Intergrain Effects in the AC Susceptibility of?Polycrystalline LaFeAsO0.94F0.06

The AC susceptibility, ??, at zero DC magnetic field of a polycrystalline sample of LaFeAsO_0.94F_0.06 (T _c ??24?K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range H _ac =0.003?Oe??4?Oe) and the frequency (in the range f=10?kHz??100?kHz). The ??(T) curve exhibits the typical two-step transition arising from the combined response of superconducting grains and intergranular weak-coupled medium. The intergranular part of ?? strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T _c down to T=4.2?K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.     

Dynamic Instabilities and Current-Voltage Curves of Polycrystalline Superconducting Y1Ba2Cu3O7−x /Ag

We performed systematic current-voltage measurements (I–V curves) in Ag—added the polycrystalline Y₁Ba₂Cu₃O_7?x sample (YBCO/Ag) as a function of the transport current (I), temperature (T), external magnetic field (H), and sweeping rates (dI/dt). Standard and reverse procedures were used in the measurements of I–V curves. The obtained results were compared to I–V curves of the YBCO and polycrystalline sample of MgB₂. Upon cycling transport current, the I–V curves of the YBCO/Ag sample exhibit hysteresis effects for both procedures and are sensitive to the variation of dI/dt. The experimental data reveal that the irreversibilities in the I–V curves of YBCO are more prominent than those of YBCO/Ag. Furthermore, there are no considerable hysteresis effects in the I–V curve of MgB₂ and this behavior is attributed to the absence of the weak-link structure in MgB₂. We suggest that the physical origin of the rather small hysteresis effects in the I–V curves of the YBCO/Ag sample can be related to the destruction of weak-link structure due to the addition of Ag into the superconducting matrix. Ag destroys partly the intergranular pinning properties of the YBCO ceramic by increasing grain coupling and, therefore, the irreversibilities in the I–V curves of the YBCO/Ag decrease considerably. The instabilities and short- and long-lived plateau regions observed in I–V curves were explained in terms of plastic flow of flux lines along easy motion channels, which are considered mainly as metallic silver paths in the YBCO/Ag sample. At moderate dissipation levels, we could not observe any difference between the data obtained by using the standard and reverse procedures. This suggests that the surface effects are also weakened by the addition of Ag into the superconducting structure.     

AC Susceptibility Study of Superconducting YBa2Cu3O7:Ag x Bulk Composites (x=0.0–0.20): The Role of Intra and Intergranular Coupling

We report the effect of silver addition on superconducting performance of bulk YBCO (YBa₂Cu₃O₇) superconductor. All the studied samples are prepared by conventional solid-state reaction method. Rietveld fitted X-ray diffraction data confirmed the single phase formation for all the studied samples. Detailed AC susceptibility measurements as a function of driven AC amplitude (1 Oe–17 Oe) of these samples revealed the enhancement of grains coupling with increasing Ag content in YBCO + Ag _x composite system. 10 wt% Ag added YBCO superconductors exhibited the optimal intergranular coupling. The Scanning Electron Microscopy (SEM) observations indicate an increase in the grains connectivity in terms of narrow grain boundaries for doped samples. The average grain size is found to increase with Ag doping. It is concluded that limited addition of Ag in bulk YBCO superconductor significantly improves the grains coupling and, as a result, the optimal superconducting performance. YBCO + Ag composites could prove to be potential candidates for bulk superconducting applications of the studied high-T _c system.     

Magnetovoltage Measurements and Hysteresis Effects in Polycrystalline Superconducting Y1Ba2Cu3O7−x /Ag in Weak Magnetic Fields

