Showing papers in "Physica C-superconductivity and Its Applications in 2014"

Basic mechanism of self-healing from thermal runaway for uninsulated REBCO pancake coils

Abstract: This paper clarifies the basic mechanism of self-healing from natural thermal runaway for uninsulated REBCO pancake coils. Based on the numerical simulation and experimental results, it is demonstrated that current flow patterns for an uninsulated REBCO pancake coil sequentially changes with the overcurrent value. Due to natural thermal runaway above the coil critical current, the current flow changes from “multi-turn-coil” mode to “single-turn-coil” mode, reducing the conductor current density and Joule heating, and thus the thermal runaway shrinks and the coil is self-healed; i.e. the REBCO coil is self-protected. For higher overcurrents, however, transverse currents across both electrodes become dominant, here called “terminal-to-terminal current” mode. It is dangerous for REBCO coils as the winding volume connecting both electrodes is extremely overheated, damaging the conductor winding.

Synthesis and post-annealing effects of alkaline-metal-ethylenediamine-intercalated superconductors Ax(C2H8N2)yFe2−zSe2 (A = Li, Na) with Tc = 45 K

Abstract: New iron-based intercalation superconductors A x (C 2 H 8 N 2 ) y Fe 2− z Se 2 ( A = Li, Na) with T c = 45 K have successfully been synthesized via intercalation of dissolved alkaline metal in ethylenediamine The c -axis lengths of A x (C 2 H 8 N 2 ) y Fe 2− z Se 2 ( A = Li, Na) are 2074(7) A and 219(1) A, respectively, and are about 50% larger than that of K x Fe 2 Se 2 , indicating that not only alkaline metal but also ethylenediamine is intercalated between the Se–Se layers of FeSe It seems that the high- T c of A x (C 2 H 8 N 2 ) y Fe 2− z Se 2 ( A = Li, Na) is caused by the possible two-dimensional electronic structure due to the large c -axis length Through the post-annealing in an evacuated glass tube, it has been found that T c decreases with increasing post-annealing temperature and that deintercalation of EDA from the as-intercalated sample takes place at low temperatures below 250 °C

Fluctuation conductivity and pseudogap in HoBa2Cu3O7-δ single crystals under pressure with transport current flowing under an angle 45° to the twin boundaries

Abstract: The influence of hydrostatic pressure up to 048 GPa on the fluctuation conductivity σ ′ ( T ) and pseudogap (PG) Δ ∗ ( T ) of slightly doped HoBa 2 Cu 3 O 7 - δ single crystals with T c ≈ 62 K and δ ≈ 035 is studied with current passing under an angle 45° to the twin boundaries It is shown that near T c the conductivity σ ′ ( T ) is well described by the Aslamasov–Larkin and Hikami–Larkin fluctuation theories demonstrating 3D–2D crossover with the increase of temperature Δ ∗ ( T ) displays two representative maxima at T max 1 ≈ 219 K and T max 2 ≈ 241 K likely caused by the phase stratification of the single crystal Pressure leads to disappearance of these maxima and linear Δ ∗ ( T ) with a positive gradient at high temperatures Essentially, with the removal of pressure the maxima are restored The comparison of our results with those obtained for YBa 2 Cu 3 O 7 - δ sheds more light on the role of magnetic subsystem in the high-Tc superconductors

Assessment on the influence of resistive superconducting fault current limiter in VSC-HVDC system

