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The theoretical and experimental studies of the main physical parameters in the case of superconductivity of a metallic friction element, which is under the unilateral action of thermal, electric and magnetic fields, made it possible to establish the following. The energy of an electron in a crystal lattice cannot take on any values due to the fact that there are sets of discrete energy levels (zones) separated by more or less significant distances. Heat capacity and thermal conductivity have components that relate to electrons and the crystal lattice; the heat capacity in the state of superconductivity turned out to be less than the heat capacity of a normal metal, and the heat conductivity increased due to the local interaction of microprotrusions of friction pairs. In the surface layer of the metal under the action of a magnetic field, a stationary electric current arises, which has its own magnetic field opposite to the applied field, which, as a result, leads to a zero value of induction in the surface layer of the metal. At a critical value of the magnetic field strength, the subsurface layer of the metal friction element goes into a normal state. [ABSTRACT FROM AUTHOR]

In this work, the resistive state of a mesoscopic superconducting sample in presence of an external transport electric current, Ja, and at zero magnetic field was studied. A non-centered channel of depleted superconductivity (of width m and at lower critical temperature Tc) is positioned at a distance d of the center of the sample. We vary the width a of the metallic contact in which the external current is applied. We study the superconducting electronic density, the vortex-antivortex velocity, and the annihilation rates of the supercurrent for several widths of the metallics contacts and position of the channel. Our investigation show that critical currents and the velocity of the v-Av of the studied system depend strongly on the size of the contact and of the position of the channel in the sample. [ABSTRACT FROM AUTHOR]

Because charge, current, resistance, and voltage are understood based on the perspective of electricity, they can describe various electrical phenomena well, but they cannot explain their origins. Therefore, this article attempts to propose a new understanding of these phenomena from the perspective of mechanics to obtain a feasible way of explaining their origin and to solve problems that cannot be explained from the perspective of electricity. Specifically, this paper attempts to explain the origin of electric charge to obtain a new understanding of the origin of current and to obtain a new understanding of voltage and resistance by explaining the origin of current. Finally, from the perspective of mechanics, the origin of charge can be understood as a manifestation of electron momentum, the origin of current can be understood as a momentum flow, the origin of resistance can be understood as a momentum resistance, and the origin of voltage can be understood as a potential pressure (potential difference) of electron orbital potential. This new understanding of the origin of current from a mechanical perspective can provide a new theoretical explanation for high-temperature superconductivity. [ABSTRACT FROM AUTHOR]

To understand the spin-torque effect on the noise in tunneling magnetoresistive (TMR) read heads, the GHz range noise spectra of TMR read heads with a narrow track width (w=36 nm), and various stripe heights (h) are investigated as a function of the external magnetic field (Hex) and dc bias current density (j). The strong noise peak intensity depends on both Hex and j, indicating that the spin-torque affects the thermal mag-noise under a positive (negative) j for a positive (negative) Hex, regardless of h in the TMR heads. Due to the increased shape anisotropy, the critical current density (jc), where the non-thermal fluctuation noise originates from the spin-torque, increases markedly as the head dimension is reduced, and the maximum value of jc is approximately +1.5 × 1012 A/m² for a head with w=36 nm and h=15 nm. These results demonstrate that the non-thermal fluctuation noise originating from the spin-torque in the TMR head can be suppressed in the current density range below 1012 A/m², as the head dimension is reduced and the shape anisotropy is increased. [ABSTRACT FROM AUTHOR]

In this work, the central cross crack problem is investigated for a long cylindrical high-temperature superconductor under electromagnetic forces. The distributions of both the current density and the magnetic flux density in the cylinder are obtained analytically in either the Bean or the Kim critical state model for both the zero-field cooling and the field cooling magnetization processes. Based on the finite element method, lots of the stress intensity factors at the crack tips for decreasing magnetic fields are numerically calculated. Numerical results obtained show that the field cooling activation process has more significant influence on the stress intensity factors than the zero-field cooling activation process, and that for every activation process, as the applied field decreases, the superconducting cylinder is most dangerous when the currents in the crack region are just be influenced. The safety of the superconducting cylinder with a cross crack depends on not only the magnetic process but also the crack length. Additionally, the introduced dimensionless parameter p in the Kim model has different effects on the stress intensity factors obtained from the most dangerous field and from the trapped field. The present study should be helpful to the design and application of high-temperature superconductors. [ABSTRACT FROM AUTHOR]

