Narrow Results

The flux creep phenomena and the J_c characteristics have been investigated in the YBa₂Cu₃O₇ (YBCO) thin films grown by metal organics chemical vapor deposition (MOCVD) technique. The anisotropy of the current density (J_c) is discussed on the basis of the intrinsic pinning model and the anisotropic critical magnetic field (H_c2). The properties of the high J_c and the pinning energy are related with the fine precipitates observed by tunneling electron microscopy (TEM). The strong pinning mechanism affects the amount of the flux motion.

Temperature dependence of the critical current density j_c(T) for the superconducting magnesium diboride MgB₂ is studied in the vicinity of T_c by using a two-band Ginzburg–Landau (G–L) theory. The result is in good agreement with experimental data for a bulk MgB₂. Comparision of obtained results with single-band GL theory are presented.

A series of superconducting materials having compositional formula, (Nd-Eu-Gd)-Ba-Cu-O (123) + x%(Nd-Eu-Gd)Ba-Cu-O (211) or NEG-123 + NEG-211 (where x ranges from 0–50%) were prepared by the Oxygen Controlled Melt Growth (OCMG) process. After characterizing the materials by studying their microstructure, measuring J_c and T_c values, the ultrasonic longitudinal velocity (V_l) measurements were carried out both as a function of composition (x% of 211 phase) as well as the temperature, over a temperature range 80–300 K. It has been found that the variation of ultrasonic longitudinal velocity is similar to that of J_c with increasing x values (concentration of NEG-211). These results indicate that by dispersing 40% fine NEG-211 particles in the superconducting matrix of NEG-123, both the critical current densities as well as the ultrasonic velocity values are found to exhibit highest values. It is known that the lattice defects of ceramic superconductors are manifested in the form of elastic anomalies, especially in the normal temperature region (100–250 K). In order to understand the complete scenario of elastic anomalies of HTSC materials, a systematic investigation of the ultrasonic velocities of HTSC materials prepared by the OCMG technique has been undertaken and using the results an attempt has been made to generalize the scenario of the elastic anomalies in HTSC materials.

Gd is successfully embedded into YBa₂Cu₃O_7-δ by the solid state reaction. Rietveld refinements show that Gd replaces Cu in the unit cell. By magnetization and resistivity measurements, the effects of Gd-doping in YBCO on the critical current density J_c and the critical temperature T_c in YBa₂Cu₃O_7-δ(YBCO) have been investigated. J_c varies with the applied field and the content of Gd and has a maximum at x = 0.03. The characteristic behavior of J_c with varying the Gd content can be explained in terms of the competition between two different effects caused by Gd-doping in YBCO: the inhomogeneity of grains distribution in nano-scale and the variation of superconductivity.

We evaluated the magnetic alignment and superconducting magnetic anisotropies of high-T_c superconductor Yb₁Ba₂Cu₃O_7-δ by both X-ray diffraction and magnetization measurements. The sample was grain aligned under a high magnetic field of 6 T. X-ray diffraction pattern revealed normal alignment in Yb-123 by showing the c axis aligned perpendicular to the magnetic field. At low temperatures, magnetic hysteresis ΔM with magnetic field perpendicular to a–b plane is larger than magnetic hysteresis parallel to a–b plane. A crossover was observed at higher temperature in magnetic hystereses, i.e., ΔM_⊥a - b < ΔM_‖a - b at certain applied field. This crossover can be explained due to difference in vortex structure along both field directions.

We investigated the effects of different operating temperatures on the performance of transport critical current density, J_c, for MgB₂ + 10 wt%C₄H₆O₅MgB₂/Fe wires. It was shown that the J_c values of the malic acid doped wires sintered at 900°C reached 10⁴ Acm^-2 at 20 K and 5 T. The J_c value extrapolated to 2 T and 20 K exceeds the practical level of 10⁵ Acm^-2. According to the Kramer plots, the pinning force, F_K = J_c^1/2 x B^1/4, is expected to be a linear function of magnetic field B. The irreversibility field, B_irr, at which extrapolated F_K reaches zero, was 1.8 T at 32.8 K, 2.8 T at 30 K, 5.7 T at 25 K, 8.6 T at 20 K, and 12.5 T at 15 K, respectively.

