Granular High-temperature Superconductors Research Articles

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.

Magnetoresistance of Superconducting Granules and Grain Boundaries in Ceramic YBa2Cu3O7−δ High-Temperature Superconductors in Weak Magnetic Fields

The objective of the work is to establish the contribution of superconductive granules and grain boundaries (weak links) in magnetoresistance ρ value of YBa2Cu3O7−δ granular high-temperature superconductor (HTS) at T<Tc. The current-voltage characteristics (CVCs) \(E(j)_{H_{\mathrm{ext}} = \mathrm{const}}\) of YBa2Cu3O6.95 ceramic samples were measured in Hext (0≤Hext≤≈500 Oe) magnetic fields. The CVCs \(E(j)_{H_{\mathrm{trap}} = \mathrm{const}}\) of the samples magnetized in Htreat magnetic fields were measured at Hext=0. Based on the CVCs, \(\rho(j)_{H_{\mathrm{ext}} = \mathrm{const}}\), ρ(Hext)j=const and \(\rho(j)_{H_{\mathrm{treat}} = \mathrm{const}}\) dependences were reestablished. The comparative analysis of \(\rho(j)_{H_{\mathrm{ext}} = \mathrm{const}}\) and ρ(Hext)j=const dependences indicates the magnetic field redistribution between grain boundaries and superconductive granules influence on transport and galvanomagnetic properties of granular HTS. The superconductive grain magnetoresistance ρg was established to be significantly lower than ρJ value of Josephson medium.

Specific features in the hysteretic behavior of the magnetoresistance of granular high-temperature superconductors

The behavior of the hysteresis of the magnetoresistance R(H) of granular high-temperature superconductors has been investigated under the conditions where the resistive response of the subsystem of grain boundaries close to saturation. The hysteretic dependences R(H) have been measured for Y1 − xPrxBa2Cu3O7 samples at x = 0.11 and 0.04 with the transition temperatures TC ≈ 85.5 and 91.0 K, respectively. The evolution of the field width of the hysteresis R(H) has been examined by varying the measuring current. The limit of the applicability has been established for the concept of the effective field in the intergranular medium, which was previously proposed for the description of the hysteretic behavior of the magnetoresistance R(H) and thermal magnetic prehistory of the granular high-temperature superconductors. In the studied samples, the approximation of the effective field in the intergranular medium is applicable until the magnetoresistance of the subsystem of grain boundaries exceeds (90 ± 5)% of the maximum value.

Peculiarities in galvanomagnetic properties of granular YBa2Cu3O7−δ high-temperature superconductors in very weak magnetic fields

The character of the evolution of a system of weak links in granular high-temperature superconductors under the action of an external magnetic field Hext has been studied by measuring the current-voltage characteristics \(E{(j)_{{H_{ext}} = const}}\) of YBa2Cu3O7 − δ (δ ≈ 0.05) ceramic samples. The measurements have been performed at T = 77.3 K in a range of very weak magnetic fields 0 < Hext ≲ 0.5Hc2J, where Hc2J is the upper critical field of the Josephson weak links. The results have been used to construct the field dependences of the magnetoresistance Δρ(Hext) of the superconducting ceramics. It has been established that the parameters of the power equation E = A(j − jcJ)ν and the magnetoresistance Δρ are nonmonotonic functions of the external magnetic field. The presence of extrema in the curves A(Hext), jcJ(Hext), ν(Hext), and Δρ(Hext) indicates that different systems of weak links between grain boundaries, which are capable of forming extended Josephson contacts, undergo sequential transitions to a resistive state with an increase in Hext.

Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics of granular YBa2Cu3O6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields Hext with a strength of up to Hext ≈ 500 Oe for low transport current densities (0.1 A/cm2 ≤ j ≤ 0.6 A/cm2). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j)Hext=const) and Hext(ρ(Hext)j = const). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at Hext ≥ Hc1g, where Hc1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)Hext = const and ρ(Hext)j = const has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field Hc1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field Hc2J.