Abstract: Due to fewer risk of commutation failures, harmonic occurrences and reactive power consumptions, Voltage Source Converter (VSC) based HVDC system is known as the optimum solution of HVDC power system for the future power grid. However, the absence of suitable fault protection devices for HVDC system hinders the efficient VSC-HVDC power grid design. In order to enhance the reliability of the VSC-HVDC power grid against the fault current problems, the application of resistive Superconducting Fault Current Limiters (SFCLs) could be considered. Also, SFCLs could be applied to the VSC-HVDC system with integrated AC Power Systems in order to enhance the transient response and the robustness of the system. In this paper, in order to evaluate the role of SFCLs in VSC-HVDC systems and to determine the suitable position of SFCLs in VSC-HVDC power systems integrated with AC power System, a simulation model based on Korea Jeju-Haenam HVDC power system was designed in Matlab Simulink/SimPowerSystems. This designed model was composed of VSC-HVDC system connected with an AC microgrid. Utilizing the designed VSC-HVDC systems, the feasible locations of resistive SFCLs were evaluated when DC line-to-line, DC line-to-ground and three phase AC faults were occurred. Consequently, it was found that the simulation model was effective to evaluate the positive effects of resistive SFCLs for the effective suppression of fault currents in VSC-HVDC systems as well as in integrated AC Systems. Finally, the optimum locations of SFCLs in VSC-HVDC transmission systems were suggested based on the simulation results.

Fabrication of Eu 1 Ba 2 Cu 3 O 7−δ +BaHfO 3 coated conductors with 141 A/cm-w under 3 T at 77 K using the IBAD/PLD process

Abstract: Introduction of artificial pinning centers such as BaZrO3 (BZO), BaSnO3 (BSO), BaHfO3 (BHO) nano-rods is effective to improve in-field critical currents (Ic). In particular, the BHO doping into Gd1Ba2Cu3O7−δ had been found to exhibit high in-field Ic under wide ranges of temperatures and magnetic fields. Furthermore, a long coated conductor (CC) with high in-field properties and high uniformity was successfully fabricated by means of BHO doping. However, increase of the Ic values exhibited somewhat in a saturated manner with thickening the superconducting films above 3 μm. In this work, a new combination of Eu1Ba2Cu3O7−δ (EuBCO) + BHO system was investigated using the IBAD/PLD process to improve in-field performance especially in thick films. As a result, it was found that a EuBCO + BHO film with 3.6 μm in thickness showed extremely high property of 141 A/cm-w measured at 77 K, 3 T. This high in-field Ic value was tentatively explained due to suppression of a-axis oriented grains even in thick films. A 200 m long EuBCO + BHO CC with 55.5 A/cm-w and a 93.7 m long CC with 108 A/cm-w measured at 77 K, 3 T were successfully fabricated.

Fabrication of single domain GdBCO bulk superconductors by a new modified TSIG technique

Abstract: Single domain GdBCO bulk superconductors have been fabricated with new and traditional solid phases by a top seeded infiltration and growth (TSIG) process technique. In the conventional TSIG process, three types of powders, such as Gd2BaCuO5, GdBa2Cu3O7−x and Ba3Cu5O8, must be prepared, but in our new modified TSIG technique, only BaCuO2 powders are required during the fabrication of the single domain GdBCO bulk superconductors. The solid phase used in the conventional process is Gd2BaCuO5 instead of the solid phase (Gd2O3 + BaCuO2) utilized in the new process. The liquid phase used in the conventional process is a mixture of (GdBa2Cu3O7−x + Ba3Cu5O8), and the liquid phase in the new process is a mixture of (Gd2O3 + 10BaCuO2 + 6CuO). Single domain GdBCO bulk superconductors have been fabricated with the new solid and liquid phases. The levitation force of the GdBCO bulk samples fabricated by the new solid phase is 28 N, which is slightly higher than that of the samples fabricated using the conventional solid phases (26 N). The microstructure and the levitation force of the samples indicate that this new method can greatly simplify the fabrication process, introduce nanometer-sized flux centers, improve the levitation force and working efficiency, and greatly reduce the cost of fabrication of single domain GdBCO bulk superconductors by the TSIG process.

