Weak-link Network Research Articles

Flux trapping in transport measurements of YBa2Cu3O7- x superconductors : A fingerprint of intragrain properties

A study of flux trapping in polycrystalline YBa 2Cu 3O 7- x was made through measurements of transport J c versus H characteristics. It was shown that most of the trapped flux which provokes hysteresis in such experiments comes from the superconducting grains, and not from persistent loops in the weak link network. The lower critical field of the grains and the intragrain critical current were calculated for oxygenated and partially deoxygenated samples, showing good coincidence with the commonly accepted values. Further evidence of the intragranular character of the flux trapping resulted from the study of the time variation of the voltage associated with flux creep, from which good estimates of the effective pinning energy of the grains were obtained.

The contribution of the intergrain currents to the low field hysteresis cycle of granular superconductors and the connection with the micro- and macrostructures

Abstract Granular superconductors exhibit a low field hysteresis cycle ( H H c1,g , the first critical field of the grains) which is related to the weak link network and to the macroscopic shape of the sample. We report measurements and calculations of the relationship between the low field cycle, the magnetic critical current density J wl,mag of the weak links, the radius R of the sample, the average radius r g of the grains and the normal state resistivity ϱ of the weak link network. It is found that the intergrain current J wl,mag is related to the low H cycle by a relation of the form 15( M + − M − )/( Rf ( x )), with f ( x )=2〈 λ L / r g 〉 in most experimental conditions. The ratio 2〈λ L / r g 〉 is related to the slopes of the low field magnetization cycle M ( H ) in the linear and reversible region and in the irreversible region curve in the linear and reversible regions. We show that for thick samples ( R ⪢ R 0 ≈50 μm typically) J wl,mag varies approximately as 1/( Rr g ), whereas for thin samples ( R ≤ R 0 ) it approaches a maximum limit inversely proportional to the resistivity ϱ of the Josephson junctions.

Flux-flow and phase-slip dissipation in crystalline Bi 2Sr 2CaCu 2O 8+δ and YBa 2Cu 3O 7−gd

We have employed a microwave technique to determine the temperature dependences of dissipative mechanisms is textured polycrystallin Y123 and in single phase crystalline BSCCO. The measurements were carried out over the temperature range of 5–100 K, with the applied magnetic field typically varying to a maximum of 1.9 T, applied parallel to the c-axis. The RF current was in the c-plane. In addition to small flux-flow signals, a thermally activated field dependent signal, which apparently arises from phase-slip in a network of weak links, was identified. Phase-slip resistivity is the dominant loss mechanisms near T c. The data have been successfully fit to an extension of a theory of Tinkham, yielding estimates of the phase-slip pinning energy and H c2.

Vortex convection non-linearity of AC response of superconductors with weak links

The electrodynamics of a high-Tc superconductor with a network of numerous weak links (the Josephson medium) in the low-field mixed state have been analysed. The AC response to an electromagnetic wave is governed by the motion of vortices of very large size (hypervortices). The nonlinear effect of the vortex convection has been taken into account, and the second harmonic amplitude calculated.

Relation of critical current density to diamagnetic magnetization in polycrystalline BiPbSrCaCuO

The relation of the critical current density J c to the lower critical field H g of bulk granular superconductors is quantitatively described. H g is the critical field of the full shielding effect of the weak link network and represents the magnetization characteristics of the entire bulk superconductor. The maximum self-field produced by transport critical current in a granular superconductor was found to be approximately equal to H g. D.C. magnetization measurements show that H g ≤ 12 Oe at 77 K for BiPbSrCaCuO superconducting materials prepared by the cast-annealing method. It is a linearly decreasing function of temperature: d H g/d T ≈ −0.34 Oe K −1. Many experiments show that the critical current density derived from H g for sintered samples with rectangular section agrees with the real transport J c. This procedure for evaluating J c is also suitable for other high T c polycrystalline materials.

The magnetization studies of cast-annealing Bi 1.5Pb 0.28Sr 1.72Ca 2.0Cu 3.5O y ceramic sample

The sample with nominal composition Bi1.5Pb0.28Sr1.72-Ca2.0Cu3.5Oy was prepared by cast-annealing method and identified to contain almost pure 110K phase from X-ray powder diffraction pattern. The A.C. magnetic susceptibility and the Meissner diamagnetic magnetization in various magnetic fields were measured as the functions of temperature down to 77K. With the increase of the field the magnitude of D.C. magnetic susceptibility Xdc is found to be reduced and Xdc transition is significantly broadened. The magnetization curve demonstrates that the present ceramic sample is a weak-pinning type-II superconductor as a whole. The slope of (-4 M) versus magnetic field shows that the value of the diamagnetic volume fractions of the whole sample and the superconducting grains is 34.5% and 18.5%, respectively, which means existence of great shielding effect from weak-link networks. Flux creep was detected at nitrogen temperature and studied directly by the magnetization measurement as a function of time in various constant magnetic fields. The result indicates that in weaker fields the magnetization changes logarithmically in time. The logarithmic creep rate is found to rise gradually as applied field is increased until the magnetic field exceeds the magnetization peak field. In higher field, however, the logarithmic dependence of flux creep on time can no longer be found.

