Irreversibility Field Research Articles

Synthesis of High Critical Current Density MgB2 Bulks by an Improved Mg-diffusion Method

MgB2 bulks with a density of 1.95 g/cm3, were prepared by an improved Mg-diffusion method. The effects of different heat treatments on superconducting transitions temperature Tc and critical current density Jc of MgB2 were investigated. The MgB2 synthesized by the optimal heat treatment has the Tc of 38.1 K and Jc of 0.53 MA/cm2 at 10 K, self-field. In order to improve poor high field properties in a diffusion method, nano-Pr6O11 and C doping effects on Jc and the irreversible field, Hirr of MgB2 were also studied. The results show that at 10 K and 6 T the Jc for 15 at% C-doped MgB2 reaches 104 A/cm2, which is almost two orders of magnitude higher than that of the undoped one, and better than that of the nano-C doped MgB2 prepared by solid-state reaction. For the nano-Pr6O11-doped MgB2, the Jc also increases by a factor of 9.4 than that of the pure sample at 10 K and 2 T. According to a lot of experimental results and theoretic analyses, we propose that the combination of the improved Mg-diffusion method and chemical doping of nanoparticles or C, may be an efficient approach to make the MgB2 have excellent Jc and Hirr capability in whole magnetic field.

Effect of magnetic field on the flux pinning mechanisms in Al and SiC co-doped MgB2 superconductor

MgB2 superconductor samples co-doped with 0.02 wt. Al2O3 and 0–0.05 wt. SiC were studied by magnetization – magnetic field (M-H) loop measurements at different temperatures. The critical current density has been calculated by the Bean model, and the irreversibility field, Hirr, has been obtained by the Kramer method. The pinning mechanism of the co-doped sample with 2% Al and 5% SiC was investigated in particular due to its having the highest Hirr. The normalized volume pinning force f = F/Fmax as a function of reduced magnetic field h = H/Hirr has been obtained, and the pinning mechanism was studied by the Dew-Houghes model. It was found that the normal point pinning (NPP), the normal surface pinning (NSP), and the normal volume pinning (NVP) mechanisms play the main roles. The magnetic field and temperature dependence of contributions of the NPP, NSP, and NVP pinning mechanisms were obtained. The results show that the contributions of the pinning mechanisms depend on the temperature and magnetic field. From the temperature dependence of the critical current density within the collective pinning theory, it was found that both the δl pinning due to spatial fluctuations of the charge-carrier mean free path and the δTc pinning due to randomly distributed spatial variations in the transition temperature coexist at zero magnetic field in co-doped samples. Yet, the charge-carrier mean-free-path fluctuation pinning (δl) is the only important pinning mechanism at non-zero magnetic fields.

Ex Situ Spark Plasma Sintering of Short Powder-in-Tube MgB2 Tapes with Open and Closed Ends

Short powder-in-tube tapes of MgB2 in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB2 core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB2 crystallite size, a weak contact between Fe and MgB2 core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB2 cores, the detected impurity phase is MgO, while at the interface with Fe, MgB4 also occurs. Impurity phases found at interface, MgO and MgB4, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB2 powder has an important contribution to observed differences, and its removal or control is recommended.

Formation of artificial flux pinning centers in Bi-2223 cuprate superconductor with Ni impurities and enhanced resistant to thermal fluxon motions of correlated 2D pancake vortices in new matrix

This comprehensive study attempts to find out the evaluation of magnetoresistivity performance and flux pinning centers in the Bi-2223 superconductor with nickel impurities and diffusion annealing temperature interval 650 °C–850 °C by means of magnetotransport measurements (0–5T). It is found that the cooper-pair probabilities as well as pinning of thermal fluxon motions of two-dimensional (2D) pancake vortices and interlayer Josephson coupling between the isolated grains considerably improve with the annealing temperature up to 700 °C as a result of enhancement in the effective flux pinning regions. However, from 700 °C onwards the potential energy barriers decrease significantly due to the increment of permanent structural problems in the crystal system. Namely, the presence of excess Ni impurities develops the recoupling linelike/discrete pancakelike nature. In this respect, the bulk sample exposed to the diffusion annealing temperature of 850 °C exhibits the worst magnetic performance characteristics. The flux lines in the material more easily jump to neighboring states due to the cooper pair-breaking mechanism, and thus the thermal fluxon motions of 2D pancake vortices lead to the energy dissipation. We also discuss the differentiation of vortex lattice elasticity for the materials with the aid of the theoretically calculated quantities such as the thermodynamic critical field (μ0Hc), irreversibility field (μ0Hirr), lower critical field (μ0Hc1), upper critical field (μ0Hc2), penetration depth (λ), coherence length (ξ) and Ginzburg Landau parameter (κ). According to the computational values, the optimum annealing temperature of 700 °C strengthens the pinning of 2D pancake vortices, vortex lattice period and elasticity as a result of the decrease in the inter-plane interaction energy. To sum up, 700 °C favors the applications of bulk Ni surface-layered Bi-2223 compounds in rather higher magnetic fields for a major innovation.