High field magneto-transport study of YBa2Cu3O7:Agx (x = 0.00–0.20)

Abstract: We report high field (up to 13 T) magneto transport [ ρ ( T ) H ] of YBa 2 Cu 3 O 7 (YBCO):Ag x ( x = 0.0, 0.1 and 0.2) composites. The transport properties are significantly improved by Ag doping on the insulating grain boundaries of YBCO. Pure and Ag diffused YBCO superconducting samples are synthesised through solid state reaction route. Both pure and Ag doped YBCO are superconducting at below 90 K. Though, the T c ( ρ = 0) of YBCO:Ag samples under applied field of 13 T is around 65 K, the same is 45 K for pure YBCO under same applied field. The upper critical field [ H c 2 (0)], being estimated from ρ ( T ) H is around 70 T for pristine sample, and is above 190 T for Ag doped samples. The boarding of the resistive transition under applied magnetic field is comparatively less and nearly single step for Ag doped samples, while the same is clearly two step and relatively much larger for the pristine YBCO. The resistive broadening is explained on the basis of changed inter-granular coupling and thermally activated flux flow (TAFF). The TAFF activation energy ( U 0 ) is found to be linear with applied magnetic field for all the samples, but with nearly an order of magnitude less value for the Ag doped samples. Summarily, it is shown that inclusion of Ag significantly improves the superconducting performance of YBCO:Ag composites, in particular under applied field.

Investigation on the levitation force behaviour of malic acid added bulk MgB2 superconductors

Abstract: The effects of malic acid addition (from 0 to 15 wt% of the total MgB 2 ) on the levitation force properties of bulk MgB 2 have been investigated. All samples were prepared from magnesium powder, amorphous boron powder, malic acid (C 4 H 6 O 5 ) and toluene (C 7 H 8 ) by using two-step solid state reaction method. Vertical and lateral levitation force measurements that are under both zero-field-cooled (ZFC) and field-cooled (FC) regimes were carried out at different temperatures of 24, 28 and 32 K for samples with various adding level. It was found that the reasonable malic acid adding has a positive impact on the levitation properties. At 24 K and 28 K, the 4 wt% and 6 wt% malic acid added samples exhibits a higher levitation force than pure sample. In the case of the optimally additive 4 wt% sample, the maximum levitation force corresponds to 18.60 N, whereas the pure sample shows 16.95 N at 24 K for ZFC regime. In this study the enhancing effect of malic acid adding on the levitation force properties of MgB 2 has been first time investigated and reported.

A new 3D levitation force measuring device for REBCO bulk superconductors

Abstract: A new 3D levitation force measuring device for ReBa2Cu3O7−x (REBCO) bulk superconductors has been designed and constructed. Three pull pressure load cells are orthogonally set on a fixing bracket to test the interaction force between a bulk superconductor and a magnet in three dimensions. To realize the simple, rapid and accurate measurement of the levitation force, a non-magnetic hollow cylinder flange, three pull pressure load cells, a piece of iron plate, a NbFeB permanent magnet (PM) and some steel balls are elaborately constructed with the fixing bracket, thus the magnet or REBCO bulk superconductor can be well and rigidly connected with the load cells, and the mutual interference from the three pull pressure load cells can be effectively avoided during the levitation force measuring processes. This device can be used to measure the interaction (or levitation) force between a superconductor and a magnet, that between a magnet and a magnet, or the magnetic force among magnetic materials in three dimensions.

Fabrication of Nb3Al superconducting wires by utilizing the mechanically alloyed Nb(Al)(ss) supersaturated solid-solution with low-temperature annealing

