arXiv subjects

D. M. Gokhfeld

Publications and source records attributed to D. M. Gokhfeld.

13 recordsLinked to original sources

arXiv2025

Effect of decorating NiO nanoparticles on superconducting properties of YBCO

The influence of adding 23 nm NiO nanoparticles on the magnetic hysteresis loops and critical current density of the high-temperature superconductor YBa2Cu3O7-d has been investigated. The samples were prepared using a fast annealing method that prevents chemical interaction between the components and does not reduce the critical temperature of the superconductor. Compared to the undoped YBCO sample, the critical current density increases in the samples doped with NiO nanoparticles in magnetic fields greater than 6 kOe. The sample containing 0.5 weight percent of NiO nanoparticles exhibits the greatest enhancement in critical current density at 10 kOe (1.36 times greater than the undoped sample).

cond-mat.supr-con↗

arXiv2025

Accounting the size distribution of HTS granules for the critical current density from magnetic measurements

The determination of the critical current density from magnetic hysteresis loops is widely used to characterize and compare superconducting samples. Magnetic hysteresis loops for tapes and single crystals depend on both the critical current density and sample size. The latter sets the scale of supercurrent circulation. However, in polycrystalline high-temperature superconductors prepared by solid-phase synthesis or by sol-gel method, the magnetization is determined by the circulation of supercurrents in individual grains. The paper discusses the effect of the grain size distribution on the effective scale of current circulation. Log-normal and Weibull distributions are both considered as possible for grain sizes. The effective size for calculating the intragrain current density has been shown to be significantly larger than the average grain size.

cond-mat.supr-con↗

arXiv2025

Larger grains in high-Tc superconductors synthesized by the solid-state reaction route

Solid-state synthesis is widely used in exploratory research to study various structural modifications that affect the properties (critical temperature, critical current density, irreversibility field, etc.) of superconductors. The popularity of this method is due to its relative simplicity and availability of the necessary equipment. Combining solid-state synthesis and top-seeded melt growth allows us to increase the grain size in a Tm- and Nd-based 1-2-3 superconductor. Samples with a grain size up to 0.1 mm have been obtained. X-ray diffraction, scanning electron microscopy and magnetization measurements have been used for investigating this superconducting material. The magnetization width ΔM has increased significantly in the synthesized samples. However the temperature dependence of the intragrain critical current density and the pinning force scaling give evidences that the pinning mechanism in the obtained superconductor is essentially the same as in polycrystalline superconductors synthesized by standard solid-state technology. The increase in grain size in the synthesized samples is the main reason for the high values of ΔM.

cond-mat.supr-con↗

arXiv2024

Spontaneous rotation of magnets levitating above high-Tc superconductor

The levitation of a cylindrical permanent magnet over a high-temperature superconductor cooled by liquid nitrogen can be accompanied by spontaneous oscillations and rotation. The reason for spontaneous rotation of the magnet is magnetization inhomogeneity induced by the temperature gradient. An experiment was carried out on the levitation of Nd-Fe-B magnets over a composite high-temperature superconductor. The experimental results confirm that the rotation frequency depends on the difference in the magnetization values in the upper and lower halves of the magnet. Methods for controlling the rotation frequency of a levitating magnet are proposed.

cond-mat.supr-con↗

arXiv2023

Highly Porous Superconductors: Synthesis, Research, and Prospects

This paper gives a review of studies of superconductors with a porosity above 50%. The pores in such superconducting materials provide refrigerant penetration, efficient heat dissipation and stable operation. Methods for the synthesis of the main groups of porous superconductors are described. The results of studies of the structural, magnetic, and electrical transport properties are presented, and the features of the current flow through porous superconductors of various types are considered.

cond-mat.supr-con↗

arXiv2022

Anisotropy and crystallite misalignment in textured superconductors

A misalignment of anisotropic crystallites causes small values of anisotropy and decreases the critical current density of textured polycrystalline superconductors. To relate the crystallite misalignment and out-plane anisotropy, the magnetic properties of the textured Bi2223 polycrystalline superconductor were investigated. A distribution of orientation angles of crystallites was determined using different data: scanning electron microscopy images and hysteresis magnetization loops when an external magnetic field was applied at different angles with respect to the texturing plane of the sample. It was demonstrated that the standard deviation of the distribution and the magnetic disorder angle of crystallites in textured samples can be determined from the magnetization data in perpendicular directions. These data may be either the irreversible magnetization measured for two different orientations of the sample or the simultaneously measured magnetization projections parallel and perpendicular to the magnetic field.

