L. V. Karnatsevich

Neutron scattering determination of condensate in liquid 4He

Abstract The results obtained during the last years on the temperature dependence of the Bose condensate in liquid 4 He are presented. Measurements of the Bose condensate density n 0 are performed with use of a deep inelastic neutron scattering method. A comparison with the other data obtained by the inelastic neutron scattering method is given. One can see a good agreement with all n 0 s obtained from inelastic neutron data by the developed and improved methods employed by other authors.

Equation of state of an equimolar 3He–4He mixture

Analytical forms of the empirical equations of state of the system are obtained for an equimolar 3He–4He mixture in the homogeneous liquid and dense fluid phases at temperatures 1.5–14 K and pressures 0–10 MPa on the basis of the existing experimental P–V–T data. This is done by choosing approximating expressions, setting up a computer program, and calculating the fitting coefficients of the expressions. The quality of the approximation corresponds to the accuracy of the experimental determinations and is on average 0.5%.

Neutron scattering study of liquid helium. Analysis of new data

A new analysis of neutron scattering data obtained earlier for liquid 4He is presented. The experiments were made on the time-of- flight spectrometer DIN-2PI in the pulsed reactor IBR-2. The results are analyzed by using a consistent data processing technique including the representation of the dynamic structural factor S(Q,ω) with a constant wave vector Q. The one-phonon component of S(Q,ω) is approximated by using the damped harmonic oscillator function taking into account the instrumental resolution. It is shown that the experimental values of S(Q,ω) are in good agreement with the fitting model, i.e., have a simple one-component structure. The presented results indicate a peculiarity in the temperature dependence of S(Q,ω) for liquid helium in the wave vector region 0.5–0.8 A−1. Various explanations of such a peculiarity are discussed.

On a structure of superfluid helium-4 elementary excitation spectrum

Abstract New experimental data of the liquid helium inelastic neutron scattering are reported. With the help of two Gaussian fitting procedure it was shown that for wave vectors q>q0=0.48&-1, the sharp phonon-maxon-roton peak consists of two modes, thus, elementary excitation spectrum possesses a complex structure. We can assume that one of these modes belongs to the neutron scattering as on the usual, “normal” fluid, while the other mode being connected with the liquid excitations induced by the presence of the bose-condensate.

A discussion of the dispersion curve of energy excitations in liquid 4He

An investigation of the dispersion of excitations in a quantum liquid, superfluid 4He, is carried out. An attempt is made to systematize the published experimental data that indicate a substantially different nature of excitations with wave vectors corresponding to different parts of the dispersion curve of liquid 4He. Neutron spectroscopy data are analyzed in relation to a certain physical hypothesis concerning the formation of such a spectrum, and it is found that the majority of the known experimental facts can be explained in framework of that hypothesis. Particular attention is paid to a comparison of the experimental data obtained on the DIN-2PI time-of-flight spectrometer (at the IBR-2 Reactor, Dubna) with the results obtained at foreign research centers.

Unified equation of state of 3He–4He liquid mixtures over the whole concentration range at temperatures of 2.25–4.2 K and pressures up to 10 MPa

A unified empirical equation of state for 3He–4He mixtures is found in analytical form on the basis of the existing experimental P–V–T data for pure 3He, 4He, and their mixtures in the homogeneous liquid phase in the temperature interval 2.25–4.2 K and pressure interval 0–10 MPa. An algorithm for calculating the fitting coefficients of an approximating expression for P(V,T,c) is constructed. The average absolute error for the determination of P is ±0.015 MPa.