Sections of the superconducting magnets of the SIS100 particle accelerator, under construction at the Facility for Antiproton and Ion Research (FAIR), the Society for Heavy Ion Research (GSI), Darmstadt, are going to be connected with the by-pass lines. Each line will be used to transfer a two-phase helium flow and an electric current. The electric current will be carried by four pairs of superconducting Nuclotron-type cables. Fast-ramping currents are expected to cause the generation of heat within the cables. In this work the results of a numerical thermal analysis of a bus-bar are presented. The amount of heat transferred from the environment was found based on geometric dimensions of the line and applied insulation. The amount of hysteresis loss, generated in the cable during the operation under most demanding regime of the operation of the accelerator, was calculated. According to the amount of the generated heat, the amount of the hysteresis loss is low in relation to the heat generated in the superconducting magnets. Also it was found that the cable used in the line still retains a large margin of current-carrying capacity. [ABSTRACT FROM AUTHOR]

To evaluate the inhibition of the REBa 2 Cu 3 O 7-δ (REBCO, where RE is rare earth) film thickness effect, a series of Y 0.5 Gd 0.5 Ba 2 Cu 3 O 7-δ (YGBCO)/interlayer/YGBCO tri-layer films with Gd-Cerium oxide (CeO 2) film were prepared under different experimental conditions in this study. The results illustrate that the electric current from the top YGBCO layer could transmit to the bottom YGBCO layer by adding Gd into the CeO 2 interlayer with the approximate thicknesses limits of 10 nm. For further increase the electric current carrying capacity, a series of YGBCO/Gd-CeO 2 /YGBCO tri-layers were fabricated under different oxygen partial pressures and laser energies. The better crystallinity, and texture, and the smoother surface levels completely eliminated the thickness effect. However, the samples' superconductivity declined sharply at low laser energies. Because the Gd particles deposited on the substrate were fully oxidized, this led to particle disappearance in the interlayer film. Additionally, YGBCO samples with a Gd-CeO 2 interlayer were more likely to be damaged than a pure YGBCO sample in the measurement process, which could be solved by increasing the Gd content in the CeO 2 +Gd target. Hence, by depositing a Gd-CeO 2 interlayer, YGBCO tri-layer films with high superconductivity and low damageability were obtained. [ABSTRACT FROM AUTHOR]

A theoretical model is formulated for the description of the nonequilibrium state of a superconductor on the basis of the nonequilibrium distribution function of phonons and quasiparticles on a finite momentum lattice convenient for numerical simulation. The simulation is performed under conditions of pulsed optical impact demonstrating the role of excess quasiparticles in suppressing the order parameter and the main stages of the relaxation process in the absence of electric current. [ABSTRACT FROM AUTHOR]

The thickness dependence of critical current density (Jc) has been investigated in YBa2Cu3O7-x films with BaZrO3 (BZO) nanorods (YBCO/BZO) aligned along the film normal. With the presence of linear defects in the YBCO/BZO films, the thickness dependence of Jc becomes negligible at self field and weak applied magnetic field (H), suggesting these linear defects playing a key role to the elimination of the thickness dependence of Jc. The effect of temperature (T) and H on Jc thickness dependence have been investigated and comparisons have been made between YBCO and YBCO/BZO samples. Since BZO nanorods alignment is greatly influenced by growth temperature, the quantitative difference in terms of nanorod's density and average length is reflected on Jc angular dependence of H and Jc thickness dependence as thermally assisted flux motion (TAFM) becomes important above a threshold H determined by the BZO nanorod density. With further increasing H, a monotonically increasing Jc vs. thickness trend was observed in YBCO/BZO films, in contrast to an opposite trend when collective pinning is dominant. This result suggests the thickness dependence of Jc is dictated by the microstructure and hence pinning mechanism in YBCO films. [ABSTRACT FROM AUTHOR]