The magnetovoltage measurements (V–H curves) with different sweeping rates (dH/dt) of the external magnetic field in Ag-added polycrystalline YBa₂Cu₃O_7?x sample (YBCO/Ag) were investigated. The measurements of V–H curves were carried out as functions of the transport current (I) and temperature (T). Upon cycling H, all V–H curves measured for different values of I exhibit a clockwise hysteresis effects in forward region. The hysteresis effects in the V–H curve were interpreted in terms of two-level magnetic system, which considers the superposition of the external magnetic field and the local magnetic fields in the intergrain boundaries induced by magnetic dipole moment of neighbor superconducting grains. The analysis of magnetovoltage data showed that the flux trapping in the junction network has a negligible effect on the evolution of the V–H curves and the irreversibilities arise mainly from the flux trapping inside the grains. It is shown that the width of V–H curves shows a universal scaling behavior with respect to the applied current below the critical temperature T _c . The comparison of V–H curves of the YBCO/Ag sample with those of YBCO shows that adding Ag to the superconducting structure weakens the pinning properties of Josephson medium and provokes the instabilities in measured dissipation. The presence of Ag in the superconducting matrix causes marked decrease in hysteresis effects and makes the V–H curves dH/dt dependent. At high values of dH/dt, the instabilities and plateau regions in V–H curves increase significantly as compared to those of observed in YBCO. In addition, the interrelation between the evolution of V–H curves obtained for different values of I and the critical current I _c was demonstrated experimentally.     

Improvement of Current Density of Different Atmosphere-Sintered Y358 Superconductors

Y₃Ba₅Cu₈O₁₈ (Y358) which is the last family member of YBCO superconducting material was produced by a solid-state reaction technique. During the production process, superconducting samples were sintered in different atmosphere conditions. XRD measurements of the samples were performed. As a result of the measurements, it was seen that the superconducting phase peaks were formed. Hysteresis loops were calculated and analyzed in the framework of a critical state model including the Meissner current. Meissner currents (I _M) were calculated as 0.0062, 0.0037, and 0.0119 for S100, S50, and S0, respectively. It was observed that S50 has the highest critical current density among the other calculated values.     

AC Losses in YBCO-Added Bi<Subscript>1.6</Subscript>Pb<Subscript>0.4</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductors

The effect of YBCO adding on the superconducting of BPSCCO system with nominal starting composition of (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O _δ )_1?x(YBCO) _x with x = 0.00?0.10 was studied. The preparation methods used to prepare the samples are the conventional solid-state techniques. From the XRD data, it is observed that the percentage of the Bi-2223 phase increases and Bi-2212 decreases for addition x = 0.02–0.04, respectively. The effect of YBCO addition on the BPSCCO system also has been investigated in term of AC susceptibility study. We have studied the various applied field dependence of the AC susceptibility in polycrystalline samples. The AC susceptibility measurements in the range 0.05 to 2.00 Oe show that as the applied fields increases, the intergranular AC loss peaks move to lower temperature, and also height decreases and width increases. The effect of YBCO addition for the intergranular critical current J _cm, and the presence of weak links that coupled the superconducting grains were defined.     

Experimental evaluation of the nonlinear surface resistance of YBa2Cu3O7−x step-edge grain boundary Josephson junctions

The temperature, microwave current, and dc magnetic field dependencies of the effective surface resistanceR _s of YBa₂Cu₃O_7?x grain boundary Josephson junctions have been investigated at 6 GHz. The junctions were prepared on stepped LaAlO₃ substrates and incorporated into tapered linear half-wavelength microstrip resonators. The characteristic parameters of the heterogeneous resonator were analyzed theoretically considering the junction as a lumped element of complex impedance. The microwave properties of the junction could then be extracted and related to their characteristic dc transport properties. The critical currentI _c of the junction was found to limit the linear power dissipation in the superconducting strip. At microwave currents aboveI _c , highly nonlinear microwave losses occurred, which displayed a characteristic magnetic field modulation. At even higher currents,R _s saturated at a level corresponding to the normal resistanceR _n of the junction.     