Mechanism of formation of a negative magnetoresistance region in granular high-temperature superconductors

The field dependences of the magnetoresistance of Bi1.8Pb0.3Sr1.9Ca2Cu3Ox samples with different densities, which have a foam microstructure and exhibit different diamagnetic responses, were studied at 77.4 K to identify the mechanism responsible for the formation of a negative magnetoresistance region in granular high-temperature superconductors. A region with negative magnetoresistance was found to exist in samples with magnetizations highest in absolute magnitude. This behavior finds a reasonable interpretation as due to the effect exerted by dipole moments of high-temperature superconductor grains on the effective intergranular field. The strength of this effective field has been estimated.

Mechanism of the hysteretic behavior of the magnetoresistance of granular HTSCs: The universal nature of the width of the magnetoresistance hysteresis loop

The hysteretic behavior of the magnetoresistance R(H) of granular high-temperature superconductors (HTSCs) of the Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and La-Sr-Cu-O classical systems is investigated for transport current densities lower and higher than the critical density (at H = 0). All systems exhibit universal behavior of the width of the magnetoresistance hysteresis loop: independence of transport current under identical external conditions. This means that flux trapping in HTSC grains is the main mechanism controlling the hysteretic behavior of the magnetoresistance of granular HTSCs, while pinning of Josephson vortices in the intragranular medium makes no appreciable contribution to the formation of magnetoresistance hysteresis (when transport current flows through the sample). Experimental data on relaxation of residual resistance after the action of a magnetic field also confirm this conclusion.

Resistive transition process in granular superconductingMgB2films by analysis and simulation of the generated current noise

An original model for the interpretation of the noise produced during the resistive transition of disordered granular $\mathrm{Mg}{\mathrm{B}}_{2}$ superconductive films is presented and tested by comparison with extended experiments on these types of films. Both the amplitude and frequency behavior of the noise power spectrum, simulated on the basis of this model, are in very good agreement with the experimental results, practically without the introduction of adjustable parameters. The model is based on the onset of correlated transitions of large sets of grains, forming resistive layers through the film cross-section area during the transition process. The strong nonlinear behavior and correlation of the grains produces abrupt resistance variations, giving rise to the large noise, of the $1∕{f}^{3}$ type, observed in experiments. These results show that, even in the case where the grain interface behaves as a strong link, as in $\mathrm{Mg}{\mathrm{B}}_{2}$, if the grains are randomly oriented and are strongly anisotropic in regard to their critical current density, the transition models based on fluxoids depinning and motion, used for single crystals or grain oriented films, cannot be applied, at least when the transition takes place near the critical temperature at low bias currents. These models would justify a current noise power spectrum 2 or 3 orders of magnitude lower than the observed one in the low frequency range. Since the intrinsic superconductive properties of the material do not enter in its development, it is believed that this model may be generally applicable to disordered granular high-temperature superconductors, at least when the conditions of temperature and bias current specified above are met.

Effect of an external magnetic field and a trapped magnetic flux on the current-voltage characteristics of a granular high-temperature superconductor YBa2Cu3O7−δ

A comparative study of the current-voltage characteristics of the high-temperature ceramic superconductor YBa2Cu3O∼6.95 at T = 77.3 K is performed over wide ranges of external magnetic fields H ext and “treatment” fields H treat. It is found that the field dependences of the parameters a and j c involved in the exponential equation E = a(j − j c)v describing the current-voltage characteristics depend substantially on the method used for applying the magnetic field, whereas the exponent v ∼ 2 depends on neither the method of application nor on the magnetic field strength. The field dependence of the trapped magnetic field H trap is determined.

Coexistence of superconductivity and magnetism, crystallographic coupling and chemical stability in Sr 2FeMnO 6/YBa 2Cu 3O 7− δ composites

We report a crystallographic and magnetic characterization of the Sr 2FeMnO 6 novel polycrystalline complex manganite produced by the ceramic method. This material was mixed in volume percentages from 0% to 100% with the granular high temperature superconductor YBa 2Cu 3O 7− δ to conform Sr 2FeMnO 6/YBa 2Cu 3O 7− δ composites. Studies of chemical stability reveal that the Sr 2FeMnO 6 does not react with the YBa 2Cu 3O 7− δ superconductor. X-ray results show an excellent coupling between the respective lattice parameters. DC susceptibility measurements indicate that the superconductivity of the YBa 2Cu 3O 7− δ and the exotic magnetic properties of the Sr 2FeMnO 6 coexist up to 80% volume of this material in the composite. These characteristics permit to infer that these compounds would be utilized to the production of antiferromagnetic-superconductor multi-layer devices.