The Effect of Using Different Y2O3 Layers on the Activation Energy and Irreversibility Line of MPMG YBCO Bulk at 1050 °C Growth Temperature

In this study, three kinds of YBCO samples which are named Y1, Y2 and Y3 were fabricated by a melt–powder–melt–growth (MPMG) method. The Y1 sample was placed into a platinum (Pt) crucible without Y2O3, the Y2 sample was located on a Al2O3 crucible with a freely poured Y2O3 powder and the Y3 sample was located on a Al2O3 crucible with a 1-mm-thick buffer layer of Y2O3. YBCO samples were investigated by magnetoresistivity (ρ–T) measurements in dc magnetic fields (parallel to the c-axis) up to 5 T. The effect of the Y2O3 layer on the activation energy and irreversible flux of the samples was studied. The activation energies (U) were determined using the Arrhenius activation energy law from ρ–T. The power law relationship for U with H−α was investigated. α was calculated to find out which defects were dominant in the samples. Irreversibility fields (Hirr) and upper critical fields (Hc2) were obtained using 10 and 90% criteria of the normal-state resistivity value from ρ–T curves. Irreversibility lines (ILs) were estimated from the equation Hirr ∼ (1 − Tirr(H)/Tirr(0))n. The fitting results to giant flux creep and vortex glass models were discussed.

Hydrostatic pressure-induced huge enhancement of critical current density and flux pinning in Fe1−xCoxSe0.5Te0.5 single crystals

We performed a systematic study of the hydrostatic pressure (HP) effect on the supercon-ducting transition temperature (Tc), critical current density (Jc), irreversibility field (Hirr), upper critical field (Hc2), and flux pinning mechanism in un-doped and 3 at.% Co-doped FeSe0.5Te0.5 crystals. We found that Tc is increased from 11.5 to 17 K as HP increases from 0 to 1.2 GPa. Remarkably, the Jc is significantly enhanced by a factor of 3 to 100 for low and high temperature and field, and the Hirr line is shifted to higher fields by HP up to 1.2 GPa. Based on the collective pinning model, the δl pinning associated with charge-carrier mean free path fluctuation is responsible for the pinning mechanism of Fe1−xCoxSe0.5Te0.5 samples with or without pressure. A comprehensive vortex phase diagram in the mixed state is constructed and analysed for the 3 at.% Co-doped sample.

Inhomogeneity and irreversibility field of superconducting Nb-Ti tapes

Magnetizations and current-voltage characteristics of superconducting tapes from Nb-50% wt.Ti alloy are studied. In the region of external magnetic fields close to the upper critical field, there is a so called irreversibility field, which is often associated with the phase transition of vortex matter. However, in this study it is shown that the whole volume of experimental data can be described if we abandon the hypothesis of a phase transition and go over to the hypothesis of macroscopic inhomogeneity. This hypothesis implies the existence of two superconducting components with different upper critical fields, which naturally arise as a result of the production process.

Update on Microinvasive Glaucoma Surgery.

Update on Microinvasive Glaucoma Surgery.