Abstract: High-performance Nb 3 Al superconducting wire is a promising candidate to the application of high-field magnets. However, due to the production problem of km-grade wires that are free from low magnetic field instability, the Nb 3 Al wires made by rapid heating, quenching and transformation (RHQT) are still not available to the large-scale engineering application. In this paper, we reported the properties of the in situ powder-in-tube (PIT) Nb 3 Al superconducting wires, which were made by using the mechanically alloyed Nb(Al) ss supersaturated solid solution, as well as the low temperature heat-treatment at 800 °C for 10 h. The results show that Nb 3 Al superconductors in this method possess very fine grains and well superconducting properties, though a little of Nb 2 Al and Nb impurities still keep being existence at present work. At the Nb 3 Al with a nominal 26 at.% Al content, the onset T c reaches 15.8 K. Furthermore, a series of Nb 3 Al wires and tapes with various sizes have been fabricated; for the 1.0 mm-diameter wire, the J c at 4.2 K, 10 T and 14 T have achieved 12,700 and 6900 A/cm 2 , respectively. This work suggests it is possible to develop high-performance Cu-matrix Nb 3 Al superconducting wires by directly using the Nb(Al) ss supersaturated solid-solution without the complex RHQT heat-treatment process.

Electronic properties of γ-U and superconductivity of U–Mo alloys

Abstract: Fundamental electronic properties of γ-Uranium were determined using Mo doping combined with ultrafast (splat) cooling, which allowed stabilization of the bcc structure to low temperatures. The Sommerfeld coefficient γ e is enhanced to 16 mJ/mol K 2 from 11 mJ/mol K 2 for α-U. Magnetic susceptibility remains weak and T -independent, ≈5 × 10 −8 m 3 /mol. The Mo-doped γ-U exhibits a conventional BCS superconductivity with T c ≈ 2.1 K and critical field exceeding 5 T for 15 at.% Mo. This type of superconductivity is qualitatively different from the one found for pure U splat, which has T c higher than 1 K but the weak specific heat anomaly proves that it is not real bulk effect.

Practical design and operating characteristic analysis of a 10 kW HTS DC induction heating machine

Abstract: Conventional induction heaters have been in operation in metal and related industries with poor energy efficiencies of only 50–60%. Also, the efficiency of atmosphere furnace, one of the various heating facilities for metal billets, is about 20%. Hence, a high temperature superconducting (HTS) DC induction heating machine to heat a rotating metal billet under uniform magnetic field generated by the 2G HTS magnet with about 80–90% of the system energy efficiency has been researched in this paper. We presented practical design specification, operational characteristics, and temperature distribution on a 10 kW class HTS DC induction heating machine which had been built and tested. The saturated temperature of an HTS no-insulated(NI) coil in the cryostat fabricated with 100 A of the operating current reached 45.9 K and the magnetic field at the centre point between two iron cores measured 0.2 T. The rotating machine for 4.1 kg of the aluminum billet was tested on 1760 rpm of the rated rotating speed. The final temperature of the aluminum billet has risen up to 500 °C for 480 s. The research outcomes are expected to be useful for the design of a large scale HTS DC induction heating machine in industries.

Film thickness dependence of microstructure and superconductive property of PLD prepared YBCO layers

Abstract: YBCO superconducting films in the range of 0.4–2.33 μm were deposited on CeO 2 /YSZ/Y 2 O 3 /NiW substrates by PLD, and the microstructure and electric property were investigated. The results show that the obtained YBCO films are predominantly c -axis oriented, though there are small NiO peaks. With the YBCO thickening, its surface quality decline, the amount of the a -axis grains and BaCeO 3 increase, and the in- and out-plane alignments of YBCO and CeO 2 become worse. The J c values degrade with the film thickening, which is only 1.2 MA/cm 2 (77 K, SF) when the thickness is 1.17 μm.

Analysis model development and specification proposal of 154 kV SFCL for the application to a live grid in South Korea

Abstract: This study presents the development of a PSCAD/EMTDC analysis model for the 154 kV SFCL in South Korea, which will be applied to the Korean power system in 2015. The analysis model is developed by using the operating mechanism supported by the developer of the SFCL, KEPRI (Korea Electric Power Research Institute). And then this paper proposes some candidate locations and design specifications for the successful demonstration application of the 154 kV SFCL to Korean power systems.

