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The issue concerning the nature and the role of microstructural inhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35 and their correlation with transport properties of this system was addressed. Presented data demonstrate that chemical disorder originating from the kinetics of the crystal growth process significantly influences the superconducting properties of an Fe-Te-Se system. Transport measurements of the transition temperature and critical current density performed for microscopic bridges allow us to deduce the local properties of a superconductor with microstructural inhomogeneities, and significant differences were noted. The variances observed in the local properties were explained as a consequence of weak superconducting links existing in the studied crystals. The results confirm that inhomogeneous spatial distribution of ions and small hexagonal symmetry nanoscale regions with nanoscale phase separation also seem to enhance the superconductivity in this system with respect to the values of the critical current density. Magnetic measurements confirm the conclusions drawn from the transport measurements., Comment: 20 pages, 5 figures

The magnetization anisotropy of a layered superconductor FeTe0.65Se0.35 sample is experimentally studied in a magnetic field directed either along the layers of the plane, or perpendicular to them. The value of the vortex pinning potential in a weak magnetic field, and the critical current density ratio are determined for these directions. The results are discussed within the framework of presenting the sample as layers of fine single crystals, divided by weak interlayer superconducting bonds with magnetic inclusions., Comment: 4 pages, 6 figures

In this paper we present our results on the low density magnetic flux pinning characteristics, obtained though HTSC samples magnetization relaxation measurements M(t) near the phase transition temperatures (0,8 < T/Tc > 0,99).As the main object of investigation we have chosen pure YBa2Cu3O7-x (YBCO) single crystal samples with optimal oxygen saturation. We note the presence of strong pinning in the system of unidirectional planar defects and a significant nonmonotonicity of the Up(T) dependence behavior for low density trapped magnetic flux., Comment: 2 Pages, 2 figures

Low density flux dynamics in Nd based cuprate (NdBa2Cu3O7-x) HTSC single crystals trapped in 1 Oe range DC magnetic fields were investigated for the first time. Complex measurements of time, temperature and field dependence of magnetization were performed. Using the results obtained and the Anderson-Kim model the normalized magnetization relaxation rate S and the averaged effective pinning potential U were calculated. We show a significant (one or even two orders) increase in the effective pinning potential in these samples in comparison with measurements performed in high magnetic fields for similar cuprate single crystals formerly. In a wide enough range of temperatures close to Tc (0.72 < T/Tc < 0.93) it has been shown that the doping of NdBa2Cu3O7-x single crystals with calcium atoms leads to the increase of the averaged effective pinning potential with an insignificant decreasing of the critical temperature., Comment: Submitted to journal "Functional Matherials"

The Abricosov vortex and bundles dynamics was experimentally investigated in Earth's magnetic field range. Isothermal relaxation features in YBCO single crystal samples with strong pinning centers were studied for different sample-field orientation. The normalized relaxation rate S obtained allowed to estimate the effective pinning potential U in the bulk of the YBCO sample and its temperature dependence, as well as the critical current density Jc. A comparison between the data obtained and the results for similar measurements in significantly higher magnetic fields was performed. To compare different Jc measuring techniques magnetization loop M(H) measurements, were made. These measurements provide many important parameters of the sample under study (penetration field Hp, first critical field Hc1, etc.) that contain the geometrical configuration of the samples., Comment: 3 pages, 4 figures

An experimental study of low density captured magnetic flux dynamics in YBCO single crystal samples near the superconducting phase transition point has been performed. Vortex dynamics similar to thermally assisted flux flow regime (TAFF) has been observed for the first time in such weak fields. It will be shown that Zeldov's thermoactivated creep model successfully describes the behavior of the magnetization in the YBCO single crystals in our experiments. Within the framework of this model the estimation of normalized relaxation rate was made., Comment: 3 pages, 4 figures, full text will be published in "Low Temperature Physics"

Trapped low magnetic flux dynamics including local ones are investigated in YBCO single crystals in the strong thermal fluctuations domain near the superconducting phase transition temperatures. The essential difference from quasi-logarithmic isothermal relaxation of a magnetization behavior (observed earlier in high magnetic fields) was established. Within the framework of classic model of a thermal activated vortices creep the estimation of an effective pinning potential was made at these temperatures., Comment: The full text of this paper will be pulished in "Low Temperature Physics"