Microwave study of the critical state in granular YBa 2Cu 3O x thin films

Modulated microwave absorption measurements at low magnetic fields in YBa 2Cu 3O x thin films in a wide temperature range are reported. The microwave losses are proposed to arise from the weak link network which we treat as a medium in the critical state. A modification of the standard critical state model is introduced considering the mutual interaction of intergranular vortices and vortices trapped within the grains: the interaction of both these media described in terms of the modified critical state model can account for the irreversible features of microwave as well as magnetization or transport measurements.

Temperature and magnetic field dependence of the critical current density of YBCO with fine grains

Abstract We measured the critical current of thin strips of sintered YBCO deposited by electrophoresis or sedimentation on an Ag substrate. The typical granulometry of deposited samples was about 1 μm. The critical current of the samples was measured as a function of the temperature (from the transition temperature down to LHe) and of the magnetic field. The current density versus temperature and magnetic field behavior shows a number of interesting features. The current increases quadratically on decreasing temperature and the half width of the I versus B curves increases slightly, with a value in agreement with the junction size. We show that these effects can be explained in the framework of BCS theory, by modelling the material as a network of weak links with small dimensions having a normal barrier.

AC susceptibility of high temperature superconductors in a critical state model

Abstract A critical state model for a granular superconductor is employed to calculate the temperature and AC and DC magnetic field dependence of the complex susceptibility, χ = χ ′ + iχ ″, of a sintered bulk YBa 2 Cu 3 O 7- δ superconductor. Inter granular Josephson vortices are assumed to sweep in and out of the weak-link network while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis losses. The predictions of the model for χ′ and χ″ are consistent with experimental data and model parameters which characterize a high temperature granular superconductor can be determined. These parameters are the inter- and intragranular pinning force densities, the fraction of the superconducting grains, the grain size distribution and a London penetration depth which neglects grain anisotropy.

Effect of measuring process on diamagnetic behaviour and remanent magnetization in bulk Bi-Pb-Sr-Ca-Cu-O superconductor

The diamagnetic behaviour and the remanent magnetization were studied in two different applying field ways (that is, cooling the sample before or after applying magnetic field). X-ray powder diffraction pattern and a.c. susceptibility have proved that the large fraction of the sample is 110K phase and that the low Tc phase in the material has transition temperature below 77K. From the initial part of magnetization curve, it is estimated that about 30 percent of the sample volume is superconducting phase at 77K. However, the fraction(-4 ΠM/ H) from Meissner effect is only 8%. The reason for this may be attributed to the shielding effect and pinning behaviour to vortex lines in superconducting grains. In the process of cooling the sample before applying magnetic field, the remanent magnetization showed that in the weak field sample at first displayed full diamagnetic shielding effect and no remanent magnetization was observed, then the flux passed into the intergrain regions surrounded by weak-link network and finally penetrated into the superconducting grains. On the other hand, in cooling the sample after applying field, the relative great remanent magnetization can be measured at very weak fields, implying that some flux can be pinned by pinning forces or trapped by weak-link network when removing the magnetic field.

Voltage versus current characteristics of high Tc superconductors described by a statistical series-parallel model

A statistical series-parallel model is presented to estimate the voltage versus current characteristics of high T c superconducting samples. It consists of ‘excellent’ and ‘poor’ superconducting sections with parallel paths of normal material connected together in series. Unconventional statistics taking into account lower and upper bound for the local critical current is used to analyse the experimentally observed characteristics. As a special case, the influence of the proximity effect on the magnetic field dependence of the thickness of the ‘poor’ sections is calculated. It is shown that the flux line pinning model better describes the current carrying mechanism in these superconductors than the weak-link network.

EVIDENCES OF JOSEPHSON WEAK LINK NETWORK IN SINTERED BULK Y1Ba2Cu3O7−x OXIDE

Measurements of point-contact tunneling, AC susceptibility vs transport current, zero voltage supercurrent under microwave irradiation and critical current as a function of temperature have been carried out on high Tc oxide superconductor Y 1 Ba 2 Cu 3 O 7−x. The results of these measurements can be interpreted by the assumption of existence of Josephson weak link network in the oxide.

A study of I–V characteristics of YBa2Cu3O7− X

We have measured I–V characteristics of sintered bulk Y 1 Ba 2 Cu 3 O 7− x superconductor at different temperatures. Two distinct features have been found on the I–V curves. From these two features and their temperature dependence, we have concluded that the destruction of superconductivity in oxide superconductor can be divided into two steps, the breakdown of superconductivity of the Josephson weak link network and of the oxide superconducting grain itself.