Relaxation of the remanent resistance of granular HTSC Y-Ba-Cu-O + CuO composites after magnetic field treatment

The hysteresis of magnetoresistance R(H) and relaxation of the remanent resistance R _rem with time after magnetic field treatment of HTSC (Y-Ba-Cu-O) + CuO composites are studied. Such a composite constitutes a network of Josephson junctions wherein the nonsuperconducting component (CuO) forms Josephson barriers between HTSC grains. By comparing the experimental R _rem(t) and R(H) dependences, it is shown that the relaxation of the remanent resistance is caused by the decreased magnetic field in the intergrain medium due to relaxation of magnetization. The reason is uncovered for the differences between the published values of pinning potentials determined by measuring the relaxation of magnetization or resistance and fitting them by the Anderson law.     

The nature of the flux lattice in granular superconducting networks

By exploiting an analogy with electric circuit theory, we study the Ginzburg-Landau equations of a superconducting network in a uniform magnetic field. For the geometry of a Sierpinski gasket (which serves as a model of a granular superconductor close to the percolation threshold) general recursion relations are derived. By numerical evaluation we determine the distribution of vortices in the gasket and the resulting diamagnetic moment as a function of the applied field. The effect of the destruction of self similarity is also discussed.     

Anisotropy of the superconducting transition in magnetic fields in a Nd1.85Ce0.15CuO4 single crystal

The anisotropy of superconducting properties of a Nd_1.85Ce_0.15CuO₄ single crystal is studied by resistance measurements over the temperature range 2–30 K in magnetic fields of 0, 1, 2, 4, and 6 T parallel to the a-b plane. A strong anisotropy of T _c (H) and H _c 2(T) is observed for different orientations of magnetic field in the a-b plane. This anisotropy leads to a twofold symmetry of T _c (H) and H _c 2(T), and the gap node direction is determined. An analysis of experimental data shows that this result can be attributed to a change in the local symmetry of the copper atom environment, which manifests itself as a reduction from tetragonal to orthorhombic symmetry in the low-temperature region. The comparison with La_1.85Sr_0.15CuO₄ suggests that the mechanisms of superconductivity in electron and hole doped superconductors are similar, and the difference observed in the experiment is related to the structural features of these materials.     

Influence of magnetic field on the superconducting transition in a Nd1.82Ce0.18CuO4−δ film

The temperature and magnetic-field dependences of the resistivity ρ and Hall effect R(j‖ab, B‖c) in a Nd_1.82Ce_0.18CuO_4−δ single crystal film (T _c =6 K) is investigated at temperatures 1.4≤T≤20 K and magnetic fields 0≤B≤5.5 T. At the lowest temperature T=1.4 K the resistive state (exhibiting resistivity and the Hall effect) arises in a magnetic field B=0.5 T. A transition to the normal state is completed at B _c 2≃3 T, where the Hall coefficient becomes nearly constant. The negative magnetoresistance due to the weak-localization effect in the normal state is observed for B>3 T. The nonmonotonic behavior and the inversion of the sign of R(B) in the mixed state are explained in a reasonable way by the flux-flow model with the effect of pinning taken into account. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 407–411 (25 September 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Simulation of flux dynamics in a superconducting bulk magnetized by multi-pulse technique

We have simulated the time and spatial dependence of local field B_z(t, r) and temperature T(t, r) on the superconducting bulk during pulsed field magnetization (PFM) using the finite element method (FEM). A modified multi-pulse technique with step-wise cooling (MMPSC) was performed to the cryo-cooled bulk, which was experimentally confirmed to the effective PFM technique to enhance the trapped field B_z higher than 5 T. In the simulation, the B_z value at the center of the bulk surface was enhanced at the 2nd stage of the MMPSC method, in which the results of the simulation reproduced the experimental ones. The enhancement of B_z results from the reduction in the temperature because of the already trapped flux in the bulk at the 1st stage of the MMPSC method.     

Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.     

Dependence of the magnitude and direction of the persistent current on the magnetic flux in superconducting rings

The obtained periodic magnetic-field dependences I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences I _c?(Φ/Φ₀) = I _c+(Φ/Φ₀ + Δ?) of the critical current measured in opposite directions. This shift means that the position of I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude I _{c, an}(Φ/Φ₀) = I _c+(Φ/Φ₀) ? I _c?(Φ/Φ₀) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature.     

Dominant influence of the compression effect of a magnetic flux in the intergranular medium of a granular high-temperature superconductor on dissipation processes in an external magnetic field

Experiments have been presented that demonstrate the effect of the compression of a magnetic flux in grain boundaries of a granular high-temperature superconductor in an external magnetic field on the dissipation processes. The compression of the magnetic flux is associated with the diamagnetic behavior of superconducting grains and the existence of a Josephson medium in grain boundaries. Under these conditions, grain boundaries are in an effective magnetic field that depends on the magnetic state (magnetization) of the superconducting grains. Based on the analysis of experimental data (dependences of the electrical resistance R and magnetization on the magnetic field H and temperature T, as well as current-voltage characteristics), the conclusion has been drawn that it is the temperature evolution of the effective magnetic field in the intergranular medium which primarily determines the behavior of the dependences R(T) in weak external magnetic fields of no more than ～10³ Oe. This should be taken into account in the interpretation of experiments on the magnetoresistance effect in granular high-temperature superconductors in terms of different theories. The conclusion drawn here also implies a significant correction of the previously obtained results.     