Influences of cracking of coating layer under applied tensile strain on V(voltage)-I(current) curve, critical current, and n-value of DyBa2Cu3O7-δ coated conductor were studied experimentally and analytically. The experimentally measured variations in V-I curve, critical current, and n-value with increasing applied strain and the correlation of n-value to critical current were described well by the partial crack-current shunting model of Fang et al. Also, the variations in the ratio of shunting current to overall critical current and the ratio of voltage developed in the cracked region to overall voltage with extension of crack, and the variation in critical current with the ratio of noncracked area to overall cross-sectional area of superconducting layer were revealed. [ABSTRACT FROM AUTHOR]

An approach to predict the variation in critical current with applied tensile/bending strain of Bi2223/Ag/Ag alloy composite tapes from the tensile stress-strain curves was presented. Three different fabrication-route samples were used to examine the applicability of the present approach. The damage strain parameter, referring to the difference between the tensile fracture strain and residual strain of Bi2223 filaments along the sample length direction (current transport direction), was estimated from the variation in the slope of the tensile stress-strain curve. With the estimated damage strain parameter, the irreversible tensile strain for critical current was predicted, which agreed well with the experimental result in all samples. Also by substituting the estimated damage strain parameter into the core shape—incorporated model, the critical current-bending strain curve was predicted, which described satisfactorily the experimental result in all samples. The present approach could be a useful tool for prediction of critical current-dependence on applied tensile/bending strain. [ABSTRACT FROM AUTHOR]

The critical current density for spin transfer torque switching with composite free layer structures is investigated using the macrospin Landau–Lifshitz–Gilbert equation. We consider a magnetic tunneling junction with a rigid fixed layer, and a composite free layer consisting of two coupled ferromagnetic layers in which the coupling is parallel or antiparallel. The dependence of critical current density on thickness, coupling sign and strength, spin torque efficiency, and magnetization of the composite free layer is explored. We determine that reduction in the critical current density can be achieved only through careful design of the composite free layer structures. We show the detailed spin dynamics of the composite free layer when the reduction in the critical current density is accomplished. [ABSTRACT FROM AUTHOR]

We have investigated the intrinsic Josephson properties in slightly underdoped (Hg,Re)Ba2Ca2Cu3Oy [Hg(Re)1223] intrinsic Josephson junctions (IJJs) with a dimension of 1.0×1.5×0.11 μm3. The current-voltage characteristics of the IJJs exhibit clear multiple branches with subgap structures similar to those of other cuprate superconductors. The switching current distributions P(I) from the zero-voltage to the nonzero-voltage state in the current-biased IJJs agree well with the theoretical curves of the thermally assisted escape model at temperatures above ≈5 K. The plasma frequency fp of the IJJs is estimated to be 1.3 THz from the fluctuation-free critical current density of 2.0×105 A/cm2, which is one of the highest among cuprate superconductors, reflecting the high Tc and a relatively low anisotropy of the Re doped Hg system. The P(I) gradually becomes independent of temperature below ≈5 K, which suggests a crossover of the escape process from thermal activation to quantum tunneling at such a high temperature. [ABSTRACT FROM AUTHOR]

Field, temperature, and strain dependences of the critical current for a SiC doped multifilamentary in situ MgB2 wire have been studied. Measurement results were compared with that of the undoped wire, and the origin of the difference in the critical current is discussed. The critical current can be calculated with the percolation model considering the effect of anisotropy. The temperature dependence of the fitting parameters, the upper critical field along the ab-plane, and c-axis is compared with the dirty-limit two-gap theory. To assess the validity of the fitting parameters, resistive transition has been measured especially to extract the upper critical field directly. It is shown that even the resistive broadening can be well explained by a simple parallel path model using the fitting parameters obtained from the critical current analysis. [ABSTRACT FROM AUTHOR]