Low Temperature Properties and Superconductivity of LuB12

We have investigated the low temperature properties of LuB₁₂ by measuring its magnetic susceptibility, heat capacity, and electrical resistivity, as well as by point-contact spectroscopy using both the spear-anvil type technique and mechanically controllable break junctions. Our specific heat measurements and point - contact spectroscopy results indicate that LuB₁₂ is a simple weak-coupling BCS-type superconductor with T_C ≈ 0.4 K, a superconducting energy gap of 2 Δ ≈ 0.12 meV, and a very small critical field B_C ≈ 1 mT. From the dU/dI(U) characteristics in the superconducting state, the energy gap 2Δ, the critical current I_C and the Andreev-reflection excess current I_ex as a function of normal-state point contact resistance R_N have been determined. At low R_N all three parameters are strongly suppressed, possibly due to the self-magnetic field. At contacts with large R_N the model of resistively and capacitively shunted Josephson junctions (RCSJ) allowed us to estimate the superconducting current plasma frequency and the Josephson coupling energy. Moreover, from the d²U/d I²(U) spectra in the normal state the (point-contact) electron-phonon interaction function and the characteristic phonon energies of LuB₁₂ have been determined.     

Magnetic-field-induced non-linear effects of Josephson coupled superconductor Bi2Sr2CaCu2O8+δ

The dc current-voltage (I-V) relation along the c-axis of single crystalline Bi₂Sr₂CaCu₂O_8+δ has been measured in magnetic fields parallel and perpendicular to the c-axis. In zero field a clear and sharp jump with large hysteresis in theI-V curve was observed, indicative of the dc-Josephson effect. In magnetic field below a characteristic fieldB ^*≈0.4 T (at T=0 K) parallel to the c-axis the magnetic field suppresses the hysteresis and reduces the critical currentI _c drastically, whereas aboveB ^* theI-V curve becomes broad and featureless behavior. The characteristic field scaleB ^* can be interpreted as an energy scale of the Josephson coupling between superconducting layers in Bi₂Sr₂CaCu₂O_8+δ and is argued with emphasis on the correlation length of pancake vortices in this system.     

Hysteretic Critical State in Coplanar Josephson Junction with Monolayer Graphene Barrier

Coplanar Al/graphene/Al junctions fabricated on the same graphene sheet deposited on silicon carbide (SiC), show robust Josephson coupling at subKelvin temperature, when the separations between the electrodes is below 400 nm. Remarkably, a hysteretic Critical State sets in when ramping an orthogonal magnetic field, with a sudden collapse of the Josephson critical current I _c when turning the field on, and a revival of I _c when inverting the sweep. Similar hysteresis can be found in granular superconducting films which may undergo the Berezinskii-Kosterlitz-Thouless transition. Here, we give quantitative arguments to prove that this odd behavior of the magnetoconductance gives evidence for an incipient Berezinskii-Kosterlitz-Thouless transition with drift and pinning of fluctuating free vortices induced by the current bias.     

Critical current in YBa2Cu3O7 ceramics

Measurements of critical currents using a four-point direct current (dc) method and an alternating magnetic field method have been performed on several superconducting YBa₂Cu₃O₇ ceramics at 77 K. In the presence of a constant magnetic field, the critical currents obtained with the alternating field method are several orders of magnitude larger than the critical currents measured by the dc method. Also, we observed a minimum in the dc critical current as a function of applied transverse magnetic field. Several authors have suggested that these ceramics behave as individual superconducting grains coupled by Josephson junctions. In this paper, we explain the two observations above using that model.     

Quantum Oscillations and π-States in Multiply Connected Ferromagnet-Superconductor Hybrids

On the basis of Usadel equations, we consider superconductivity nucleation and Josephson current in multiply connected mesoscopic superconductor/ferromagnet (S/F) hybrids. We demonstrate that the exchange field can provoke an increase in the critical temperature T _c of the superconducting transition in the magnetic field. We study the Josephson effect in S/F composites and demonstrate that the negative sign of the critical current (π state) can be realized in such structures despite a dispersion of the distances between different segments of superconducting electrodes.     

Modelling the Resistive State in a Transition Edge Sensor

We have developed a model for the resistive transition in a transition edge sensor (TES) based on the model of a resistively shunted junction, taking into account phase-slips of a superconducting system across the barriers of the tilted washing board potential. We obtained analytical expressions for the resistance of the TES, R(T,I), and its partial logarithmic derivatives α _I and β _I as functions of temperature and current. We have shown that all the major parameters describing the resistive state of the TES are determined by the dependence on temperature of the Josephson critical current, rather than by intrinsic properties of the S-N transition. The complex impedance of a pristine TES exhibits two-pole behaviour due to its own intrinsic reactance.     

