Specific Heat of Helium Confined to Cylinders Near the Lambda Point

We report measurements of the specific heat of helium-4 confined to very uniform 8-micron diameter cylinders and to 0.26-micron diameter Anopore cylinders in the temperature region near the lambda point. The data allow improved tests of the emerging theory of finite size phenomena and cross-over to lower dimensional states. We find generally good agreement with scaling using exponents derived by renormalization methods. We also find reasonable agreement with more detailed numerical calculations of cross-over behavior with explicit inclusion of boundary conditions. However, in the region near the peak, the 8-micron data do not scale well relative to Anopore. We also compare the results with previous work on 1-dimensional systems.     

Thermal Conductivity and Critical Boundary Resistance of Helium Near the Lambda Point

We report measurements of the temperature dependence of the thermal conductivity of helium in the region just above the lambda point and the critical boundary resistance on both sides of the transition. The data were obtained using a cell with a gap that was varied from 0.12 to 0.73 mm. The range of the data was from about 2 mK below to 8 mK above the transition, with a maximum resolution of about 5×10⁻⁹ K, well within the gravity rounded region. We find good agreement with the expected behavior of the thermal conductivity based on the dynamic renormalization group theory. The boundary resistance results above the transition are in fair agreement with the theoretical prediction of Frank and Dohm. Below the transition the wide range results are in approximate agreement with additional predictions and other measurements at higher powers. Closer to the transition on the low temperature side phenomena are observed which appear to be apparatus-dependent.      

液氦自动补加实验

一种简易的液氦自动补加装置业已制成并通过了试验。该装置由一个１００Ｌ液氦贮存容器、一个蒸发率约５Ｌ／ｄ的低温恒温器、一条１ｍ长的柔性输送管、两个电磁阀、一个５Ｗ电加热器、一个容器压力传感器和控制器以及两个液面传感器和控制器组成。它没有旁通管和输送冷却的阀门。由于用加热器通过液氦蒸发使容器的压力升高，所以安全装置具有防止超压和无液加热的作用。这一简易可靠的装置，它的工作性能已通过模型试验和正常使用所     

Nucleation and Boiling Near the Gas–Liquid Critical Point

Nucleation near the gas–liquid critical point depends sensitively on whether the pressure or the volume is fixed. We consider near-critical fluids close to the coexistence curve. (i) Upon decompression to a constant pressure with a fixed boundary temperature, bulk nucleation can well be induced from a gas state, whereas from a liquid state boiling is easily triggered in the thermal diffusion layer near the boundary. In this case, bulk nucleation in a metastable gas is described by the classical Lifshitz–Slyozov theory. (ii) Upon cooling of the boundary temperature under the fixed-volume condition, bulk nucleation can be realized in a liquid and a modified Lifshitz–Slyozov theory follows. However, if a gas is cooled from the boundary at a fixed volume, liquid droplets readily appear in the thermal diffusion layer, apparently suggesting no metastability in a gas in agreement with previous experiments. (iii) On the other hand, if a liquid is heated at the boundary wall, boiling readily occurs both at a fixed volume and at a fixed pressure.     

Properties of Electron Bubbles in Liquid Helium

We present calculations of a number of properties of electron bubbles in liquid helium. The size and shape of bubbles containing electrons in different quantum states is determined based on a simplified model. We then find how the geometry of these bubbles changes with the applied pressure. The radiative lifetime of bubbles with electrons in excited states is calculated. Finally, we use a quantum Monte Carlo method to determine the properties of a bubble containing two electrons. We show that this object is unstable against fission.     

Logarithmic Singularity of Specific Heat in Liquid Helium II at the λ Point

The experimental width of the second sound peak in Brillouin scattering is less than the instrumental width. It is also less than the width of the first sound peak near the λ point. The theoretical width of the second sound peak becomes infinitely large at the λ point in the traditional viewpoint. This discrepancy suggests that the second sound peak detected in Brillouin scattering is an elementary excitation. We calculate the specific heat of liquid helium near the λ point by considering the second sound peak. The calculated temperature-dependence of the specific heat has a logarithmic divergence at the λ point. Furthermore, the calculated values of the specific heat agree well with data derived from experiments.     