Magnetic characterisation of large grain, bulk Y–Ba–Cu–O superconductor–soft ferromagnetic alloy hybrid structures

Abstract: Large grain, bulk Y–Ba–Cu–O (YBCO) high temperature superconductors (HTS) have significant potential for use in a variety of practical applications that incorporate powerful quasi-permanent magnets. In the present work, we investigate how the trapped field of such magnets can be improved by combining bulk YBCO with a soft FeNi, ferromagnetic alloy. This involves machining the alloy into components of various shapes, such as cylinders and rings, which are attached subsequently to the top surface of a solid, bulk HTS cylinder. The effect of these modifications on the magnetic hysteresis curve and trapped field of the bulk superconductor at 77 K are then studied using pick-up coil and Hall probe measurements. The experimental data are compared to finite element modelling of the magnetic flux distribution using Campbell’s algorithm. Initially we establish the validity of the technique involving pick-up coils wrapped around the bulk superconductor to obtain its magnetic hysteresis curve in a non-destructive way and highlight the difference between the measured signal and the true magnetization of the sample. We then consider the properties of hybrid ferromagnet/superconductor (F/S) structures. Hall probe measurements, together with the results of the model, establish that flux lines curve outwards through the ferromagnet, which acts, effectively, like a magnetic short circuit. Magnetic hysteresis curves show that the effects of the superconductor and the ferromagnet simply add when the ferromagnet is saturated fully by the applied field. The trapped field of the hybrid structure is always larger than that of the superconductor alone below this saturation level, and especially when the applied field is removed. The results of the study show further that the beneficial effects on the trapped field are enhanced when the ferromagnet covers the entire surface of the superconductor for different ferromagnetic components of various shapes and fixed volume.

Delamination behaviour in differently copper laminated REBCO coated conductor tapes under transverse loading

Abstract: Laminated HTS coated conductor (CC) tapes having a unique multi-layer structure made them vulnerable when exposed to transverse loading. Electromechanical transport properties of these CC tapes can be affected by excessive transverse stresses. Due to the coefficient of thermal expansion (CTE) mismatch and incompatibility among constituent materials used in coil applications, delamination among layers occurs and causes critical current, Ic degradation in the CC tapes. In this study, the delamination behaviors in copper (Cu) solder-laminated CC tapes by soldering and surround Cu-stabilized ones by electroplating under transverse tension loading were investigated. Similarly to the surround Cu-stabilized CC tapes in our previous reports, the Cu solder-laminated CC tapes also showed an abrupt and gradual Ic degradation behavior. However, the Cu solder-laminated CC tapes showed different delamination morphologies as compared to the surround Cu-stabilized CC tapes; the superconducting side and the substrate side of the Cu solder laminated CC tapes were totally separated by delamination. On the other hand, the brass laminate did not show any significant effect on the delamination strength when it is added upon the surround Cu-stabilized CC tapes.

The study on SiO2 nanoparticles and nanowires added YBCuO: Microstructure and normal state electrical properties

Abstract: The effects of nanosized silicon oxide nanoparticles and nanowires additions on the microstructure and the normal state transport properties of polycrystalline YBa 2 Cu 3 O y (YBCO, or Y-123) were systematically studied. Samples were synthesized in air using a standard solid state reaction technique by adding nanosized entities up to 3 wt.%. Phases, microstructure, superconductivity, have been systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrical measurements. When nanosized SiO 2 entities are added to the YBCO the orthorhombic structure is maintained. SEM results reveal that the grain size is reduced with increasing the content of SiO 2 . TEM investigation shows the presence of inhomogeneities embedded in the superconducting matrix along with the presence of columnar defects in the case of SiO 2 nanoparticles added samples. Nanowires tend to agglomerate by entangling with each other in the intergrain regions. To analyze the normal state properties of the samples, the percolation theory based on localized states is applied. A change from Coulomb gap to variable-range hopping mechanisms is observed as a result of increasing the nano-entities concentration. The SiO 2 nanowires addition modifies the electrical behavior of samples from metallic to insulating with a much lower concentration comparatively to SiO 2 nanoparticles addition. The localization length d and the range hopping R of samples are estimated.