The effect of processing atmosphere on the microstructure and superconducting properties of MgB₂ was studied for bulk samples made using the in situ reaction technique under argon gas with three different levels of purity: ultra-high, high, and welding grade. The critical temperature, T_c, decreased by 0.5 K, while the full width at half maximum (FWHM) of the MgB₂ peaks, and the resistivity showed an increase in the sample processed in welding Ar. The critical current density, J_c, and the upper critical field, H_c2, for the welding Ar sample were improved in comparison with the samples treated in ultra-high and high purity argon. Optimal J_c in MgB₂ can be achieved by using readily available and economical welding grade Ar as the protective atmosphere instead of using ultra-high or high purity Ar.

The effects of quantum and thermal fluctuations on collective pinning and critical current density J_c are investigated for bulk type-II superconductors by utilizing quantum statistics. It is shown that for a constant magnetic field, J_c is nearly independent of temperature in the quantum limit; however, in the classical limit, J_c decreases weakly with increasing temperature when T < T_dp (depinning temperature); when T_dp < T < T_f (boundary fluctuation temperature), J_c is power-law-decaying, and when T > T_f, J_c decays exponentially. For constant temperature, J_c first decreases, then increases after reaching a maximum, and finally decreases again. These results are in agreement with the experiments.

The influences of microstructure, connectivity, and disorder on the critical current density, $J_c$, are discussed to clarify the different mechanisms of $J_c(H)$ in different magnetic field ranges for in situ and combined in situ/ex situ${\mathrm{\text{MgB}}}_2∕\mathrm{\text{Fe}}$ wires with nano SiC and malic acid codoping. Sintering temperature plays a very important role in the electromagnetic properties at different temperatures and under various magnetic fields. Connectivity, upper critical field, $H_{c2}$, and irreversibility field, $H_{\mathrm{\text{irr}}}$, are studied to demonstrate the mechanism of $J_c$ dependence on magnetic field. The combined in situ/ex situ process is proved to be a promising technique for fabrication of practical ${\mathrm{\text{MgB}}}_2$ wires.

Ca-doped (x = 0.00, 0.07 and 0.20) epitaxial Y_1-xCa_xBa₂Cu₃O_7-δ thin films were prepared on SrTiO₃ (100) by PLD technique. The superconducting transition temperature is found to decrease with increasing Ca content. The AC-susceptibility data were used to determine the intergranular critical current density, when varying the field amplitude H_AC. The temperature dependence of critical current density was determined from the imaginary part of the complex susceptibility, χ′′(T), using Xing's relation. The results were analyzed in terms of superconductor-insulator-superconductor (SIS) and superconductor-normal-superconductor (SNS) type models.

AC susceptibility measurements have been done to determine the intergranular properties for some 123-superconducting ceramics. From the imaginary part of the complex susceptibility, χ″(T), the temperature dependence of the critical current density was determined by using Bean's model. The intergranular current density was experimentally determined from AC susceptibility data by varying the field amplitude. All intragrain critical parameters are gradually degraded with increasing the concentration of the dopant substituting on the rare-earth side and with decreasing the sintering temperature during the sample preparation.

Bulk polycrystalline samples of nano-Mo doped MgB₂ were synthesized by Fe tube encapsulation at ambient pressure under argon annealing (850°C). Mo substitution takes place successfully at the Mg site in Mg_1-xMo_xB₂ only till x=0.2. For higher (x>0.2) Mo content the same did not enter the MgB₂ lattice but rather forms an isomorphic lattice in the host with decreased c but an increased a-parameter. The ρ(T) measurements showed superconducting transition temperature (T_c) of around 36 K for all the samples till x=0.3 and slightly decreased values of 35 and 34 K for x=0.4 and 0.5 samples, respectively. Resistivity under magnetic field [R(T)H] experiments showed distinct single peaks in dρ/dT for all applied fields up to 8 Tesla. The estimated upper critical field H_c2 is 8 Tesla for pristine and x=0.2 samples at 15.6 and 19 K, respectively. Thus H_c2 increases up to x=0.20 samples and decreases afterwards. Magnetic susceptibility measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 38 K (T_c) in zero field-cooled measurements. Commendable current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements invoking the Bean's critical state model for pristine samples. For higher fields above 2.5 Tesla the J_c(H) characteristics of x=0.1 and 0.2 samples were found to be slightly superior to that for pristine samples with enhanced H_irr (irreversibility field).