1000L卧式液氦供气杜瓦的研制

低温液体储运具有效率高，供气质量好，压力稳定，供气简便及使用安全的特点，液氦供气杜瓦不仅具有常规杜瓦的要求，而且气压可控，本文主要论述了1000L液氦供气杜瓦的工作原理和结构设计，及主要研制工艺和试验情况，介绍了液氦供气系统的特点，产品于1997年9月投入正常使用，全部性能指标达到设计要求，液氦日蒸发率为0．9％。     

Isochoric Heat Capacity Measurements for Heavy Water Near the Critical Point

Isochoric heat capacity measurements of D₂O are presented as a function of temperature at fixed densities of 319.60, 398.90, 431.09, and 506.95 kg·m^–3. The measurements cover a range of temperatures from 551 to 671 K and pressures up to 32 MPa. The coverage includes one- and two-phase states and the coexistence curve near the critical point of D₂O. A high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter was used for the measurements. Uncertainties of the heat capacity measurements are estimated to be 2 to 3%. Temperatures at saturation T _S () were measured isochorically using a quasi-static thermogram method. The uncertainty of the phase transition temperature measurements is about ±0.02 K. The measured C _V data for D₂O were compared with values predicted from a parametric crossover equation of state and six-term Landau expansion crossover model. The critical behavior of second temperature derivatives of the vapor pressure and chemical potential were studied using measured two-phase isochoric heat capacities. From measured isochoric heat capacities and saturated densities for heavy water, the values of asymptotic critical amplitudes were estimated. It is shown that the critical parameters (critical temperature and critical density) adopted by IAPWS are consistent with the T _S – _S measurements for D₂O near the critical point.     

Nonasymptotic Transport Properties in Fluids and Mixtures Near a Critical Point

We review the critical dynamics of fluids and mixtures. Special attention in the comparison with experiment is paid to nonasymptotic effects. Our theoretical results are based on the complete model H of Siggia, Halperin, and Hohenberg including the sound mode variables. Using the dynamic renormalization group theory, we calculate the temperature dependence of the transport coefficients as well as the frequency-dependent sound velocity and sound attenuation. In mixtures a time ratio between the Onsager coefficients related to the diffusive modes, which is directly related to the critical enhancement of the thermal conductivity near a consolute point, has to be taken into account. The sound mode contains, besides the dynamic parameters, a static coupling related to the logarithmic derivative of the weakly diverging specific heat. The deviation from the asymptotic value of this coupling at finite frequencies and temperature distance from T _c leads to additional nonasymptotic effects. Our theory, which derives the phenomenological ansatz of Ferrell and Bhattacharjee for pure fluids and mixtures near a consolute point, is also applicable near a plait point.     

Finite-Size Scaling for Equilibrium Properties of Confined Liquid Helium-Four Near the Lambda Point

Simple analytic closed-expression scaling functions are reported for the lambda transition of confined liquid ⁴He in equilibrium. The results follow directly from the quantization of the fluctuation modes of the order parameter and exhibit a smooth temperature dependence down to a shifted transition temperature.     

Isochoric Heat Capacity Measurements for Light and Heavy Water Near the Critical Point

The isochoric heat capacity was measured for D₂O at a fixed density of 356.075 kg·m^–3 and for H₂O at 309.905 kg·m^–3. The measurements cover the range of temperatures from 623 to 661 K. The measurements were made with a high-temperature, high-pressure, adiabatic calorimeter with a nearly constant inner volume. The uncertainty of the temperature is 10 mK, while the uncertainty of the heat capacity is estimated to be 2 to 3%. Measurements were made in both the two-phase and the one-phase regions. The calorimeter instrumentation also enables measurements of PVT and the temperature derivative (P/T)_V along each measured isochore. A detailed discussion is presented on the experimental temperature behavior of C_V in the one- and two-phase regions, including the coexistence curve near the critical point. A quasi-static thermogram method was applied to determine values of temperature at saturation T_S() on measured isochores. The uncertainty of the phase-transition temperature measurements is about ±0.02 K. The measured C_V data for D₂O and H₂O are compared with values predicted from a recent developed parametric crossover equation of state and IAPWS-95 formulation.     

Effect of Molecular Structure on the Thermodynamics of Nitromethane+Butanol Isomers Near the Upper Critical Point

Densities, isentropic compressibilities, and isobaric molar heat capacities were determined over the whole composition range for nitromethane+(2-butanol or isobutanol) at atmospheric pressure and at the temperatures 288.15, 293.15, 298.15, and 308.15 K. These results allowed us to obtain isobaric thermal expansivities, isothermal compressibilities, and isochoric molar heat capacities at the temperature 298.15 K. The excess quantities for the given properties were obtained. In addition, liquid–liquid phase separation temperatures were also determined, locating upper critical solution temperatures near the experimental temperatures. The variation of the properties among isomers is discussed. Also, the effect of the nonrandomness of the mixtures expected near the critical point is discussed.     

