Effect of temperature and magnetic field on the evolution of a vortex structure of the granular YBa2Cu3O7 − δ high-temperature superconductor

The temperature dependences of the electrical resistivity ρ(T) _{H = const} have been measured in external magnetic fields H _ext (0 ≤ H _ext ≤ 1420 Oe) at temperatures ranging from 70 to 273 K for samples of the granular YBa₂Cu₃O_{7 ? δ} high-temperature superconductor (HTSC). Cooling of the samples to the minimum temperature T _min (70 K) has been performed in external magnetic fields (FC mode) and in the absence of a magnetic field (ZFC mode). Moreover, the dependences ρ(T) _{H = 0} for samples cooled in the FC mode have been measured in a zero field. The curves ρ(T)H = const have been converted into isotherms of the magnetore-sistance ρ(H _ext) _{T = const}. A comparative analysis of the specific features in the behavior of the curves ρ(H) _{T = const} for samples with different “magnetic prehistories” has made it possible to elucidate the nature and mechanisms of the influence of the particular scenario of the magnetic treatment of granular HTSCs on the behavior of their galvanomagnetic properties. The temperature dependences of the critical magnetic fields of superconducting grains (H _c1g , H _c2g ) and Josephson weak links (H _c2J ) have been determined, and the H-T phase diagrams of granular YBa₂Cu₃O_{7 ? δ} HTSCs have been recovered.     

Pyrylocyanines. 12. Unsymmetrical pyrylo-2-cyanines

Unsymmetrical 4,6-diphenylpyrylo-2- and 2,6-diphenylpyrylo-4-carbo- and -dicarbocyanines, as well as the analogous pyridocyanines containing 3-ethylbenzothiazolium and 1,3,3-trimethyl-3H-indolium residues, were synthesized. The pyrylo-2-cyanines are more deeply colored than the corresponding 4 isomers, and their absorption bands are distinguished by lower intensities and greater widths. On the basis of data on the increase in the width of the absorption bands of unsymmetrical dyes as the length of their polymethine chain increases it is concluded that the -pyran system has greater electron-donor character than the -pyran system. The regularities observed in the pyridocyanine series are just the opposite of these.See [1] for Communication 11.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 903–908, July, 1980.     

An investigation of the nonsteady interaction of a conducting gas cloud with a given electrical circuit

The behavior of a cloud of conducting gas obtained from a coaxial plasma gun is investigated as it passes through a constant magnetic field. The way that this cloud interacts by induction with an electrical circuit coupled to an ohmic resistance is also studied. Paticular attention is paid to the study of the energy characteristics of the interaction (the energy generated in the ohmic resistance, relations between the plasma energy and Joule dissipation) as a function of the geometry and certain parameters of the electrical circuit. The process is analyzed theoretically for small values of the magnetic Reynolds number. Experimental and theoretical results are compared.     

Influence of the Sign of Solvatochromism of Merocyanines on their Photoelectric Properties in Polymer Films

It has been established that the intensities of the longwave, electron-absorption bands of merocyanine dyes differing in the sign of solvatochromism redistribute under the action of an external electric field. This is explained by the fact that the probability of vibronic transitions changes under the action of an electric field. It has been revealed that the photoconductivity of poly-N-epoxypropylcarbazole films doped with merocyanines increases with decrease in negative solvatochromism and increase in positive solvatochromism. It is suggested that the more bipolar structure of the excited state of the above-mentioned films in the first case as compared to the structure of these films in the second case is favorable for electron transfer from the poly-N-epoxypropylcarbazole molecules to the dye molecule and the formation of a carbazole cation-radical providing hole conduction in the polymer.     

Pyrylocyanines

Depending on the reaction conditions, in the reaction of symmetrical 2,6-diphenyl-4-pyrylomonomethine-, carbo-, and dicarbocyanines with methylamine the oxygen atoms in one or both pyrylium residues are replaced to give both unsymmetrical pyrylopyrido- and symmetrical pyridocyanines. The deviations for the synthesized dyes increase as the lengths of the polymethine chromophore increases.See [1] for communication V.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 612–616, May, 1975.     

