A. Yu. Prokhorov

Geometrical surface vortex pinning in superconducting films

A model is proposed for vortex pinning in a superconducting film with a rough surface. The model relates the critical current to the steepness of the surface relief and, at a high vortex concentration, to the distance between neighboring steepness maxima on the paths of vortex motion. The dependence of the critical current density on the thickness of a high-Tc superconducting film is measured in a weak magnetic field. Its behavior can be explained by the pinning at the stepped surface relief.

Anomalous Properties of Crystalline Methane in the Range of 60-70 K

Mechanical, structural, thermal, and spectral properties of solid methane in the temperature range 0.5Ttr-Ttr (Ttr is the triple point) at the equilibrium vapor pressure have been analyzed. It has been shown that the totality of observed anomalies in the temperature range of 60–70 K can be explained by the existence of collective rotational degrees of freedom in solid methane.

Phase separation in strained cation- and anion-deficient Nd0.52Sr0.48MnO3 films

The magnetic and transport properties of anion- and cation-deficient Nd0.52Sr0.48MnO3 films with different thicknesses, as well as of two films from this system grown on different SrTiO3 and LaAlO3 substrates, are studied. Below Curie temperature TC, the films with different thicknesses exhibit phase separation: they represent magnetic clusters (drops) embedded in a nonconducting paramagnetic (at T > TN, where TN is the Néel temperature) or antiferromagnetic (T < TN) matrix. The temperature dependences of the resistivity of the films are well described in terms of the polaron mechanism of conduction. In external magnetic field H = 0.01 T, the drops may reach 15 nm in size. They consist of magnetic polarons with a small radius (1–2 nm). The drops are shown to interact with each other in the films. Because of competition between drop-drop dipole interaction and the magnetic energy, the drops disintegrate into droplets with a size comparable to that of a magnetic polaron in a field of 1 T. An explanation is given for the discrepancy between our results and the frequently observed growth of the drops with a rise in the external magnetic field. As the film gets thicker, the fraction of the ferromagnetic phase grows with thickness nonlinearly. In the film grown on SrTiO3 (compressed by 0.9%), the characteristic Néel and Curie temperatures are lower than in the film grown on LaAlO3. The diameters of ferromagnetic drops (both maximal at H = 0.01 T and minimal at H = 1 T) turn out to be roughly the same as in the films with different thicknesses.

Low-frequency internal friction as express-method for identification of cryocrystals in pores of the solids

Abstract We show that studying of low-frequency internal friction (LFIF) of solid samples at low temperatures allows determining the presence of various gases absorbed, for some reasons, in pores and caverns of the solids. The gases come over to a solid state (cryocrystals) and exist in the pores under corresponding thermodynamic conditions giving an additional contribution to the LFIF spectra. The spectra reflect the special points of the gases (temperatures of melting or phase transitions). This information gives a real opportunity for identification of gas in the matrix, i.e. the studied solids. This may be of great importance for investigations of cosmic or geological samples, for instance, asteroids, meteorites, rock formations, etc. The LFIF method allows identification of gas media surrounding the studied sample.

Internal friction and phase transitions of solid oxygen

We report an experimental study of low-frequency internal friction (LTIF) in solid oxygen at temperatures of 7–52 K. Comparison of the temperature dependence of the IF with data on x-ray diffraction and other thermodynamic and elastic properties shows that the anomalies in the temperature dependence of the internal friction arise from phase transitions in solid oxygen.

An YBCO film as a Josephson medium near Tc: Frequency and field dependences and scaling relationships

The diamagnetic response of a laser-deposited YBCO film was studied in weak exciting magnetic fields Hac=H0sinωt (H0=2–500 mOe, ω/2π=1–30 kHz). An analysis of the experimentally measured diamagnetic response showed evidence of a ring character of the electric field flowing in the sample at small H0 values. The plots of Tm (the temperature of maximum dissipative losses χ″1) versus H0 measured at various frequencies showed the existence of a certain critical field strength H*(ω) at which the (1−Tm/Tc) versus H02/3 plots exhibit a break. A logarithmic dependence of Tm on the frequency ω of the exciting field Hac is reliably valid only in the region of (ω/2π)>5 kHz. Verification of the scaling relationship showed that this relationship is valid (to within the experimental accuracy) for the frequencies ω above 5 kHz and the field amplitudes H0>H*(ω). However, the exponent n in the scaling relationship under consideration is not correlated with the exponent n in the power relationship describing the current-voltage characteristic.

Temperature dependence of the critical current of YBCO–STO–LCMO heterostructures near Tc

The complex differential susceptibility of a YBCO film and YBCO–STO–LCMO structures with different thicknesses of the STO layer (0, 2, and 7 nm) is investigated. It is shown that for a YBCO–LCMO structure the temperature of the superconducting transition Tonset and the critical current density jc are observed to decrease in comparison with a pure YBCO film. In a YBCO–STO–LCMO structure, on the contrary, Tonset increases, while jc increases for the structure with the 2 nm thickness of STO and decreases for that with 7 nm of STO. It is conjectured that there is a change of the vortex pinning mechanism as the temperature is lowered.

Manifestation of two-dimensional behavior of YBCO films in a study of their complex susceptibility

The temperature dependence of the complex magnetic susceptibility χ′+χ″ of various YBCO films is investigated at different amplitudes of the exciting field Hac. It is found that when the temperature Tm at which the maximum is observed on the χ″(T) curve is plotted as a function of Hacα (the parameter α=1, 1/2, or 2/3, depending on the character of the coupling between crystallites), a kink appears at a certain field Hac=H1*. It is suggested that the temperature Tc1 obtained by extrapolating the Tm(Hac2/3) curve from the high-field region to zero field is the Berezinskii–Kosterlitz–Thouless transition temperature TBKT. This suggestion is based on a comparison of the present results with published data from a study of the complex susceptibility of a GdBa2Cu3O6.75 single crystal for directions of Hac parallel to the c axis of the single crystal and in its ab plane, and also on the coincidence of the calculated values of certain characteristic temperatures near the BKT transition with our experimental values.

Quasiamorphous state of methane hydrate

It is established experimentally that at temperatures above 0°C formation of methane hydrate starts with the appearance of thin films of this substance on the surface of water; during isothermal storage, the films permeate throughout the whole volume of the sample. The maximal content of methane hydrate is several (volume) per cent. The viscosity of the sample is much lower than that of ice and much higher than that of water.

Cellular nanostructure of gas hydrates

It is found that the hydrates obtained by reaction of liquid water with a hydrate-forming gas have a cellular structure with a cell ∼10-5 cm in size. At T ∼ 274 K, the thickness of the walls of the methane hydrate cell is ∼10-7 cm. Formation of the cellular structure is due to a competition between the bulk (difference between chemical potentials, specific energy of elastic deformation) and surface energies. At positive temperatures, monolithic methane hydrate is formed at pressures higher than 6 MPa.