Efficacy and safety of vigabatrin in Japanese patients with infantile spasms: Primary short-term study and extension study

Vigabatrin was approved for the treatment of infantile spasms by the US Food and Drug Administration, but not in Japan at the time of initiating this clinical study because of concerns about irreversible peripheral visual field defects (VFDs). This study evaluated the efficacy and safety of vigabatrin for Japanese patients with infantile spasms. Of 15 patients (aged ≥4weeks and <2years) enrolled, with the exception of two patients who did not receive vigabatrin, 13 were treated with a titrated dosage of vigabatrin (50–150mg/kg/day; limited to 3000mg/day). Twelve out of 13 patients receiving vigabatrin had spasms that were treatment refractory; these patients were concurrently treated with at least one other antiepileptic drug. One patient received vigabatrin monotherapy. Eight of the 13 patients (61.5% [95% CI: 31.6–86.1%]) had a ≥50% reduction during the dose-adjustment phase compared with baseline in the frequency of spasms, with efficacy maintained through a 2-week maintenance phase. Spasms disappeared in six out of nine patients (66.7% [95% CI: 29.9–92.5%]) who transitioned to the maintenance phase and hypsarrhythmia on electroencephalography also resolved in four patients. Hypsarrhythmia was improved in another two patients. Six out of seven patients who continued treatment through Week 32 of an extension study reported ongoing efficacy for vigabatrin. The most common adverse events (AEs) were psychiatric disorders and nervous system disorders (n=8; 61.5%) that were generally mild in severity. No treatment-related peripheral VFDs were observed. No severe AEs or AEs resulting in discontinuation of vigabatrin therapy were reported. An abnormality in magnetic resonance images was observed in one patient during the extension period. Vigabatrin was deemed to be clinically effective and well tolerated in Japanese patients with infantile spasms.

Substitution effect of Sr2+ by Ca2+ on structure and superconducting properties of Bi2Sr1.6La0.4CuO6+δ (Bi-2201) ceramics

In this work, the effect of Ca2+ iso-valence substitution for Sr2+ on properties of Bi2Sr1.6La0.4CuO6+δ superconductors is presented. Samples series with nominal composition of Bi2Sr1.6-xCaxLa0.4CuO6+δ (x= 0, 0.2, 0.4, 0.6 and 0.8) are prepared by a solid-state reaction method. When Ca content is increased, the X-ray diffraction technique shows that the cell parameters a and c decrease, while b one is almost constant. The scanning electron microscopy analysis reveals that the substitution has no significant effect on the porosity and the grain size of the samples. The physical properties of the samples are studied by the analysis of the magneto-resistivity curves measured under magnetic fields in the range 0-1 T. As Ca is added, the results show that the high temperature transition appears and is pushed up to 94.87 K for x=0.8. The substitution also improves the bulk onset critical transition temperature, the transition width, the residual resistivity, the activation energy of vortices and the irreversibility field. The best results are seen for x=0.4 of Ca content.

The impact of glaucoma on quality of life in Ethiopia: a case-control study

BackgroundGlaucoma is a chronic disease characterized by irreversible optic nerve damage and visual field loss that leads to visual impairment and blindness; ultimately limiting personal independence and compromising overall quality of life of affected individuals. There is paucity of information on how glaucoma affects the quality of life of patients in low and middle-income countries where resources for both diagnosis and treatment of such conditions are limited. In this study we investigate the impact of glaucoma on quality of life in Ethiopian patients.MethodsThe quality of life of 307 glaucoma patients and 76 normal controls that were frequency matched to the age and sex profiles of the cases was assessed using Amharic version of Glaucoma Quality of Life −15 questionnaire. Linear regression models and the t-test were employed to compare significant differences in GQL-15 scores and to generate mean and mean differences between cases and controls respectively.ResultsThe mean GQL-15 score in the glaucoma cases was substantially higher (indicating poorer quality of life) than the controls [cases 46.3 (95% CI, 28.8–63.8) and controls 18.6 (95% CI, 15.2–22.0), p < 0.0001]. Cases with normal visual acuity and mild glaucoma had significantly higher scores than the controls. Poorer quality of life was associated with age ≥ 71 years old 51.1 (95%CI, 26.2–75.9), rural residence 55.7 (95%CI, 49.9–61.5), monthly income of <400 Birr (53.1; 95%CI, 50.5–55.6), diagnosis time 1–5 years (49.6; 95%CI, 41.2–57.9), severe visual impairment (70.5; 95%CI, 58.1–82.8), and advanced glaucoma (50.9; 95%CI, 43.6–58.3).ConclusionThese glaucoma patients, including those with normal visual acuity and early disease, had poorer quality of life compared to normal controls. Older age, rural residence, low income and more advanced disease were significantly associated with poorer quality of life. There is a need to increase awareness of the impact of glaucoma among clinicians, patients and their families, for a better understanding of the impact this disease has on a person’s life.