The effect of an electrostatic field on the photoluminescence and photoconductivity of polymer films doped by cationic polymethine dyes with end groups with different electron-donating properties

The effect of an external electrostatic field on the photoluminescence spectra and photoconductivity of symmetric and asymmetric cationic polymethine dyes with end groups possessing different electron-donating properties (basicity) in polymer films based on photoconductive poly-N-epoxypropylcarbazole and nonphotoconductive polyvinylethylal is studied. It is found that application of an electric field to films can both decrease and increase their luminescence intensity. The strongest decrease in the luminescence intensity is observed for symmetric dyes in poly-N-epoxypropylcarbazole films with either a decrease in the electron-donating ability of the end groups or with a decrease in the length of the polymethine chain. Colored polyvinylethylal films exhibit both a negative and a positive effect of the field on the photoluminescence intensity. The character of changes in the luminescence spectra does not depend on the electron-donating ability of the end groups. The spectral effects are explained by a change in the probability of vibronic transitions due to the redistribution of the electron density in the chromophore of the dye in the ground and excited states in an external electric field.     

Influence of a Photoconductive Polymer Matrix on the Photoluminescence of Cationic Polymethine Dyes

Using symmetric cationic indopolycarbocyanines HIC and HIDC as an example, the authors detected the enhancement of their photoluminescence in films of photoconductive polymers poly–N–epoxypropyl carbazole and poly–N–vinyl carbazole as compared to nonphotoconductive polymers, i.e., polyvinyl butyral, polystyrene, and polyethylene. The excitation was carried out on the shortwave edge of the absorption band of the dye and did not affect the absorption region of the polymer. It is shown that the effect of enhancement of the luminescence increases with decrease in the excitation wavelength and becomes weaker with increase in the molecular mass of carbazole–containing polymers. Its enhancement is interpreted as the recombination luminescence of electron–hole pairs formed in photogeneration of charge from the dye molecules.     

Electric Conductivity of Sandwich Structures with Films of Poly-N-epoxypropylcarbazole Doped with Organic Dyes of Various Ionicity

The conducting characteristics of samples of a sandwich structure with films based on the photoconducting polymer poly-N-epoxypropylcarbazole doped with cationic, anionic, cationic–anionic, neutral, and bipolar organic dyes were investigated. It was shown that the conductivity of the samples is caused by ion drift, thermofield generation of holes from uncontrollable impurity centers, thermofield generation of electrons and holes from the dopant molecules, and thermofield injection of electrons and holes from the electric contacts. The contribution from the injection currents of the charge carriers increases in the transition from a cationic dye to anionic, cationic–anionic, and intraionic dyes.     

Phase transitions and vortex structure evolution in two-level high-temperature granular superconductor YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 – δ</Subscript> under temperature and magnetic field

Temperature dependences of the resistivity ρ(T) of samples of granular high-temperature superconductor YBa₂Cu₃O_{7 – δ} are measured at various transverse external magnetic fields at 0 < H _ext < 1900 Оe in the temperature range from the upper Josephson critical temperature of “weak bonds” T _c2J to temperatures slightly exceeding the superconducting transition temperature T _c . Based on the data obtained, the behavior of the field dependences of the critical temperatures of superconducting grains and “weak bonds,” and temperature and field dependences of the magnetic contribution to the resistivity \(\left[ {\Delta \rho \left( {T,H} \right) = \rho {{\left( T \right)}_{{H_{ext}} = const}} - \rho {{\left( T \right)}_{{H_{ext}} = 0}}} \right]\). It is shown that the behavior of the magnetic contribution to the resistivity Δρ along the line of the phase transition related to the onset of the magnetic field penetration in the form of Abrikosov vortices into the subsystem of superconducting grains T _c1g (H _ext) is anomalous. The concepts on the magnetic flux redistribution between both subsystems of two-level HTSC near in the vicinity of T _c1g : the Josephson vortex decreases, and the Abrikosov vortex density increases.