A. Yu. Tsvetkov

Percolation nature of the 60-K to 90-K phase transition in YBa2Cu3O6 + δ

The temperature dependence of the heat capacity of the HTSC YBa2Cu3O6 + δ is measured in the temperature range 1.8–300 K and the doping range 0.70 < δ < 0.86. The results obtained suggest that the transition from the 60-K to the 90-K phase in YBa2Cu3O6 + δ has a percolation nature and that the underdoped 60-K phase represents a heterophase structure in which numerous superconducting clusters are embedded into an insulating matrix.

Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa2Cu3O6.93 high-Tc superconductor

The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature Tc have been measured for a series of fine-crystalline YBa2Cu3Oy samples having nearly optimum values of y = 6.93 ± 0.3 and Tc = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field Hc are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in Hc(T) and the jump of the specific heat ΔC/Tc. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-Tc superconductors.

Effect of radiation defects on the magnetotransport properties of Ba(Fe1 − xCoxAs)2 high-temperature superconductor

The effect of 200-keV He+ ion irradiation on the transport properties of films of the iron-based Ba(Fe1 − xCoxAs)2 superconductor has been studied. Contributions to the resistivity and magnetoresistance of irradiated samples from scattering by magnetic and nonmagnetic defects have been separated. It has been shown that mainly nonmagnetic defects are generated in the sample under the corresponding irradiation conditions. This result is important in view of the use of the radiation technique for the study of the effect of defects on the properties of iron-based superconductors.

Investigation of the properties of superconducting plates with a thickness of the order of the coherence length ξ in the framework of the Ginzburg-Landau theory

The properties of superconducting plates with a thickness of the order of the coherence length ξ have been investigated by numerically solving the system of one-dimensional Ginzburg-Landau equations. The equations have been solved using boundary conditions of the general form for the order parameter, which makes it possible to take into account the influence of the boundaries of the plate on its superconducting properties. The behavior of the critical current and critical magnetic field as a function of external parameters has been analyzed. It has been shown that the inclusion of the influence of the boundary in the calculations leads to results that are in better agreement with the experimental data.

On the role of the boundary conditions in the Ginzburg-Landau theory

The effect of the boundary conditions for solutions on the Ginzburg-Landau (GL) equations for superconducting plates in the vortex-free limit is studied by numerical methods. Based on the self-consistent solution of the system of GL equations, the dependence of the critical current Ic on the external magnetic field and the distribution of the order parameter over the plate thickness are determined. When solving the equations with general boundary conditions, it was found that the critical temperature and critical current density decreased in comparison with those obtained by solving equations with ordinary boundary conditions. According to the results of this study, the use of general boundary conditions leads to a number of interesting results which were not observed when using ordinary boundary conditions. The range of the applicability of the vortex-free limit for the films of thickness of the order of the coherence length ξ are discussed. The effect of boundary conditions on the applicability of this limit is analyzed.

Calculation of critical states of superconducting multilayers based on numerical solution of the Ginzburg-Landau equations for superconducting plates

Numerical methods are used to analyze the Ginzburg-Landau equations for a superconducting plate carrying transport current in a magnetic field. Critical current is calculated as a function of the applied magnetic field strength for superconducting plates with different thicknesses. The relations between the field dependence of critical current and the distributions of order parameter, magnetic field, and supercurrent in a plate are analyzed. The field-dependent critical currents computed for plates are used to determine the critical current as a function of the applied magnetic field strength and local magnetic field and current distributions for multilayers in parallel magnetic fields. The constituent superconducting layers are assumed to interact only via magnetic field. A simple method is proposed for analyzing the critical states of multilayers in magnetic fields of arbitrary strength, based on elementary transformations of the critical current-density distribution over individual layers in zero applied magnetic field. The method can be used to analyze experimental results.

Scaling of the current-voltage characteristics of superconducting films in the flux creep model

On the basis of the magnetic flux creep model, taking viscous vortex motion and the spatial shape of the pinning potential into account, we have constructed a model explaining the shape of the current-voltage characteristics (CVC’s) of high-temperature superconducting films and the sign change of the curvature of these characteristics with change in temperature. The model given also allows one to explain the scaling of these curves.

Superconducting gap symmetry in BaFe

T.E. Kuzmicheva, S.A. Kuzmichev, 1 A.V. Sadakov, S.Yu. Gavrilkin, A.Yu. Tsvetkov, X. Lu, H. Luo, A. N. Vasiliev, 4, 5 V. M. Pudalov, Xiao-Jia Chen, and Mahmoud Abdel-Hafiez 7, ∗ P.N. Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia M.V. Lomonosov Moscow State University, Faculty of Physics, 119991 Moscow, Russia Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China National Research South Ural State University, Chelyabinsk 454080, Russia National University of Science and Technology (MISiS), Moscow 119049, Russia Center for High Pressure Science and Technology Advanced Research, Shanghai, 201203, China Center for High Pressure Science and Technology Advanced Research, Beijing, 100094, China (Dated: June 8, 2018)

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The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS3 on the direction and magnitude of the magnetic field B have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at T < 100 K. Below T0 ≈ 50 K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance (B ∥ c) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at T0. The negative magnetoresistance (at B ∥ c) below T0 is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at B ∥ ab) is explained by the effect of the magnetic field on the spectrum of electronic states.

Ni

The effect of boundary conditions in the Ginzburg–Landau theory on the critical state of superconducting layered structures is studied. The method is based on the numerical solution of the Ginzburg–Landau nonlinear equations describing the behavior of a superconducting plate carrying a transport current in a magnetic field, provided the absence of vortices in it. The use of the general boundary condition for the Ginzburg–Landau system of equations leads to a change in the order parameter over the thickness of thin superconducting plates. The calculated dependences of the critical current of plates on the magnetic field applied in parallel to layers are used to determine the critical current of multilayered structures. It is assumed that the mutual influence of superconducting layers occurs only through the magnetic field induced by them.