Critical current density versus normal-state resistivity in granularhigh-temperature superconductors with different average grain size

The resistivity, ρ, and the critical current density at zeroapplied magnetic field, Jc, of ceramic Y1Ba2Cu3O7-δ samples with different average grain size, a (from 5 to14 µm), are reported. These results show that at a given temperatureJc depends only on the specific contact resistance of the average grainboundary junction, ρ□, so that the product Jcρ□is sample independent. The relevance of ρ□ and not of a onJc has been further tested on a subset of samples having very differentaverage grain size but similar ρ□ for which negligiblevariations of Jc are observed.

Low-field electrodynamics of granular high-temperature superconductors

Studies of the low-field electrodynamics of YBaCuO ceramic superconductors are described. An analytical treatment and a numerical simulation are carried out in the model of pinning and viscous flow of hypervortices in high-Tc superconductors (HTSCs) to find the dependence of the real and imaginary components of the surface impedance on the modulus of a static magnetic field and on the amplitude and frequency of an alternating field. Experimental results are presented on the magnetic-field dependence of the impedance components. The studies were carried out in the frequency range from 30 Hz to 1 kHz and at amplitudes of the alternating component of the field from 0.1 to 10 Oe. It is shown that the critical-state model is applicable to HTSCs in the region of low frequencies and small amplitudes of the alternating component of the field. The model of viscous flow of hypervortices gives a good description of the low-field electrodynamics of HTSCs over a wide range of frequencies and, in particular, predicts the experimentally observed hysteresis of the magnetic-field dependence of the surface impedance.

Transport critical current in granular high-temperature superconductors

The temperature and size dependences of critical current in three Bi-based ceramic HTSC samples with a circular cross section and in a sample with a right triangular cross section in zero magnetic field are studied by a contactless technique. It is shown that the critical current of ceramic HTSC can be presented as the product of the temperature- and size-dependent factors. The temperature-dependent factor describes individual properties of the Josephson net of each sample, while the size-dependent factor is a homogeneous function whose exponent does not depend on the shape of the sample cross section. An analysis of experimental data is used to find the radial distribution of critical current density in round samples and to determine its dependence on the magnetic induction in granular HTSC.

QUANTUM JOSEPHSON RELATIONS FOR MACROSCOPIC JOSEPHSON SYSTEMS

The quantum version of the Feynman two-state model in describing the Josephson systems is shown to be equivalent to a quantum two-mode coupled oscillator. Such equivalence permits us to treat the Josephson systems in a fully quantum mechanical manner in the two-state model and to reveal secondary macroscopic quantum effects therein. The noncommuting nature of macroscopic variables is crucial in doing so. An interesting application of the present investigation into granular superconductor and high-temperature superconductor is briefly addressed.

Crossing point of the magnetization versus temperature curves and the Meissner fraction in granularLa1.9Sr0.1CuO4superconductors: Random orientation and inhomogeneity effects

In this paper, we first calculate the extrinsic random orientation effects on the Meissner fraction in granular layered superconductors with highly anisotropic single crystal grains. Then we present detailed measurements of the crossing point of the excess magnetization versus temperature curves, (Formula presented) in the reversible mixed state and of the Meissner fraction in two granular (Formula presented) samples, before and after grain alignment. The analyses of these experiments on the grounds of the calculations indicated above allow us to disentangle the different extrinsic effects on the excess magnetization at the crossing point, (Formula presented) and on the field-cooled susceptibility at the crossing point temperature, (Formula presented) random orientation, demagnetization, and inhomogeneities. We show then that once these two measured observables are adequately and separately corrected from random orientation and demagnetizing effects, the remaining extrinsic effects on (Formula presented) (associated with structural and, mainly, stoichiometric inhomogeneities) may be taken into account through (Formula presented) This seems to apply at least in samples with relatively good magnetic response [let us say, with (Formula presented) The resulting intrinsic crossing point may be explained in terms of the theoretical approaches proposed by Bulaevskii and co-workers and by Tesanovic and co-workers and based on thermal fluctuations of vortices. These last results extend to granular high-temperature superconductors (HTSC) our recent conclusions for highly anisotropic single crystalline HTSC. © 1999 The American Physical Society.