Photonic band gaps of a two-dimensional square lattice composed by superconducting hollow rods

Abstract: In this paper by means of the plane wave expansion method, we have calculated the photonic band structure of 2D photonic crystals consisting of high temperature superconducting hollow cylinders arranged in a square lattice. Band structures were obtained at low frequencies and assuming TM polarization of the incident wave, for different inner radii of the cylinders and for two different temperatures (5 K and 15 K), showing the tunability of photonic band gaps with respect to these parameters. Interesting features, such as the decreasing of cutoff frequency and separation of photonic modes were observed by increasing both the temperature and inner radius. Permittivity contrast and the difference between the inner and outer radius lead to the appearance of new band gaps when compared with the case of solid cylinders. These band gaps can be modulated by the width of the shell and temperature, which may be used for the development of novel optical devices.

New superconductor (Na0.25K0.45) Ba3Bi4O12: A first-principles study

Abstract: A new superconductor (Na 0.25 K 0.45 )Ba 3 Bi 4 O 12 , having an A -site-ordered double perovskite structure, with a maximum T c ∼ 27 K has very recently been discovered through hydrothermal synthesis at 593 K. The structural, elastic, electronic, and thermal properties of the new synthesized compound have been investigated theoretically. Here we have employed the pseudo-potential plane-wave (PP-PW) approach based on the density functional (DFT) theory, within the generalized gradient approximation (GGA). The elastic constants ( C ij ), Pugh‘s ratio, Cauchy‘s pressure and other elastic parameters are derived first time. We have discussed the bonding nature in the light of the electronic valence charge density. Both electron and hole-like Fermi surfaces are present in the compound under study which indicate the multiple-band nature of (Na 0.25 K 0.45 )Ba 3 Bi 4 O 12 . The compound is indicated to be a strongly coupled superconductor which is based on the estimated e-ph coupling constant. The thermodynamic properties at elevated temperature and pressure are calculated and analyzed for the first time by using quasi-harmonic model which includes vibrational term.

Strong vortex matching effects in YBCO films with periodic modulations of the superconducting order parameter fabricated by masked ion irradiation

Abstract: We report on measurements of the magnetoresistance and of the critical current in thin films of the high-temperature superconductor YBa 2 Cu 3 O 7 - δ (YBCO). A square array of regions with suppressed superconducting order parameter has been created in these films by introducing point defects via irradiation with He + ions through a silicon stencil mask. In such a structure distinct peaks of the critical current can be observed at commensurate arrangements of magnetic flux quanta with the artificial defect lattice. Concurrently, the magnetoresistance shows pronounced minima. Both observations demonstrate that the strong intrinsic pinning in YBCO can be overcome by a periodic array of ion-damage columns with 300 nm spacing.

Measurements of tunneling barrier thicknesses for Nb/Al–AlOx/Nb tunnel junctions

Abstract: The tunnel barrier thicknesses of Nb/Al–AlO x /Nb tunnel junctions were measured using transmission electron microscopy (TEM) and X-ray Reflection (XRR). By investigating the barrier thickness dependence of current density J c , the barrier height for Nb/Al–AlO x /Nb junctions was calculated. Nb/Al–AlO x /Nb junctions with different J c were fabricated by controlling the O 2 exposure in Al oxidation. The junctions show good tunneling properties with subgap leakage factor V m larger than 30 mV in the range of J c from tens of A/cm 2 to several kA/cm 2 . TEM images showed clear interface and indicated the AlO x thicknesses ranging from 0.8 nm to 1.9 nm, and the average barrier height was estimated to be 0.17 eV for Nb/Al–AlO x /Nb tunnel junctions.