The critical current density (Jc) of Nb–AlOx–Nb Josephson-junction (JJ) arrays was found to depend on their wiring structure. The Jc values of all JJs wired with a niobium electrode covered with a palladium layer increased by about 20%, while the Jc values of those with electrodes without palladium coverage stayed unchanged (except for that of the two junctions directly connected to the pads of an electrical probe covered with palladium.) To explain this Jc increase, we propose a “hydrogen mechanism,” that is, the hydrogen inclusion into niobium electrodes occurs during fabrication, and its desorption occurs after fabrication. Hydrogen atoms incorporated in the electrodes are thought to influence the mechanical and the electronical properties of niobium, resulting in the deviation of critical current density. Hydrogen desorption analysis and measurements on niobium-film properties verified the occurrence of hydrogen incorporation into the niobium films during the fabrication process for superconducting JJ circuits. The incorporation and desorption processes were confirmed to proceed, even in air, if the niobium film is covered with palladium. As hydrogen diffuses quickly in niobium but stops in aluminum or aluminum oxide, differences in hydrogen concentration can happen within a circuit consisting of electrically connected multiple junctions. This hydrogen concentration difference can explain the observation that two junctions with increased Jc exist in the serial junction array without palladium coverage. [ABSTRACT FROM AUTHOR]

The anisotropy of the critical current density Jc was determined from angular critical current measurements for YBa2Cu3O6+x (YBCO) thin films laser deposited from a nanograined and a BaZrO3-doped nanograined target. In quantitative analysis using Blatter scaling the effective Jc anisotropy was found to be much smaller than for standard YBCO films. [ABSTRACT FROM AUTHOR]

We introduce an approach to the Josephson effect in the superconductor-insulator-superconductor (SIS) tunnel junctions. The Josephson coupling energy is calculated from the overlap of real space Cooper pair wavefunctions in two superconductors through an insulating barrier. It is shown that the Josephson tunneling is limited by the size of the Cooper pair and its shrinking during the tunneling. Therefore, the Josephson coupling energy and the critical current become extremely small in high Tc superconductors, including MgB2. This shrinking also causes the observed direct current (dc) supercurrent in low Tc superconductors, such as Nb, Pb, and Sn, to fall off much faster than 1/Rn for tunneling resistance Rn above a few ohms. Consequently, there is a material-dependent threshold resistance, above which the supercurrent decreases much faster with increasing resistance. Spectacular confirmation is provided by the MgB2 break and tunnel junctions, where only small gap shows the supercurrents, while the big gap does not. The impurity-induced shrinking is also shown to limit the critical current. Furthermore, the (weak) temperature dependence of the Cooper pair size is found to contribute to the temperature dependence of the dc supercurrent. This understanding may lead to the discovery of better materials for SIS junctions other than Nb and the optimum miniaturization of the SIS junctions for the petaflops superconducting supercomputers. [ABSTRACT FROM AUTHOR]

Improvement in the interplane and intergranular coupling achieved by Mg doping at Ca site in high temperature Cu0.5Tl0.5Ba2Ca2(Cu0.5Zn2.5)O10-δ superconductor is studied by infield magnetic measurements. The decrease of c-axis length and volume of the unit cell of Cu0.5Tl0.5Ba2(Ca2-yMgy) (Cu0.5Zn2.5)O10-δ (y=0, 0.5, 1.0, and 1.5) superconductor with the increase of Mg doping shows the improved interplane coupling among CuO2/ZnO2 planes. The presence of Mg at the termination ends of the crystals of Cu0.5Tl0.5Ba2(Ca2-yMgy) (Cu0.5Zn2.5)O10-δ compound has been found to improve intergrain coupling, which is evident from enhanced critical current density (Jc) of these samples. The exponent of temperature variation of thermodynamic critical field Hc∼(1-TP/Tc)n suggests that the weak links form superconductor-insulator-superconductor type of junctions in all the samples. The presence of insulating material at the intergrain sites possibly increases the area of shielding current flow, which promotes the enhancement in Jc of final compound. [ABSTRACT FROM AUTHOR]