Collective vortex pinning and merging of the irreversibility line and second peak effect in optimally doped Ba[formula omitted]KxBiO3 single crystals

Measurements on magnetization and relaxation have been carried out on an optimally doped Ba1−xKxBiO3+δ single crystal with Tc = 31.3 K. Detailed analysis is undertaken on the data. Both the dynamical relaxation and conventional relaxation have been measured leading to the self-consistent determination of the magnetization relaxation rate. It is found that the data are well described by the collective pinning model leading to the glassy exponent of about μ ≈ 1.64–1.68 with the magnetic fields of 1 and 3 T. The analysis based on Maley’s method combining with the conventional relaxation data allows us to determine the current dependent activation energy U which yields a μ value of about 1.23–1.29 for the magnetic fields of 1 and 3 T. The second magnetization peaks appear in wide temperature region from 2 K to 24 K. The separation between the second peak field and the irreversibility field becomes narrow when temperature is increased. When the two fields are close to each other, we find that the second peak evolves into a step-like transition of magnetization. Finally, we present a vortex phase diagram and demonstrate that the vortex dynamics in Ba1−xKxBiO3 can be used as a model system for studying the collective vortex pining.

Field induced metastable ferroelectric phase in Pb0.97La0.03(Zr0.90Ti0.10)0.9925O3 ceramics

Pb0.97La0.03(Zr0.9Ti0.1)0.9925O3 (PLZT 3/90/10) ceramics prepared by solid-state reaction with the compositions near the antiferroelectric/ferroelectric (FE/AFE) phase boundary were studied. From the polarization–electric field P(E) dependence and ex situ X-ray study, an irreversible electric field induced AFE-to-FE phase transition is verified at room temperature. Dielectric and in situ temperature dependent X-ray analysis evidence that the phase transition sequence in PLZT 3/90/10-based ceramics can be readily altered by poling. A first order antiferroelectric-paraelectric (AFE-to-PE) transition occurred at ∼190°C in virgin sample and at ∼180°C in poled sample. In addition, a FE-to-AFE transition occurs in the poled ceramic at much lower temperatures (∼120°C) with respect to the Curie range (∼190°C). The temperature-induced FE-to-AFE transition is diffuse and takes place in a broad temperature range of 72–135°C. The recovery of AFE is accompanied by an enhancement in the piezoelectric properties.

Vortex pinning properties and microstructure of MgB2 heavily doped with titanium group elements

The doping of titanium group elements (TGE; Ti, Zr, Hf) to MgB2 is known to improve vortex pinning properties. However, the optimal doping level of each TGE is still a controversial issue. In this study, to improve the critical current density, , in high magnetic-field by the TGE-doping, we have studied the vortex pinning properties of (MgTGEx)B2 samples with x ranging in the interval . The of 1.2 kA cm−2 at 20 K in 3 T for the pristine sample was maximized to 2.9 kA cm−2 for the Ti20%-doped sample, 4.9 kA cm−2 for the Zr30%-doped one, and 5.2 kA cm−2 for the Hf30%-doped one under identical temperature and magnetic field. The irreversibility field, , of 3.8 T for the pristine sample was also shifted to a higher field of 4.2 T, 4.3 T and 4.6 T by the Ti20%-, Zr30%- and Hf30%-doping, respectively. Microstractural observations suggested that the grain size, shape and distribution of the MgB2 and impurity phases depended strongly on the species of TGE. We discuss the mechanism of the enhanced vortex pinning properties of the TGE-doped MgB2.

Point defect induced giant enhancement of flux pinning in Co-doped FeSe0.5Te0.5 superconducting single crystals

Point defect pinning centers are the key factors responsible for the flux pinning and critical current density in type II superconductors. The introduction of the point defects and increasing their density without any changes to the superconducting transition temperature Tc, irreversibility field Hirr, and upper critical field Hc2, would be ideal to gain insight into the intrinsic point-defect-induced pinning mechanism. In this work, we present our investigations on the critical current density Jc, Hc2, Hirr, the activation energy U0, and the flux pinning mechanism in Fe1-xCoxSe0.5Te0.5 (x = 0, 0.03 and 0.05) single crystals. Remarkably, we observe that the Jc and U0 are significantly enhanced by up to 12 times and 4 times for the 3at.% Co-doped sample, whereas, there is little change in Tc, Hirr, and Hc2. Furthermore, charge-carrier mean free path fluctuation, δl pinning, is responsible for the pinning mechanism in Fe1-xCoxSe0.5Te0.5.