Nanostructuring of high-TC superconductors via masked ion irradiation for efficient ordered vortex pinning

Abstract: We studied vortex dynamics in a YBa 2 Cu 3 O 7−δ thin film with two different sources of pinning: intrinsic random defects and an artificial square array of defects created by masked ion irradiation. We study commensurability effects between the vortex lattice and the pinning array as a function of the vortex velocity v and the temperature. We find that at low temperatures the commensurability effects (magneto-resistance drop at the matching fields) are stronger at low velocities, in contrast with the behavior previously observed in low-critical-temperature superconductors.

Resistive state triggered by vortex entry in YBa2Cu3O7-delta nanostructures

Abstract: We have realized YBa2Cu3O7-delta nanowires and nano Superconducting Quantum Interference Devices (nanoSQUID). The measured temperature dependence of the wire resistances below the superconducting transition temperature has been analyzed using a thermally activated vortex entry model valid for wires wider than the superconducting coherence length. The extracted zero temperature values of the London penetration depth, lambda(0) similar or equal to 270 +/- 15 nm, are in good agreement with the value obtained from critical current modulations as a function of an externally applied magnetic field in a nanoSQUID implementing two nanowires.

Critical current density and pinning behaviour of mono-core MgB2 wires prepared by internal magnesium diffusion and in-situ powder-in-tube method

Abstract: Mono-core MgB 2 wires were produced by internal magnesium diffusion (IMD) and in-situ powder-in-tube (PIT) technique. Hysteresis loops and magnetic relaxations were measured to calculate critical current density, J cm (via Bean’s critical state model) and mean effective activation energy (via Anderson’s flux creep theory). To calculate J cm of the IMD sample, the formulae for typical cylindrical sample geometry must have been modified properly with regard to IMD hollow cylinder geometry. Results for J cm were compared to direct transport measurements. Also studied was the irreversibility field, B irr , and upper critical field, B c2 . It was found that the dominant difference between wires is in J c . For the IMD sample, J c was higher and less field-dependent than J c of the PIT sample.

Chemical pressure effect on Tc in BiS2-based Ce1−xNdxO0.5F0.5BiS2

Abstract: We have investigated the crystal structure and superconducting properties of the BiS 2 -based layered superconductor Ce 1− x Nd x O 0.5 F 0.5 BiS 2 . Bulk superconductivity was observed for x ⩾ 0.4, and the transition temperature was enhanced with increasing Nd concentration. The highest transition temperature was 4.8 K for x = 1.0. With increasing Nd concentration, the length of the a axis decreased, while the length of the c axis did not show a remarkable change. The chemical pressure along the a axis upon Nd substitution seemed to be linked with the inducement of bulk superconductivity. We found that the chemical pressure effect did not completely correspond to the external pressure effect.

Role of MgO impurity on the superconducting properties of MgB2

Abstract: We address the effect of MgO impurity on the superconducting properties of MgB 2 . The synthesis of MgB 2 is very crucial because of sensitivity of Mg to oxidation which may lead to MgO as a secondary phase. Rietveld refinement was performed to determine the quantitative volume fraction of MgO in the samples synthesized by two different techniques. Both the samples were subjected to magnetization measurements under dc and a.c. applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB 2 having more amount of MgO (with T c = 37.1 K) whereas the pure sample MgB 2 having minor quantity of MgO shows diamagnetic Meissner effect with T c = 38.8 K. M – H measurements at 10 K reveal a slight difference in irreversibility field which is due to MgO impurity along with wide transition observed from ac magnetic susceptibility measurements. The magnetotransport measurements ρ ( T ) using ρ N = 90%, 50% and 10% criterion on pure sample of MgB 2 has been used to determine the upper critical field whereas the sample having large quantity of MgO does not allow these measurements due to its high resistance.

