Физика низких температур, 2015, № 07

The standardization and certification procedures of cryogenic equipment in Kazakhstan

2018-01-01T01:03:30Z

The standardization and certification procedures of cryogenic equipment in Kazakhstan Shinbayeva, A.; Drobyshev, A.; Drobyshev, N. The complex of procedures on stepwise carrying out actions of standardization and certification of liquid nitrogen, argon and oxygen gasificator G-200 is considered. In the process of organization of standardization and certification procedures the set of regulatory documents was developed that allow starting small-scale production of gasificators in Kazakhstan. Thus the emphasis on the potential incorporation of the requirements of such documents is placed from the Eurasian Economic Union.

Nanotube-based source of charges for experiments with solid helium at low temperatures

2018-01-12T19:26:38Z

Nanotube-based source of charges for experiments with solid helium at low temperatures Borisenko, D.N.; Walmsley, P.M.; Golov, A.I.; Kolesnikov, N.N.; Kotov, Yu.V.; Levchenko, A.A.; Mezhov-Deglin, L.P.; Fear, M.J. Methods of preparation of the field-emission sources of charges from carbon nanotubes suitable for study of injected charges in solid helium at low temperatures T < 1 K are presented. The sources have been prepared by arc discharge deposition of nanotubes onto a flat copper substrate or by mechanical rubbing of nanotubes into porous metal surface. The test study of the voltage-current characteristics of a diode cell with the nanotube source in superfluid He II have shown that at voltages above 120 V one can observe a relatively large current I ≥ 10⁻¹³ A of negative charges in liquid helium. The field and temperature dependences of positive and negative currents in solid ⁴He were studied in samples grown by the blocked capillary technique. Usage of the nanotube based source of injected charges had permitted us for the first time to observe motion of the positive charges in solid helium at temperatures below 0.1 К. The current-voltage dependence could be described by a power law I ~ Uα, with the value of the exponent α >> 2, much higher than what one would expect for the regime of space charge limited currents.

Thermal conductivity of donor-doped GaN measured with 3ω and stationary methods

2018-01-01T01:03:09Z

Thermal conductivity of donor-doped GaN measured with 3ω and stationary methods Churiukova, O.; Jeżowski, A.; Stachowiak, P.; Mucha, J.; Litwick, Z.; Perlin, P.; Susk, T. The thermal conductivity of three single crystal samples of n-type gallium nitride with electron densities of 4.0⋅10¹⁶, 2.6⋅10¹⁸, and 1.1⋅10²⁰ cm⁻³ has been determined in the temperature range 4–320 K. The measurements were carried out within the ab plane using the stationary method. The thermal conductivity depends strongly on the donor concentration. The analysis within the Callaway approach and the Debye model shows a significant influence of phonon–electron scattering on the thermal conductivity of the samples. In addition, some preliminary results obtained along the c axes of GaN layered samples are presented. The latter measurements have been carried out using the 3ω method.

Phonon spectrum and vibrational characteristics of linear nanostructures in solid matrices

2018-01-01T01:03:01Z

Phonon spectrum and vibrational characteristics of linear nanostructures in solid matrices Manzhelii, E.V.; Feodosyev, S.B.; Gospodarev, I.A.; Syrkin, E.S.; Minakova, K.A. The atomic dynamics of linear chains embedded in a crystalline matrix or adsorbed on its surface is studied. A linear chain formed by substitutional impurities in a surface layer and at the same time offsetting from this layer was analyzed particularly. This system models the actively studied experimentally structures in which gas molecules are adsorbed on the walls of the bundles of carbon nanotubes located in certain medium. It is shown that the quasi-one-dimensional features are typical for the chains in which the interatomic interaction is higher than the interaction between the atoms of the chain and the atoms of the crystal matrix. On the local phonon density of atoms of the chain the transition to quasi-one-dimensional behavior has the form of the kink. In other words, it is the first (lowest-frequency) van Hove singularity, which in 3D structures (the system under consideration is generally three-dimensional) corresponds to the transition from closed to open constant-frequency (quasi-plane) surfaces. The local phonon densities of atoms in the chain have one-dimensional character at frequencies higher than the frequency of the van Hove singularity. The rms amplitude of embedded chains atoms vibrations is calculated and the behavior of the atomic vibrations contribution in the low-temperature heat capacity of the system is analyzed.