Nematic topological superconducting phase in Nb-doped Bi2Se3

A nematic topological superconductor has an order parameter symmetry, which spontaneously breaks the crystalline symmetry in its superconducting state. This state can be observed, for example, by thermodynamic or upper critical field experiments in which a magnetic field is rotated with respect to the crystalline axes. The corresponding physical quantity then directly reflects the symmetry of the order parameter. We present a study on the superconducting upper critical field of the Nb-doped topological insulator NbxBi2Se3 for various magnetic field orientations parallel and perpendicular to the basal plane of the Bi2Se3 layers. The data were obtained by two complementary experimental techniques, magnetoresistance and DC magnetization, on three different single crystalline samples of the same batch. Both methods and all samples show with perfect agreement that the in-plane upper critical fields clearly demonstrate a two-fold symmetry that breaks the three-fold crystal symmetry. The two-fold symmetry is also found in the absolute value of the magnetization of the initial zero-field-cooled branch of the hysteresis loop and in the value of the thermodynamic contribution above the irreversibility field, but also in the irreversible properties such as the value of the characteristic irreversibility field and in the width of the hysteresis loop. This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic topological superconductor similar to the Cu- and Sr-doped Bi2Se3.

Vortex dynamics and the critical current density in NdFeAsO0.9F0.1 single crystals

The importance of type-II superconductors with strong pinning comes from their ability to carry large transport currents in the presence of a magnetic field. We report on the results of the bulk magnetization measurements in the superconducting state in clean single crystals of NdFeAsO0.9F0.1 at various temperatures. The zero-temperature superconducting critical current density, Jc, where the superconducting transition temperature Tc reaches a maximum of 45(1) kelvin, displays a remarkable high value. The field dependence of the critical current density was analyzed within the collective pinning model. A remarkably good agreement between the experimental results and theoretical δ1 pinning curve is obtained. This indicates that pinning in NdFeAsO0.9F0.1 crystal originates from spatial variation of the mean free path. Additionally, the normalized pinning force density, Fp, curves verses H/Hirr (Hirr is the irreversibility field) were scaled using the Dew-Hughes model. Our analysis suggests that point pinning alone cannot explain the observed field variation of Fp.

Defining the electric field threshold optimizing prostate cancer treatment with irreversible electroporation

International group of researchers determine the irreversible electroporation electric field threshold needed to safely remove human prostate cancer tissue.

Strongly enhanced irreversibility field and flux pinning force density in SmBa2Cu3Oy-coated conductors with well-aligned BaHfO3 nanorods

A high flux pinning performance was obtained for a 3.8 vol % BaHfO3-doped SmBa2Cu3Oy superconducting film on a metallic substrate. At a temperature of 77.3 K, an irreversibility field of 16.8 T and the maximum flux pinning force density of 32.5 GN/m3 in fields applied parallel to the c-axis of the film were achieved, which are the highest values reported thus far for REBa2Cu3Oy films, to our knowledge. The introduction of well-aligned BaHfO3 nanorods with a high number density into REBa2Cu3Oy films with little or no degradation of the critical temperature is an effective method for improving the flux pinning performance.

Upper critical and irreversibility fields in Ni- and Co-doped pnictide bulk superconductors

A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe0.95Ni0.05)2As2 (large grain and small grain samples), Ba(Fe0.94Ni0.06)2As2, Ba(Fe0.92Co0.08)2As2, and Ba(Fe0.92Co0.09)2As2 polycrystalline bulk pnictide superconductors was made in pulsed fields of up to 65T. The full magnetic field-temperature (H-T) phase diagrams, starting at 1.5K, were measured. The higher temperature, upper critical field Hc2 data are well described by the one-band Werthamer, Helfand, and Hohenberg (WHH) model. At low temperatures, the experimental data depart from the fitted WHH curves, suggesting an emergence of a new phase that could be attributed to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The large values of the Maki fitting parameter α indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant in these materials. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model. Table of measured physical parameters allows us to compare these pnictide superconductors for different Co- and Ni- doping levels and granularity.