cond-mat.supr-con↗

arXiv2019

Analog of the Intertype Superconductivity in Nanostructured Materials

Magnetization hysteresis loops of tin samples with an inverted opal structure are presented. The sample formed by tin particles with the size of 70 and 128 nm is found to be a type-I superconductor. The tin sample formed by 80 and 42 nm particles demonstrates an analog of intertype superconductivity: features of both type-I and II superconductors are observed on the magnetization isothermal curves. Behaviors of the irreversible and reversible magnetization support coexistence of type-I and II superconducting nanoparticles in this sample.

cond-mat.supr-con↗

arXiv2013

Pinning in a Porous Bi2223

The current-voltage characteristics of a porous superconductor Bi2Sr2Ca2Cu3Ox (Bi2223) have been measured at temperature range from 10 to 90 K. The experimental dependences have been analyzed within the model allowing for pinning by clusters of a normal phase with fractal boundaries, as well as the model taking into account phase transformations of vortex matter. It has been found that the electrical resistance of the superconductor material significantly increases at temperatures of 60-70 K over the entire range of magnetic fields under consideration without changing in the sign of the curvature of the R(I) dependence. The melting of the vortex structure occurs at these temperatures. It has been assumed that this behavior is associated with the specific feature of the pinning in a highly porous high-temperature superconductor, which lies in the fractal distribution of pinning centers in a wide range of self-similarity scales.

cond-mat.supr-con↗

arXiv2010

Magnetization asymmetry of type-II superconductors in high magnetic fields

Inhomogeneous distribution of the pinning force in superconductor results in a magnetization asymmetry. A model considering the field distribution in superconductor was developed and symmetric and asymmetric magnetization loops of porous and textured Bi_{1.8}Pb_{0.3}Sr_{1.9}Ca_{2}Cu_{3}O_{x} were fitted. It is found that the thermal equilibrium magnetization realizes in crystals smaller than some size depending on temperature and magnetic field.

cond-mat.supr-con↗

arXiv2009

Asymmetry of magnetization curves of textured BSCCO

The dependence of magnetization on magnetic field M(H) was measured for textured BSCCO samples for H || c, H || ab at different temperatures. Inclusion of the pinning parameter in the Valkov-Khrustalev model [V.V. Valkov, B.P. Khrustalev, JETP 80 (1995) 680] allowed to describe successfully the asymmetric M(H) dependencies. The temperature and magnetic field dependencies of critical current and pinning parameter for H || c and H || ab were estimated.

cond-mat.supr-con↗

arXiv2008

Pinning Enhancement by Heterovalent Substitution in Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$

The intragrain pinning in high-$T_c$ superconductor compounds Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ with low concentration of RE (La, Ce, Pr) was investigated. Magnetic and transport measurements reveal that the pinning is maximal for the concentration of heterovalent RE such that the average distance between the impurity ions in the plane of rare-earth elements close to the diameter of Abrikosov vortices in YBCO.

cond-mat.supr-con↗

arXiv2005

Magnetization loop and critical current of porous Bi-based HTSC

The magnetization of porous $Bi_{1.8}Pb_{0.3}Sr_2Ca_2Cu_3O_x$ have been investigated. The experimental magnetization hysteretic loops of $M(H)$ were described in the frames of Val'kov -- Khrustalev model developed for type II granular superconductors.

cond-mat.supr-con↗

arXiv2004

Andreev reflection in Bi_{1.8}Pb_{0.3}Sr_{1.9}Ca_2Cu_3O_x break junction

The current-voltage characteristic (CVC) of break junction made from polycrystalline Bi_{1.8}Pb_{0.3}Sr_{1.9}Ca_2Cu_3O_x is investigated. The experimental CVC has hysteretic feature that reflects a part of a curve with negative differential resistance. The CVC is discussed within the framework of the Kummel-Gunsenheimer-Nicolsky theory considering multiple Andreev reflections in superconductor/normal-metal/superconductor junctions.

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