Density Of Pinning Centers Research Articles

The Influence of High Isostatic Pressure on Critical Current Density in C-Doped MgB2 Wires

This article reports the influence of isostatic pressure (from 0.1 MPa to 1.1 GPa), low annealing temperature of 570 ∘C, and annealing time for the formation of high-field pinning centers in 2% C-doped MgB2 wires. Measurements indicate that 1.1 GPa pressure significantly increases the density of high-field pinning centers below 20 K. However, lower pressure (0.6 GPa) slightly increases the density of high-field pinning centers. Increasing the annealing time from 120 to 210 min leads to a reduction of critical temperature (Tc), irreversibility field (Birr), critical current density (Jc), and upper critical field (Bc2), suggesting that a long annealing time leads to a reduction of high-field pinning center density and the number of connections between superconducting grains. The high pressures and low annealing temperature lead to a high critical current density of 1000 A/mm2 in 7.2 T and 100 A/mm2 in 12.5 T at 4.2 K in MgB2 wires.

Improvement of Flux Pinning Properties for Hafnium-Doped Gd123 Thin Films Fabricated by a Fluorine-Free MOD Method

In order to enhance the flux pinning force density $F_{p}$ in magnetic fields for REBa2Cu3O $_{y}$ (RE123)-coated conductors, Hf and Zr doping are the very valid way of introducing the perovskite nanostructure as artificial pinning centers (APCs) into the superconductors. However, the studies for Hf-doped RE123 thin films fabricated by the fluorine-free metal organic deposition (FF-MOD), which is the easiest production process, have not been done yet. In this study, we have successfully fabricated Hf-doped FF-MOD GdBa2 Cu3O $_{y}$ (Gd123) thin films on LaAlO3 substrates and investigated their flux pinning properties. Critical temperature for Gd123 thin films indicated around 92 K, and $T_{c}$ varied little by Hf doping. Hf 10 mol% doped film achieved high critical current densities of 2.72 mA⋅cm −2 at 77.3 K under 0 T, and 0.27 mA⋅cm−2 at 77.3 K under 1 T. With increasing Hf-doping amount, $F_{p}$ gradually increased, and the peak of $F_{p}$ shifted to the high magnetic field side. The elementary pinning force and the effective pinning center density also increased. We believe that effective APCs, probably BaHfO3 are introduced into FF-MOD Gd123 thin films by Hf doping.

Superconducting vortex dynamics on arrays with bicrystal-like structures: matching and rectifier effects

Hybrid nanostructures of magnetic Ni dots embedded in superconducting Nb films have been fabricated. The dot arrays show bicrystal-like structures with an interface which separates two different dot arrangements in the same array. Vortex lattice dynamics is studied on these particular bi-arrays using magnetotransport measurements. Commensurability and rectifier (ratchet) effects have been observed. Matching between the vortex lattice and the pinning bi-array is only governed by the interplay between the densities of vortices and pinning centers. In bi-arrays with dissimilar pinning densities, for vortex flow parallel to the boundary, the highest pinning center density governs the matching effects. Hybrid samples with only half of the array with asymmetric potentials show a ratchet effect; that is input ac currents yield a net flow of vortices.

Roles of pinning strength and density in vortex melting

We have investigated the role of pinning strength and density on the equilibriumvortex-lattice to vortex-liquid phase transition under several applied magnetic fields. Thisstudy was conducted using a series of molecular dynamic simulations on several sampleswith different strengths and densities of pinning sites which are arranged in periodic squarearrays. We have found a single solid–liquid vortex transition when the vortex filling factorn>1. We have found that, for fixed pinning densities and strengths, the melting temperature,Tm, decreases almost linearly with increasing magnetic field. Our results provide directnumerical evidence for the significant role of both the strength and density of pinningcenters on the position of the melting line. We have found that the vortex-lattice tovortex-liquid melting line shifts up as the pinning strength or the pinning density wasincreased. The effect on the melting line was found to be more pronounced at small valuesof strength and density of pinning sites.

Doping Effects of<tex>$rm TiC$</tex>and<tex>$rm Mo_2rm C$</tex>on Critical Current Properties of<tex>$rm MgB_2$</tex>Tapes

Doping effects of TiC and Mo2C on critical current properties and microstructures of MgB2 short tapes were systematically studied. Reduction of Tc and shortening of a-axis length were found to be derived from the substitution of carbon, independent of carbon source. Enhanced critical current properties under high magnetic fields were observed in the TiC, Mo2C doped samples. Grain growth of MgB2, which means a decrease of grain boundaries, i.e. reduction of effective pinning center density, was suppressed by TiC or Mo2C doping. Moreover, field dependence of Jc at low temperatures ~5 K was found to be strongly correlated with the actual carbon content of the MgB2 lattice. Our results suggest that control of actual carbon content in the MgB2 bulk is crucially important for improvement of Jc under magnetic fields

Self-assembling nano-diameter needlelike pinning centers in YBCO, utilizing a foreign element dopant