Hysteresis losses in MgB2 superconductors exposed to combinations of low AC and high DC magnetic fields and transport currents

Abstract: MgB 2 superconductors are considered for generator field coils for direct drive wind turbine generators. In such coils, the losses generated by AC magnetic fields may generate excessive local heating and add to the thermal load, which must be removed by the cooling system. These losses must be evaluated in the design of the generator to ensure a sufficient overall efficiency. A major loss component is the hysteresis losses in the superconductor itself. In the high DC – low AC current and magnetic field region experimental results still lack for MgB 2 conductors. In this article we reason towards a simplified theoretical treatment of the hysteresis losses based on available models in the literature with the aim of setting the basis for estimation of the allowable magnetic fields and current ripples in superconducting generator coils intended for large wind turbine direct drive generators. The resulting equations use the DC in-field critical current, the geometry of the superconductor and the magnitude of the AC magnetic field component as parameters. This simplified approach can be valuable in the design of MgB 2 DC coils in the 1–4 T range with low AC magnetic field and current ripples.

Dynamics of vortex matter in YBCO sub-micron bridges

Abstract: We have developed a fabrication process that allows us to realize pure YBCO nanowires displaying robust superconductivity at widths w as low as 160 nm We can modify the process in order to maintain a Au protective layer This allows us to scale our nanowires even further to widths as low as 50 nm We have studied how the presence of vortices and the occurrence of phase slips affect the transport properties of nanowires in the width range ξ w λ , being ξ the coherence length and λ the magnetic penetration depth Magnetoresistance curves present features which are related to the effect of screening currents Vortex entry barrier is found to scale with the width Our findings confirm that for widths ξ w λ nanowires are better protected against phase slips and vortex flow

Manifestation of percolation in high temperature superconductivity

Abstract: Emergent advanced electronic and magnetic functionalities in novel materials appear in systems with a complex lattice structure. The key point is understanding the intrinsic effect of lattice fluctuations on the relevant electronic features in the range of 10–100 meV near the Fermi level in new materials which is needed to develop advanced quantum nano-devices. This requires the control of structural inhomogeneity at multiple scales. Here we report some of the known advances in the field of percolative superconductivity. The necessity of the review is based on the growing consensus that the lack of an understanding of high temperature superconductivity is due to the few information on lattice fluctuations. In particular they could control the pseudo-gap phase, the electronic duality of holes in Fermi arcs and electrons in small Fermi pockets, multiple condensates in different points of the k-space. Moreover the emerging lattice granularity in cuprates shifts the search for the superconducting mechanism from a homogeneous superconductivity to a percolative superconductivity, therefore it is the scope of this review to provide further data to this kind of research.

Enhanced superconducting properties of rare-earth oxides and graphene oxide added MgB2

Abstract: In this paper, the effects of addition of (i) graphene oxide (GO), (ii) a series of rare-earth (RE, RE = La, Sm, Eu, Gd, Tb and Ho) oxides (REO) and (iii) a mixture of GO and rare-earth oxides (GO + REO) on the superconducting properties of MgB2, have been studied with the help of electrical transport and magnetic measurements. All the samples have been prepared following the standard solid-state reaction route. We have used an optimum value of 1 wt% REO and 3 wt% GO for addition on the basis of previous studies. X-ray diffraction studies confirm the formation of hexagonal crystal structure (space group P6/mmm) of MgB2 with small amounts of REBx (x = 4 and 6) and MgO impurity phases in all the synthesized samples. We observe that the critical current density, Jc and upper critical field Hc2(0) improve significantly in the REO-added and GO-added samples with no significant change in critical temperature, Tc. A substantial enhancement in Jc(H) and Hc2(0) is observed with the GO + REO addition in MgB2. The different flux pinning mechanisms in all the samples are studied and it is found that the point pinning is the dominant mechanism in the GO-added samples and grain boundary pinning is the dominant one in the REO added samples. We have seen the combined effect of both types of flux pinning mechanisms in GO + REO added MgB2.