Analysis of the excess resistivity induced by a low magnetic field in YBCO and BSCCO compounds

The results of a (weak) DC magnetic field induced excess resistivity in granular high-temperature superconductors are analyzed. Assuming a random mixture of superconducting and normal regions, the two characteristic excess resistivity peaks are interpreted as originating from two different mechanisms: the higher temperature peak in excess resistivity is attributed to a distribution of the critical temperature in individual grains affected by the applied magnetic field, this results in a quasi-percolative normal current energy dissipation. The lower temperature wide peak is attributed to an interplay between strong pinning due to the spatial inhomogeneity of the condensation energy on one hand and to the onset of proximity effect with decreasing temperature, on the other. The results obtained with various concentrations of Ag admixture and under varying magnetic field are consistent with the predictions of this model.

A granular high-temperature superconductive film in the field of a magnetostatic surface wave

The reflection of a magnetostatic surface wave from a strip of a granular high-temperature superconductor is investigated. An expression obtained by the method of perturbations describes the field of the scattered wave. The coefficient of reflection of the MSSW from the HTSC strip is calculated as a function of frequency and temperature. It is shown that the reflectance is very low at temperatures above critical and that the HTSC ceramic behaves as an ordinary superconductor. The reflectance rises sharply when the temperature drops below critical. As the critical current decreases, the reflectance drops off and the film loses the high-temperature properties. An expression is derived for the loss resistance of the MSSW in a granular HTSC film. The loss resistance amounts to a few kiloohms and falls off sharply as the film transitions to the superconductive state. The results can also be developed for periodic HTSC structures and used in the design of frequency-selective devices, as well as high-speed bolometric photodetectors.

Low field magnetic response of the granular superconductor La1.8Sr0.2CuO4

The properties of the low excitation field magnetic response of the granular high-temperature (HT c) superconductor La1.8Sr0.2CuO4 have been analyzed at low temperatures. The response of the Josephson currents has been derived from the data. It is shown that intergrain current response is fully irreversible, producing a shielding response, but that it does not carry Meissner magnetization. Analysis of the data shows that the system of Josephson currents freezes into a glassy state even in the absence of external magnetic field, which is argued to be a consequence of the d-wave nature of superconductivity in La1.8Sr0.2CuO4. The macroscopic diamagnetic response to very weak variations of the magnetic field is shown to be strongly irreversible, but still qualitatively different from any previously known kind of critical-state behavior in superconductors. A phenomenological description of these data is given in terms of a newly proposed fractal model of irreversibility in superconductors.

Anisotropy and structural-defect contributions to percolative conduction in granular copper oxide superconductors

A well-defined temperature-independent slope of the current-voltage characteristics (CVC) is a common feature of granular high-temperature superconductors in the paracoherent state (grains superconducting; intergranular regions normal). By analyzing the contributions of anisotropy and structural defects to percolative conduction processes both in the normal state and in the paracoherent state, we quantitatively account for the observed CVC slopes using only the normal-state resistivity values. In particular, from effective-medium theory it is found that the CVC slope of nontextured granular YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) should be equal to approximately one-third of the normal-state resistivity extrapolated to zero temperature. Simultaneous measurements of the CVC and normal-state resistivity on a batch of granular YBCO samples are also presented that verify our predictions with no free parameters. {copyright} {ital 1997} {ital The American Physical Society}

Surface acoustic waves in a piezo-electric with a resonant high temperature superconductor planar structure

We have analyzed the propagation of SAW in a periodic planar structure made from a granular high-temperature superconductor formed on the surface of a piezoelectric material. A number of essential features in the properties of the SAW were revealed. In the region of the N-S transition in a band where the Josephson current at the intergranular junctions of the superconductor is dominant the frequency and temperature dependences of the SAW parameters can have a resonant character. An estimate showed that the resonant damping peak can reach 100 dB/cm while the shift ΔV/V of the SAW velocities in the resonance region exceeds 10−2. The sharp temperature dependences of these parameters allow one to use the effect to make bolometric acoustoelectronic photodetectors and other transducers.