The author calculates precisely the induced currents in striated YBa2Cu3O7 coated conductors covered partially by segmented pieces of normal metal, connecting the individual superconducting stripes. It is shown that the induced current density is below the critical current density of the superconducting stripes and the coupling losses are acceptable low, ensuring in addition sufficient current sharing between the stripes. Thus, the proposed structure can be applicable at ac conditions without twisting procedure. The ratio of the current closed between the normal plates to the current flowing in the superconducting stripes and the losses in the substrate are also discussed. The total current flowing through the whole length of the superconductor, which has to be closed at the ends, is also acceptable low. In addition, the frequency range, in which the magnetic flux can fully penetrate both between the stripes and into the normal segments, is given. [ABSTRACT FROM AUTHOR]

[100]-tilt grain boundary Josephson junctions with misorientation of the c-axis directions have been fabricated using epitaxial (Hg0.9Re0.1)Ba2CaCu2Oy [(Hg,Re)-1212] thin films grown on SrTiO3 bicrystal substrates. Two types of the [100]-tilt junctions, that is, the mountain-type junction and the valley-type junction, of which a-b planes formed a mountain shape and a valley shape along the grain boundaries, respectively, were examined for various misorientation angles (θ). Both the types of junctions showed resistively shunted junction-type current-voltage (I-V) characteristics with relatively high characteristic voltages in a wide temperature range from 4.2 to 110 K. The valley-type junctions exhibited properties superior to those for the mountain-type junctions, such as very low excess current and much homogeneous current distribution, which is attributed to different film growth mode near the grain boundaries. They also exhibited the products of the critical current (Ic) and the normal-state resistance (Rn) as high as 3.0–4.2, 1.0–1.4, and 0.2–0.4 mV at 4.2, 77, and 100 K, respectively. The misorientation angle dependences of junction parameters for the [100]-tilt junctions were compared with those for conventional [001]-tilt junctions with misorientation of the a-b plane directions. The Rn and the IcRn products for the [100]-tilt junctions were substantially larger than those for the [001]-tilt junctions. Large hysteresis in the magnetic-field dependence of Ic was observed for the applied field larger than a certain threshold value, suggesting flux penetration between the CuO2 planes. [ABSTRACT FROM AUTHOR]

We report on the synthesis and performance of 3- and 4-μm-thick YBa2Cu3O7 films on buffered metallic tapes. The precursor films were deposited by vacuum coevaporation of BaF2, Y, and Cu on the substrates and converted to YBa2Cu3O7 by the BaF2 ex situ process at reduced processing gas pressures. The best value of critical-current density Jc for these films was ∼3.8×103 A/mm2 at 77 K and in 1 T external magnetic field perpendicular to the film plane. Also, estimated critical-current densities per width of tape Jcw at zero magnetic field were ∼60 and ∼80 A/mm for 3- and 4-μm-thick films, respectively. In order to achieve these high-Jc values, the films were processed at high growth rates (∼0.7 nm/s) and the oxygen partial pressure p(O2) was varied to minimize the growth of the “granular” c axis and randomly oriented YBa2Cu3O7 grains. A simple thermodynamic argument is also given to describe the observed dependence of the nucleation of YBa2Cu3O7 with different orientations on p(O2). This result demonstrates the feasibility of fabricating coated conductors with a single YBa2Cu3O7 layer having the critical current of the order of 100 A/mm at self-field and liquid-N2 temperature. [ABSTRACT FROM AUTHOR]

The diffusion between Nb–20%Ta (wt %) and pure Ti is studied at temperatures of 973, 1023, and 1073 K, for duration times among 25 and 121 h in an artificial pinning center (APC) wire composed of a Ti core surrounded by a Nb–20%Ta layer. The produced diffusion layer is a ternary alloy with superconducting properties, such as critical field Bc2 and critical current density JC, which intrinsically depend on the layer composition. Measurements of layer morphology and composition were performed, and the results show a preferential diffusion of Nb and Ta into Ti. There is a slight diffusion of Ti into Nb through grain boundaries. The presence of Ta also slows down the diffusion of Nb in Ti if compared to the couple formed by pure Nb and Ti. Regarding the mechanical properties of the composite wire, the use of lower temperatures to form the ternary phase is desirable in order to avoid a larger portion of the diffusion layer rich in Ti that favorites α-Ti precipitations that are detrimental to the wire ductility. The best JC value was obtained for the sample heat treated at 973 K. The improvement of the flux-line pinning was associated with a sharp change of the diffusion layer composition rather than pinning by normal layer interfaces, suggesting a new source of pinning in this kind of material. Nb–Ti–Ta ternary alloys have the potential to be used in superconducting magnets when fields above 12 T are required. [ABSTRACT FROM AUTHOR]

