pressure of roughly –15 bar is reached. We show that this result is incorrect, and that at a critical pressure P_c (–9 bar) the liquid becomes macroscopically unstable. We have made a calculation of the nucleation rate allowing for this effect. It is shown that the effect of quantum nucleation is small and probably hard to observe experimentally. Finally, we demonstrate that one can understand the pressure dependence of the phonon dispersion relation by a simple model. This model uses a parameter which also enters into the nucleation calculation.     

Nucleation in a Fermi Liquid at Negative Pressure

Experimental investigation of cavitation in liquid helium 3 has revealed a singular behaviour in the degenerate region at low temperature. As the temperature decreases below 80 mK, the cavitation pressure becomes significantly more negative. To investigate this, we have extrapolated the Fermi parameters in the negative pressure region. This allowed us to calculate the zero sound velocity, which we found to remain finite at the spinodal limit where the first sound velocity vanishes. We discuss the impact on the nucleation of the gas phase in terms of a quantum stiffness of the Fermi liquid. As a result we predict a cavitation pressure which is nearer to the spinodal line than previously thought.     

Laser-Induced Breakdown in Liquid Helium

We report on experiments in which focused laser light is used to induce optical breakdown in liquid helium-4. The threshold intensity has been measured over the temperature range from 1.1 to 2.8 K with light of wavelength 1064 nm. In addition to the measurement of the threshold, we have performed experiments to study how the breakdown from one pulse modifies the probability that a subsequent pulse will result in breakdown.     

Quantum Nucleation of Bubbles from Electrons in Liquid Helium at Negative Pressure

We consider the quantum, nucleation of bubbles associated with negative ions in liquid helium at negative pressure.