We have investigated the role of pinning density on the properties of the peak effect in the critical current density in superconducting systems. This study was conducted using a series of molecular dynamic simulations on two systems of nanostructures of periodic square arrays of pinning sites with different pining densities. We have found that the peak occurred in both systems only at zero temperature and for specific values of pinning strength. The most interesting result was that in both systems, the peak was found to occur only at nearly 0.5 fraction of the first matching field for all values of pinning strength. The properties of the peak were found to depend mainly on the initial positions of the vortices with respect to the positions of the pinning sites. The critical dipping force at the peak was found to increase linearly with the pinning strength and to have larger values for the system with the smaller density of pinning sites. The dependence of the relative height of the peak on the pinning strength was found be nearly the same in both systems. One-dimensional linear channels of moving vortices along the direction of the driving force were observed at the dip just before the peak.

The magnetic properties of the newly discovered iron-oxypnictide SmFeAsO_0.85 high-T_c superconductor with a T_c of around 55 K were investigated. Bulk SmFeAsO_0.85 was prepared by a method for high-pressure synthesis. The lower critical field H_c1 was estimated from the magnetization at low fields; H_c1(0) was measured to be 212 Oe. A linear temperature dependence instead of saturation at low temperatures in H_c1(T) revealed unconventional superconductivity with a nodal gap structure in our SmFeAsO_0.85 superconductor. The results showed that the well-known secondary peak in the temperature dependence of the critical current density J_c is absent in the SmFeAsO_0.85 high-T_c superconductor. The irreversibility line B_irr was fitted well by the power law dependence (1 - T/T_c)ⁿ with n ~ 1.5. This is indicative of the flux creep phenomena in the SmFeAsO_0.85 high-T_c superconductor. In addition, within the range of measurement temperatures in this study, no crossover was observed in the temperature dependence of the irreversibility line B_irr which may be due to low anisotropy in our SmFeAsO_0.85 superconductor.

The effect of nanosized silicon oxide nanoparticles (denoted NP-SiO₂) and nanowires (denoted NW-SiO₂) additions during the final processing stage on electrical fluctuation conductivity of polycrystalline YBa₂Cu₃O${}_y$ (Y-123 for brevity) in the mean field region has been reported. Series of samples were synthesized in air using a standard solid-state reaction technique by adding nanosized entities up to 0.5 wt.%. Phases, microstructure and superconductivity properties have been systematically investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electrical measurements. TEM investigations show the presence of inhomogeneities embedded in the superconducting matrix along with the presence of columnar defects in the case of SiO₂ nanoparticles added samples, however nanowires tend to agglomerate by entangling with each other in the intergrain regions. The fluctuation conductivity was analyzed as a function of reduced temperature using the Aslamazov–Larkin model. Using the Lawrence–Doniach equations, the Ginzburg–Landau (GL) number (NG) and equations, the coherence length, the effective layer thickness, the lower critical field $B_{c1}(0)$, the upper critical field $B_{c2}(0)$ and the critical current density $J_c(0)$ were estimated. It was found that the addition of an optimum concentration of SiO₂ nanomaterials, that depends on the shape, effectively controlled the microstructure, the grains coupling and hence improved the physical properties of Y-123 compound.

The effect of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) addition on the phase formation and the superconducting properties of GdBa₂Cu₃O${}_{7-\delta }$ phase has been studied. Therefore, composite superconductor samples of type (CNTs)${}_x$ GdBa₂Cu₃O${}_{7-\delta }$, 0.0 $\le x\le$ 0.1 wt.% have been synthesized by a standard solid-state reaction technique. The samples have been characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results of XRD show an enhancement in the phase formation up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively. SEM and TEM reveal that CNTs form an electrical network resulting in well-connected superconducting grains. The electrical properties of the prepared samples have been examined by electric resistivity and I–V measurements, and their results reinforce the XRD, SEM and TEM. Consequently, both $T_c$ and $J_c$ improve as the addition percentage increases up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively.