Although pinning centers created by irradiation presently produce the highest Jc, it is probable that ultimately these will be emulated by chemical pinning centers. The best pinning centers produced by irradiation nevertheless provide guidelines for desirable morphology of chemical pinning structures. The highest Jc produced earlier in textured HTS was obtained using isotropic high-energy ions produced by fission of 235U. This so-called U/n process produces pinning centers of diameter ⩽ 4.5 nm, with an effective length of ∼2.7 µm. Maximum Jc occurs for pinning center density of ∼1010 cm−3. We use this as a model for desired chemical pinning centers. Our approach to introducing chemical pinning centers has been to produce precipitates within the HTS containing elements not native to the HTS, and to seek needlelike (columnar) deposits of small diameter. We report here on the formation of needlelike or columnar deposits in textured Y123 containing a dopant foreign to Y123. It serves as a demonstration that self-assembling nanometer diameter columns utilizing a dopant foreign to the HTS system are a feasible goal. These deposits, however, do not fully meet the ultimate requirements of pinning centers because the desired deposits should be smaller. The self-assembling columns formed contain titanium, are ∼500 nm in diameter, and up to 10 µm long. The size and morphology of the deposits vary with the mass of admixed Ti dopant. Jc is decreased for small dopant mass. At larger dopant masses needlelike precipitates form, and Jc increases again. A small range of mass of admixed Ti exists in which Jc is enhanced by pinning. In the range of admixed Ti mass studied in these experiments there is a negligible effect on Tc. Magnetization studies of Jc are also reported.

Slow relaxations and history dependence of the transport properties of layered superconductors

We study numerically the time evolution of the transport properties of layered superconductors after different preparations. We show that, in accordance with recent experiments in BSCCO performed in the second peak region of the phase diagram (Portier et al, 2001), the relaxation strongly depends on the initial conditions and is extremely slow. We investigate the dependence on the pinning center density and the perturbation applied. We compare the measurements to recent findings in tapped granular matter and we interpret our results with a rather simple picture.

Optimal size of an insulating inclusion acting as a pinning center for magnetic flux in superconductors: calculation of pinning force

By numerically solving the two-dimensional Ginzburg-Landau equations, we have for the first time evaluated the optimal size of an insulating inclusion for pinning a single-quantum vortex in a superconductor. Although it has so far been believed that the optimal size of a pinning center is twice the coherence length, we found for the first time that, for the case of low magnetic fields and low pinning center density, a prismatic insulating inclusion with a square cross-section of the side length approximately equal to the penetration depth gives the strongest pinning force to the vortex.

Vortex lattice melting, pinning and kinetics

The phenomenology of the high Tc superconductors is discussed both at the level of the thermodynamics of melting of the Abrikosov flux lattice and in terms of the melting and kinetics of the flux lattice for a pinned system. We review results on 3D melting obtained by a Monte Carlo simulation approach in which the 2D “pancake” vortices are treated as statistical variables. We discuss pinning in the context of the strong pinning regime in which the vortex density given in terms of the applied field B is small compared to that represented by an effective field Bpin measuring the pinning center density. We introduce a new criterion for the unfreezing of a vortex glass on increase of magnetic field or temperature, in the strong pinning, small field limit. We model this limit in terms of a single flux line interacting with a columnar pin. This model is studied both analytically and by computer simulation. By applying a tilt potential, we study the kinetics of the vortex motion in an external current and show that the resulting current-voltage characteristic follows a basic vortex glass-like scaling relation in the vicinity of the depinning transition.

V-Ga/Cu-Ga-Mg合金複合加工法による超電導V<SUB>3</SUB>Gaテープの特性改善

Effects of gallium and magnesium additions on the superconducting critical temperature, Tc, the upper critical magnetic field, Hc2, and the critical current density, Jc, of the composite processed V3Ga have been studied. Specimens were prepared by fabricating a composite consisting of a V-Ga alloy core and a Cu-Ga-Mg alloy matrix into tapes, and by reacting them to form V3Ga layers. The addition of 3 and 6 at%Ga to the core and that of 0.5 at%Mg to the matrix raise Tc by 0.1-0.2 K. The simultaneous addition of gallium and magnesium increases Tc by 0.2-0.3 K; the highest Tc obtained is 15.0 K. The addition of gallium to the core increases Hc2 by 1-2T. An Hc2 exceeding 22T has been obtained at 4.2 K by the simultaneous addition of gallium and magnesium. The improvement in Hc2 can primarily be attributed to the increase in Tc by the addition of these elements. Jc′s are strongly dependent on the microstructure of V3Ga which is varied by the additional element and the heat treatment condition. The magnesium addition makes V3Ga grains much finer. This grain refinement increases the flux pinning center density and produces high Jc′s. In the V-(3-6) Ga/Cu-19Ga-0.5Mg specimens high Jc values are obtained in the entire magnetic field range; Jc′s exceeding 1×109 A/m2 have been obtained at 4.2 K and 20 T. The present study reveals that the fabrication of multifilamentary V3Ga wires with excellent stability and high-field performance can be expected by the simultaneous addition of gallium and magnesium.