Twin boundary energy (γtw) and its temperature dependence are important factors to be evaluated for engineering twin morphology for strong flux pinning and high critical current density in superconducting YBa2Cu3O7-x. Here, it has been measured as a function of oxygenation temperature from 450 to 680 °C using both the twin spacing method and a twin tip method. By comparing the twin energies obtained by the two methods, the geometrical factor α used in twin spacing method is determined. This geometrical factor is directly related to the sharpness of the twin tips. From the temperature dependence of twin boundary energy per unit area, twin boundary entropy (Δs) and enthalpy (Δh) for twin boundaries are projected to be 0.24 mJ/K m2 and 246 mJ/m2, respectively. [ABSTRACT FROM AUTHOR]

Twin boundary energy (γtw) and its temperature dependence are important factors to be evaluated for engineering twin morphology for strong flux pinning and high critical current density in superconducting YBa2Cu3O7-x. Here, it has been measured as a function of oxygenation temperature from 450 to 680 °C using both the twin spacing method and a twin tip method. By comparing the twin energies obtained by the two methods, the geometrical factor α used in twin spacing method is determined. This geometrical factor is directly related to the sharpness of the twin tips. From the temperature dependence of twin boundary energy per unit area, twin boundary entropy (Δs) and enthalpy (Δh) for twin boundaries are projected to be 0.24 mJ/K m2 and 246 mJ/m2, respectively. [ABSTRACT FROM AUTHOR]

Approximately 160-nm-thick (Hg0.9Re0.1)Ba2CaCu2Oy [(Hg,Re)-1212] epitaxial thin films were fabricated on 2°–18.4°-tilt SrTiO3 vicinal substrates with thin buffer layers. The vicinal films had superior crystallinity and superconducting properties comparable to those for conventional films with no tilt. X-ray diffraction measurements revealed that the tilt angles of the c axes of the (Hg,Re)-1212 are slightly larger by 0.5°–1.5° than the substrate tilt angles, which is attributed to deformation of crystal structure due to lattice mismatch in the c-axis direction. The resistivity and critical current density along the c-axis direction (ρc and Jc-c) were estimated to be ρc=500 mΩ cm at room temperature and Jc-c=(1.0–1.4)×105 A/cm2 at 10 K from transport measurements for different in-plane directions of the vicinal films. The ρc clearly exhibited semiconductor-like temperature dependence below 180 K. The temperature dependence of Jc-c almost agreed with the theoretical relationship for superconductor-insulator-superconductor junctions. The current–voltage characteristics in the directions across their a–b planes exhibited a multiple branch structure of intrinsic Josephson junctions with voltage jumps at intervals of approximately 20 mV at 4–40 K. [ABSTRACT FROM AUTHOR]

C-axis-oriented NdBa2Cu3O7-δ thin films (thickness ∼2000 Å) on LaAlO3 substrates have been prepared and irradiated with Ag+14 (200 MeV) ions at different inclinations (θd=0°, 15°, 30°, and 45°) to the c axis. Magnetization-field hysteresis loops have been obtained on pristine and irradiated thin films at two different temperatures in applied magnetic fields up to 5 T. Estimates of the critical current density Jc(H,θ) and the volume pinning force Fp(H,θ) from the magnetization data reveal a strong dependence on track inclination. An increase and a subsequent saturation in Fp(H,θ), instead of the usual Gaussian peak around the equivalent matching field, B[lowercase_phi_synonym], is observed. It is suggested that this dependence of Jc (or Fp) on θd is a consequence of the effective modification of vortex pinning by tilted columnar defects. [ABSTRACT FROM AUTHOR]