Superconducting current in a thin film of poly(phthalidylidene biphenylene)

We report on the Josephson oscillations observed in a weak transverse magnetic field applied to a superconductor-poly(phthalidylidene biphenylene)-superconductor structure. The weak coupling is provided by point contacts at the polymer-superconductor interface. It is experimentally established that a characteristic scale of the Josephson oscillations depends on the I/I_c ratio, where I_c is the critical current.     

What does instrinsic Josephson coupling say about the pairing symmetry in the cuprates?

Measurements of the c-axis properties of the cuprate superconductors show anomalous behavior in both normal and superconducting states. In particular, there is strong evidence that pairs of CuO₂ planes in neighboring unit cells act as Josephson junctions below the critical temperatureT _c. We present a theory based on incoherent transport along the c-axis which naturally reproduces the anisotropic normal-state resistivity and the superconducting-state Josephson coupling. Applying this theory to YBa₂Cu₃O₇- (YBCO), we make quantitative predictions for the strength and temperature dependence of the Josephson coupling as well as the variation ofT _c with disorder. Beyond the expected low-temperature behavior, the Josephson critical current does not make a clean separation between s- and d-wave superconductors, but the disorder-inducedT _c variation does. Further experimental and theoretical work along these lines may therefore help determine the order parameter symmetry in the cuprates.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

Influence of Interfacial LSMO Nanoparticles/Layer on the Vortex Pinning Properties of YBCO Thin Film

YBa₂Cu₃O_7?δ (YBCO) thin films have been deposited on bare and La_0.67Sr_0.33MnO₃ (LSMO) modified single crystal SrTiO₃ (STO) substrates. The effect of randomly distributed ferromagnetic LSMO nanoparticles and a complete LSMO layer, present at STO/YBCO interface, on the superconducting properties of YBCO thin films has been investigated by temperature dependent magnetization studies. The YBCO thin film on LSMO nanoparticles decorated STO substrate shows significant improvement in the critical current density and pinning force density as compared to the YBCO thin film deposited on bare STO substrate and this improvement is more significant at higher applied magnetic field. However, the LSMO/YBCO bilayer showed the improved flux pinning properties only up to a magnetic field of 1.5 T above which it deteriorates. In the case of LSMO/YBCO bilayer, the underlying LSMO layer gives rise to magnetic inhomogeneities due to domain structure, which leads to improved flux pinning properties limited to lower field. However, in the case of LSMO nanoparticles decorated substrate, the presence of LSMO nanoparticles at YBCO/STO interface seems to introduce magnetic inhomogeneities as well as structural defects, which might be acting as correlated pinning sites leading to improved flux pinning properties of the YBCO thin film over a wide range of applied magnetic field.     

Magnetic and transport critical current density of laser-irradiated Sm-Ba-Cu-O superconductor at 4.2 K

The low temperature (4.2 K) magnetic and transport critical current density of laser-irradiated (Q-switched ruby laser, 694 nm, 30 ns) Sm-Ba-Cu-O ceramic superconductors prepared by the coprecipitation technique have been investigated. Laser irradiation did not significantly change the structural parameter and the critical transition temperatureT _c but caused an appreciable increase in magnetic critical current densityJ _mc and transport critical current densityJ _tc . Inverse a.c. Josephson effect studies at 77 K showed a sharp decrease of microwave-induced d.c. voltage after laser irradiation. SEM studies revealed partial melting at grain boundaries and grain growth due to sintering which improves the interconnectivity in the network of superconducting grain structures after laser irradiation. These phenomenon are attributed to physical densification and consequent reduction in the total number of weak links between the superconducting grains. The significant increase ofJ _mc andJ _tc after laser irradiation is presumably connected with the creation of irradiation-induced mobile defects which act as pinning centers and, hence, stronger Josephson current paths between the superconducting intergrains.