Nonlinear Features of Phonon–Ripplon Modes in the Electron Crystal Over Liquid Helium

The resonance spectra of coupled phonon–ripplon oscillations in two-dimensional electron crystals over liquid helium are measured at different driving electric potentials. The crystals with surface electron densities n _s = (3.2 − 12) × 10⁸ cm⁻² are studied at holding electric fields E _⊥ = 590−1180 V/cm at temperature T≈ 80 mK. It is found that an increase of the driving potential leads to a change of resonance curves: the curves become more flat, asymmetric, and splintered depending on electron density and holding potential. The data obtained are used to estimate the electron effective mass in the crystal. It is shown for the first time that the effective electron mass in the crystal increases as the driving field increases.     

Recent High Resolution Measurements of the Specific Heat and Isothermal Compressibility Near the 3He Liquid-Gas Critical Point

Measurements of the specific heat at constant volume and isothermal compressibility have been made along a near critical isochore in the liquid-gas critical region of ³ He. The critical density was determined to within 0.1% from pressure-density measurements along a near critical isotherm in the single phase region. The specific heat was measured in the gravity affected region in the reduced temperature range |T/T _c – 1| 3 × 10^–4 using a slow cooling drift technique. A new electrostriction technique was developed to measure the isothermal compressibility along isochores and isotherms near the critical point. Initial measurements that validate this new technique will also be presented.     

The Viscosity Anomaly Near the Lower Critical Consolute Point

Shear viscosity measurements for a critical mixture of 3-methylpyridine + heavy water near a lower critical consolute point are reported. The background contribution was determined from viscosity measurements of mixture at a noncritical composition. In the entire investigated temperature range T _c – T 15.6 K, the viscosity of the critical mixture exceeds the background contribution, and the critical enhancement is important. The increase of the viscosity near critical is found in the temperature range T _c–T 1.82 K. The critical exponent y = 0.0415 ± 0.002 and the wave number Q = (0.40 ± 0.07) nm^–1 are determined.     

Boson Number in Bose–Einstein Condensate of Liquid Helium Near the λ Point

Bose–Einstein condensation has been well investigated in dilute atomic gases. For liquid helium system, the superfluid component is considered to be a background flow in the Landau theory. We study the relation between Bose–Einstein condensate and superfluid component. The concept of dressed bosons is introduced, which are eigenstates of the total Hamiltonian. The total energy is the sum of the kinetic energy of the center of mass and Galilean invariant terms. Therefore, the energy of the dressed boson has a nonlinear form for their number distribution function. If it is required that the excitation energy of a dressed boson be in agreement with the elementary excitation energy of the Landau theory near 0.0 K, then the functional form of the dressed boson energy can be determined. Because of this functional form, the dressed bosons with zero momentum have no friction against a vessel only in existence of a Bose–Einstein condensate. Consequently, the condensate of the dressed bosons with zero momentum is the superfluid component. The number n ₀ of dressed bosons with zero momentum is calculated. It shows temperature dependence (1−(T/T _λ ))^1/3 near the λ point, where T is the temperature and T _λ is the λ transition temperature.     

液氦温区VM-PT制冷机气量分配特性研究

VM气耦合脉冲管制冷机(VM-PT)是一种新型的液氦温区制冷机,为探索两级气耦合复杂的机理,本文采用Sage软件构建了低温调相VM-PT制冷机的整机模拟程序,研究了运行频率、平均压力、毛细管长度以及Er3Ni填充长度等参数对两级气量分配的影响。数值结果表明运行频率、平均圧力、毛细管长度以及Er3Ni填充长度均会影响两级质量流的分配,进而影响制冷机的最低温度,权衡工质的做工能力以及蓄冷器损失两方面因素,该四个参数均存在一个最佳值。搭建了实验平台并对数值模拟进行了验证。在实验中通过优化毛细管和蓄冷器,在运行频率1.6 Hz、平均压力1.4 MPa、压比1.6的情况下得到了3.86 K的无负荷制冷温度,在4.2 K可提供约10 mW的制冷量。