Физика низких температур, 2015, № 06

Investigations of thermal conductivity of simple van der Waals crystal-based nanocomposites

2018-01-01T01:03:03Z

Investigations of thermal conductivity of simple van der Waals crystal-based nanocomposites Nikonkov, R.V.; Stachowiak, P.; Romanova, T.V.; Jeżowski, A.; Sumarokov, V.V. The experimental setup for obtaining and determination of the thermal conductivity of simple van der Waals crystal-based nanocomposites is described. Preliminary thermal conductivity results of measurements carried out in the temperature range 1–40 K on two samples of methane crystals containing nanoparticles of hydroxyapatite are presented. These results confirm usability of the setup and its suitability as a proper experimental method for investigations of the thermal conductivity of the nanocomposites.

Low-temperature thermodynamics of Xe-doped fullerite C₆₀

2018-01-01T01:02:56Z

Low-temperature thermodynamics of Xe-doped fullerite C₆₀ Klochko, M.S.; Strzhemechny, M.A. Using a model of the fullerene C₆₀ molecule with carbon atoms uniformly distributed over its surface, the potential energy U(n) of a Xe atom in an octahedral void of C₆₀ is calculated. Within the framework of threedimensional harmonic oscillator, the lowest energy levels are estimated and the contribution of xenon impurity atoms to the heat capacity of the Xe–C₆₀ system is determined. The contribution of Xe dopants to the total heat capacity is shown to be essential compared to that of pure fullerite. Using the calculated energy spectrum we estimated the contribution of Xe atoms to the thermal expansivity of C₆₀ with 37% of Xe. This contribution is in a qualitative agreement with experimental findings. We estimated the Grüneisen parameter Г due to the anisotropic part of U(n) to show that the negative part of Г is negligible due to the very small width of the five lower oscillatory wave functions.

Wave turbulence on the surface of liquid hydrogen in restricted geometry: the influence of the boundary conditions

2018-01-01T01:02:54Z

Wave turbulence on the surface of liquid hydrogen in restricted geometry: the influence of the boundary conditions Brazhnikov, M.Yu.; Levchenko, A.A.; Mezhov-Deglin, L.P.; Remizov, I.A. Formation of low frequency harmonics on turbulent distribution in the system of waves on the surface of liquid hydrogen has been studied in the frequency range 1–100 Hz (capillary-gravity waves). It is shown that the geometry of the experimental cell has a significant influence on the direct cascade of capillary waves generated by monochromatic force as well as on the direction of the wave energy transfer from the range of pumping towards that of dissipation. Besides a direct turbulent cascade, single half-frequency harmonic generation was observed in a cylindrical cell under high pump power. In a square cell we observed not only a half-frequency harmonic but a number of low frequency harmonics below the driving frequency generated by the nonlinear threewave interaction. In the case of a rectangular cell we observed formation of incommensurate low frequency harmonics caused by the three-wave interaction of capillary waves and generation of a wave mode of ~1 Hz in the frequency range of gravity waves which could be attributed to the four-wave interaction.

Energy spectrum of the quantum vortices configurations

2018-01-01T01:02:50Z

Energy spectrum of the quantum vortices configurations Nemirovskii, S.K. The energy spectra of the 3D velocity field, induced by various vortex filaments configurations are reviewed. The especial attention is paid to configurations generating the Kolmogorov type energy spectrum E(k) ∝ k⁻⁵/³. The motivation of this work is related to the problem of modeling classical turbulence with a set of chaotic vortex filaments. The quantity can be exactly calculated, provided that we know the probability distribution functional P({s(ξ,t)}) of vortex loops configurations. The knowledge of P({s(ξ,t)}) is identical to the full solution of the problem of quantum turbulence and, in general, P is unknown. One of the simplifications is to investigate various truthful vortex configurations which can be elements of real vortex tangles. These configurations are: the uniform and nonuniform vortex arrays, the straight lines with excited Kelvin waves on it and the reconnecting vortex filaments. We demonstrate that the spectra E(k), generated by the these configurations, are close to the Kolmogorov dependence ∝ k⁻⁵/³, and discuss the reason for this as well as the reason for deviation.