μSR study of internal magnetic fields in superconducting La1.9Sr0.1CuO4 in the mK region

Muon-spin-rotation (μSR) experiments were performed on superconducting La_1.90Sr_0.10CuO₄ in zero external field. Below 2 K, a fast depolarization of the muon signal is observed, indicating that internal magnetic fields are present at the muon site. The average magnitude of the magnetic field in La_1.90Sr_0.10CuO₄ is in the order of 3 mT and thus approximately 14 times smaller than the corresponding field in antiferromagnetic La₂CuO₄. The present data support theoretical models of superconductivity in which magnetic correlations are responsible for pairing.     

Coupling of magnetic and ferroelectric hysteresis by a multicomponent magnetic structure in Mn2GeO4

The olivine compound Mn(2)GeO(4) is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism. Symmetry suggests that the direct coupling between the ferromagnetism and ferroelectricity is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multicomponent magnetic structures found in magnetically frustrated materials like Mn(2)GeO(4) provide a new route towards functional materials that exhibit coupled ferromagnetism and ferroelectricity.     

Improvement of the superconducting properties of an infiltrated YBCO bulk superconductor by a BaCeO3 addition

Single grain YBa₂Cu₃O_7−x (Y123) bulk superconductors with Y₂BaCuO₅ (Y211) and various amounts of BaCeO₃ (5–45 wt.% by an increment of 10 wt.%) were fabricated by a seeded infiltration process. The addition of BaCeO₃ was found to be effective for a modification of the microstructure and an improvement of the superconducting properties. The refinement effect for Y211 particles within an entire superconducting YBa₂Cu₃O_7−x (Y123) matrix was achieved by BaCeO₃ additions. The critical current density (J_c) values were increased as the BaCeO₃ contents were increased (maximum J_c at 35 wt.% BaCeO₃ addition). The J_c improvement by BaCeO₃ additions might be due to the microstructure modifications associated with the finely distributed Y211 and BaCeO₃ particles. With the addition of BaCeO₃ the onset T_c values decreased slightly, indicating highly limited Ce substitution for Y site. It can be concluded that the BaCeO₃ addition has a beneficial effect on the morphology, the size and the distribution of the Y211 inclusions and the microstructure regarding pinning improvement.     

Anisotropy of the magnetoresistive properties of granular high-temperature superconductors resulting from magnetic flux compression in the intergrain medium

To elucidate the origin of the well-known anisotropy of the magnetoresistive properties of granular high-temperature superconductors (HTSs), which is related to the mutual orientation of magnetic field H and transport current j, we investigate the hysteretic dependences of magnetoresistance R(H) of the yttrium HTS sample at the perpendicular (H ⊥ j) and parallel (H || j) configurations. The hysteretic R(H) dependences are analyzed using the concept of the effective field in the intergrain boundaries through which superconducting current carriers tunnel. The effective degree of magnetic flux compression in the intergrain medium at the perpendicular configuration was found to be twice as much as at the parallel one. This approach explains well the anisotropy of the magnetoresistive properties of granular HTSs, which was previously reported by many authors, and the temperature dependences of the resistance in the resistive transition region.     

Coulomb-Gas scaling law for a superconducting Bi(2+y)Sr(2-x-y)La(x)CuO(6+delta) thin films in magnetic fields

The electrical transport properties of epitaxial superconducting Bi(2+y)Sr(2-x-y)La(x)CuO(6+delta) thin films have been studied in magnetic fields. Using a modified Coulomb-gas scaling law, we can fit all the magnetic field dependent low resistance data with a universal scaling curve, which allows us to determine a relation between the activation energy of the thermally activated flux flow resistance and the characteristic temperature scaling parameters.     

Dependence of the specific heat of a La1.85Sr0.15CuO4 superconducting single crystal on the magnetic field direction in the a-b plane

The low-temperature specific heat of a La_1.85Sr_0.15CuO₄ superconducting single crystal was investigated in magnetic fields up to 8 T and with four orientations — in the a-b plane (along the (100) and (110) directions) and at angles of 45° and 90° with respect to the a-b plane (along the (103) and (001) directions). Anisotropy was observed in the field dependence of the specific heat in the a-b plane. The specific heat was found to be minimum with the field oriented in the direction of the a axis and maximum with the field oriented in a direction making an angle of 45° with the a axis. This can be explained by the anisotropy of the energy gap, whose minimum lies along the (110) direction. For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field strength. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 683–687 (10 December 1997)     

Crossing in the magnetic force-gap hysteresis curve of magnetic levitation systems with a high-Tc superconductor

For the magnetic levitation system consisting of a high-T_c superconductor and permanent magnet, the relation curve of magnetic force with gap between these two components is known as a hysteresis loop, that is, the approaching and departing portions envelop a complete one, and generally these two portions do not cross each other. However, in some special cases this crossing arises, and makes the complete loop broken. In this paper, by the numerical simulation of the magnetic force-gap curve in large numbers of physical and geometrical parameters, two typical crossings were found. To investigate the crossing and explore its physical causes, for one of the crossings, the current density in the superconductor was further calculated and its magnitude and vector distribution at the gaps nearby where the crossing arises were obtained. Based on these calculation results and an adequate discussion, the conclusion was induced that the crossing in the magnetic force-gap hysteresis curve results from applied magnetic field’s incomplete and insufficient penetrating in superconductor.