The effect of nanoscale-SiC doping of MgB2 was investigated in comparison with undoped, clean-limit, and Mg-vapor-exposed samples using transport and magnetic measurements. It was found that there are two distinguishable but related mechanisms that control the critical current-density-field Jc(H) behavior: increase of upper critical field Hc2 and improvement of flux pinning. There is a clear correlation between the critical temperature Tc, the resistivity ρ, the residual resistivity ratio RRR=R(300 K)/R(40 K), the irreversibility field H*, and the alloying state in the samples. The Hc2 is about the same within the measured field range for both the Mg-vapor-treated and the SiC-doped samples. However, the Jc(H) for the latter is higher than the former in a high-field regime by an order of magnitude. Mg vapor treatment induced intrinsic scattering and contributed to an increase in Hc2. SiC doping, on the other hand, introduced many nanoscale precipitates and disorder at B and Mg sites, provoking an increase of ρ(40 K) from 1 μΩ cm (RRR=15) for the clean-limit sample to 300 μΩ cm (RRR=1.75) for the SiC-doped sample, leading to significant enhancement of both Hc2 and H* with only a minor effect on Tc. Electron energy-loss spectroscope and transmission electron microscope analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, and BC at a scale below 10 nm and an extensive domain structure of 2–4-nm domains in the doped sample, which serve as strong pinning centers. [ABSTRACT FROM AUTHOR]

The effect of nanoscale-SiC doping of MgB2 was investigated in comparison with undoped, clean-limit, and Mg-vapor-exposed samples using transport and magnetic measurements. It was found that there are two distinguishable but related mechanisms that control the critical current-density-field Jc(H) behavior: increase of upper critical field Hc2 and improvement of flux pinning. There is a clear correlation between the critical temperature Tc, the resistivity ρ, the residual resistivity ratio RRR=R(300 K)/R(40 K), the irreversibility field H*, and the alloying state in the samples. The Hc2 is about the same within the measured field range for both the Mg-vapor-treated and the SiC-doped samples. However, the Jc(H) for the latter is higher than the former in a high-field regime by an order of magnitude. Mg vapor treatment induced intrinsic scattering and contributed to an increase in Hc2. SiC doping, on the other hand, introduced many nanoscale precipitates and disorder at B and Mg sites, provoking an increase of ρ(40 K) from 1 μΩ cm (RRR=15) for the clean-limit sample to 300 μΩ cm (RRR=1.75) for the SiC-doped sample, leading to significant enhancement of both Hc2 and H* with only a minor effect on Tc. Electron energy-loss spectroscope and transmission electron microscope analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, and BC at a scale below 10 nm and an extensive domain structure of 2–4-nm domains in the doped sample, which serve as strong pinning centers. [ABSTRACT FROM AUTHOR]

Melt-processed samples of YBa2Cu3O7-δ (YBCO) doped with Li additions substituting at Cu(2) plane sites were used to study the effects of doping on the superconducting temperature, critical current density, irreversibility field, upper critical field, coherence length, and magnetic relaxation as a function of temperature and magnetic field. The intrinsic superconducting properties were found only slightly influenced by Li additions at some optimal concentration (0.6 at. %). At this Li concentration the volume pinning force of doped material is about five times that of undoped YBCO in intermediate magnetic fields at 77 K. No significant change of the maximum volume pinning force with oxygen defects after long-time oxygenation experiment (∼450 h) has been observed. The normalized pinning force of the sample optimally doped with Li obeys the same scaling law as a function of magnetic field and temperature before and after additional heat treatment, indicative of a single type of pinning centers. The time dependence of the magnetization in the standard and in the Li-doped sample has been analyzed in the framework of collective pinning theory. The magnetic relaxation measurements combined with the B-T phase diagram data provide evidence of an elastic-plastic transition. The pinning parameters estimated show a considerable enhancement of the critical current density in the optimally doped sample, but only an insignificant change in the effective activation energy, pointing to a weak pinning of pointlike pinning centers, created by Li additions. [ABSTRACT FROM AUTHOR]