BiPb-2212 phase in nanoscale was added to BiPb-2223 phase with a general stoichiometry of (Bi${}_{1.7}$Pb${}_{0.4}$Sr${}_{2.1}$Ca${}_{1.1}$Cu${}_{2.1}$O${}_{8+\delta }{)}_x$/Bi${}_{1.8}$Pb${}_{0.4}$Sr${}_{2.0}$Ca${}_{2.0}$Cu${}_{3.2}$O${}_{10+\delta }$, 0.0 $\le$ x $\le$ 2.5 wt.%. All samples were prepared by the standard solid-state reaction method. The prepared nano BiPb-2212 phase was characterized by X-ray powder diffraction (XRD) and transmission electron microscope (TEM). The prepared samples were characterized by XRD and the scanning electron microscope (SEM). XRD analysis indicated that the sample with x = 1.5 wt.% has the highest relative volume fraction for BiPb-2223 phase. Samples were examined by electrical resistivity and I–V measurements. There is no significant change in the superconducting transition temperature $T_c$ for all samples. The highest critical current density $J_c$ was recorded for the sample with x = 1.5 wt.%. The normalized excess conductivity $(\mathrm{\Delta }\sigma /{\sigma }_{\mathrm{\text{room}}})$ was calculated according to Aslamazov–Larkin (AL) model. Four different fluctuating regions were recorded as the temperature decreased. The coherence length along the c-axis at 0 K ${\xi }_c(0)$, interlayer coupling strength s, Fermi velocity $v_{\mathrm{\text{F}}}$ of the carriers and Fermi energy $E_{\mathrm{\text{F}}}$ were calculated for both samples with x = 0.0 wt.% and 1.5 wt.%.

In this paper, we have extracted transport critical current density ($J_c$) of BaZrO₃ (BZO) added YBa₂Cu₃O${}_{7-\delta }$ (YBCO) by using several low electric field (E)-based criteria. By varying E-based criteria in order of magnitude, we have extracted $J_c$(T) and granular critical current density, $J_{cG}$(T) by using JE (IV) characteristics. The temperature (T) dependence of $J_c$ and $J_{cG}$ is found to be affected strongly as a result of the choice of E as criterion. The sensitiveness of $J_c$ and $J_{cG}$ becomes prominent with the lowering of T. The Ambegaokar–Baratoff (AB) and the Ginzburg–Landau (GL) descriptions of $J_c$(T) and $J_{cG}$(T) have been studied to understand how varying criteria may change the extraction of several associated coefficients. Extrapolated $J_c$($T=0$) and $J_{cG}$($T=0$) is also highly sensitive to criteria used.

Bulk melt-textured YBa₂Cu₃O₇ irradiated by fast neutrons has been investigated. We observed: (1) a critical current density (J_c) determined magnetically at 77 K exceeds 5×10⁵A/cm² in 0.1 T, 2×10⁵A/cm² in 1 T, and 1.2×10³A/cm² in 5.0 T; (2) at 5 K, J_c is >2.1×10⁷A/cm² in 0.5 T, ~1.8×10⁷A/cm² at 1 T and >8×10⁶A/cm² in 5.0 T and (3) the J_c-anisotropy decreases rapidly with the external field (H), e.g. at 77 K, it becomes 1 at ~4.8 T with J_c ~3×10³A/cm². We also found that (1) such a J_c-enhancement is most effective in low H but less so in high H, (2) the enhancement is temperature independent at low H, (3) the pinning potential remains unchanged to within ~10% as J_c varies by a factor of 15, and (4) thermal annealing up to 600°C recovers the un-irradiated state. These results and their implications are presented and discussed.

dc susceptibility has been used to interpret the percolation behavior of several Ag-sheathed 3-layer Bi(Pb)-compound wires. The wires have been synthesized to obtain relatively inferior transport critical current densities (J_c’s) from 0 to 8×10³A/cm² at 77 K and 0 T. Combined with SEM and X-ray data, our observations suggest that the superconductive cores in the wires are highly textured grains with a considerable fraction of voids. The neighboring Bi-grains in the same plane are frequently bridged by grains in the next layer. The transport supercurrent, therefore, flows in a zig-zag pattern across the grain boundaries along the c-direction. The large and relatively clean grain boundaries in the c-planes help to achieve a rather large J_c under optimum conditions. However, in such a percolation figure, the intergrowth or layered inclusions become one of the main factors to limit J_c.