Current injection into superconducting microbridges has been studied for state-of-the-art YBa[sub 2]Cu[sub 3]O[sub 7-x]/SrTiO[sub 3]/Au (YBCO/STO/Au) planar heterostructures based on c-axis oriented YBCO layers. By injecting a gate current I[sub g] into the YBCO channel of these three-terminal devices the critical current I[sub c] could be reduced up to a gain factor G≡-dI[sub c]/dI[sub g]∼5 at 77 K. G is found to be roughly proportional to the superconducting quality of the samples as expressed by the critical current density J[sub c] of the YBCO channel. At least for our high-quality samples, this I[sub c] suppression is shown to be mainly due to the perturbation of the electronic superconductive equilibrium state by quasiparticles associated with the injection of the gate current. However, this basic mechanism for the current-injection-based transistor principle was in our experiments always accompanied by substantial parasitic effects, such as current summation and Joule heating due to electric power dissipation in the ultrathin STO barrier. This reduces the efficiency of the transistor mechanism and thus the hopes for THz operation of such devices. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This article presents information on a study conducted to examine the local transport characteristics in the transverse direction of bismuth strontium calcium copper oxide (BSCCO) tapes. The authors comment on the superconductivity of the BSCCO tapes as well as the anisotropy in the dissipation of the transport currents. Problems related to overheating are also explored.

In this work, we consider a reduced Ginzburg-Landau model in which the magnetic field is neglected and the magnitude of the current density is significantly stronger than that considered in a recent work by the same authors. We prove the existence of a solution which can be obtained by solving a nonconvex minimization problem away from the boundary of the domain. Near the boundary, we show that this solution is essentially one-dimensional. We also establish some linear stability results for a simplified, one-dimensional version of the original problem. [ABSTRACT FROM AUTHOR]

The main purpose of this article is to show an easier and more accurate method for analyzing high-field pinning centers. Previous methods did not allow for precise analysis and research on high-field pinning centers, they studied only the dominant pinning mechanism, and they required a maximum of pinning force (Fpmax) to analyze pinning centers. The main advantage of this method can be used in samples without the maximum pinning force required by other methods to analyze the pinning mechanism. In addition, this method allows us to observe even small changes in the density of individual pinning centers. This method is based on analysis of critical current density values in different ranges of reduced magnetic field. The method of analysis of high-field pinning centers allows primarily to develop methods and directions for creation of high-field pinning centers. The surface pinning centers are strong pinning centers in low magnetic fields and very weak pinning centers in middle and high magnetic fields. Additionally, the analysis of pinning centers shows that dislocations, strains, substitutions in the crystal lattice, and precipitation inside the grains create strong pinning centers in high magnetic fields and very weak pinning centers in low and middle magnetic fields. Research indicates that strains, substitutions in the crystal lattice, and precipitates inside grains form pinning centers that operate in the high magnetic field range. [ABSTRACT FROM AUTHOR]

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The critical current density Jc is a crucial parameter to establish the actual technological potential of a superconducting (SC) material. Furthermore, being proportional to the SC gap parameter, it can reveal important information about the microscopic nature of the SC state in a given material. The FeAs-based class of SC materials has been a focus of intense scientific investigation lately, but direct investigation of Jc by transport measurements is rather scarce in literature. For these materials, it is very interesting to map Jc as a function of their distinct SC tuning parameters such as applied pressure and chemical substitution. In this work, detailed investigation of the field, temperature, and pressure dependences of transport critical current density Jc for Cu-substituted BaFe2As2 single crystals is reported. In this particular material, Cu-substitution has a strong magnetic pair breaking effect. However, with increasing pressure, this sample shows an almost twofold increase of Tc, from 3.2K to 6.9 K, which is followed by an increase in Jc. These observations are discussed considering the presence of magnetic pinning centers in the Fe-As plane, which, in principle, could suggest effective routes to increase Jc in the this class of materials. [ABSTRACT FROM AUTHOR]