Secondary Peak Effect Research Articles

Effects of Zn doping on superconducting properties of melt textured Y–Ba–Cu–O bulk superconductors

Zn doping which may induce a second peak effect in YBa2Cu3O7−δ (YBCO) has been opted as a regular technique to improve the trapped field and flux pinning stability of YBCO bulk superconductors for practical applications, such as quasi-permanent or maglev magnets. Here, the effects of Zn doping on the flux trapping performance of single domain YBCO bulk superconductors have been systematically investigated. The doping amount x varies from 0 to 1.0 mol%. We analyzed the critical transition temperature (T c), critical current density (J c), trapped field (B t) and the flux relaxation at various temperatures, and measured in addition the levitation force (F L) for comparison. The doping amount of 0.6 mol% has been proven to be the optimal value, which enhances self-field J c by 243.3% at 30 K with an obvious secondary peak effect, resulting in great improvements in trapped field and flux stability. The holistic and local superconducting performances exhibit good agreement, demonstrating Zn-doping an effective method to improve flux pinning properties for practical applications.

High-Brightness InGaN/GaN Micro-LEDs With Secondary Peak Effect for Displays

The green InGaN/GaN micro light-emitting diode (Micro-LED) is a significant component in micro-display whereas lack of size dependence and high injected current analysis. In this letter, different size Micro-LEDs were fabricated and the electro-optical characteristics were measured. The size-dependence phenomenon for EQE “efficiency droop”, luminous flux and brightness were observed and discussed. All sizes Micro-LEDs exhibited an extremely high brightness as 37.5k and 67.6k nits at 1 A/cm2, 2.89M and 3.81M nits at 300 A/cm2 for 25 and $200~\mu \text{m}$ respectively. In addition, the elevating current densities from 160 to 6400 A/cm2 were injected into $25~\mu \text{m}$ Micro-LED and the electroluminescence (EL) spectra transformation of different current densities were depicted. The green to blue shift and secondary peak effect, which promise an opportunity to modulate the emission wavelength for higher quality and lower the barrier of mass transfer technology, were observed and analyzed through the spectra.

Influence of Sm2O3 microalloying and Yb contamination on Y211 particles coarsening and superconducting properties of IG YBCO bulk superconductors

Single grain YBa2Cu3O7−x (YBCO or Y123) bulk superconductors were produced by an infiltration growth process. The solid phase precursor was prepared by solid state synthesis from Y2O3 + BaCuO2 powders. The influence of the addition of Sm2O3 and YB contamination from the substrate on the microstructure and superconducting properties was analyzed. The dependences of Yb concentration on the distance from the bottom of the samples measured by energy dispersive spectroscopy microanalysis used in conjunction with scanning electron microscopy confirmed the contamination of the samples during the melting stage of the sample preparation. It is shown that the addition of Sm in low concentration and its combination with Yb from the substrate modify the coarsening of the Y211 particles as well as lead to the appearance of a secondary peak effect in the field dependences of the critical current density.

Doping dependence of the vortex dynamics in single-crystal superconducting NaFeCoxAs

We investigate the doping dependence of flux pinning in superconducting NaFeCoxAs (x = 0.01, 0.03, 0.05 and 0.07) single crystals grown by the Bridgman method. The electronic specific heat displays a pronounced anomaly in a sample series at superconducting transition temperature, which hardly shows any residual part at low temperature. We found that Co doping plays an important role in signifying the secondary peak in the magnetic hysteresis of optimally doped (x = 0.03) and heavily doped (x = 0.05, 0.07) crystals. Furthermore, the dependence of the relaxation rate S = ∣d ln M/d ln t∣ on magnetic field and temperature exhibits a decreasing trend within a certain range corresponding to the secondary peak effect in the optimally and heavily doped samples. The magnetic relaxation rate combined with the Maley analysis of the current-dependent creep energy shows a single-vortex pinning in the lightly doped sample dominant at low applied fields, and plastic pinning at high applied fields, without showing a secondary peak. However, in the optimally and heavily doped samples, the magnetic relaxation rate and U(J) isothermal analysis show that the collective pinning that dominates below Hpeak crosses over to plastic pinning for fields above Hpeak.

Equilibrium states of pancake vortices in layered superconductors: coupling of inter-layer ordering and in-layer ordering

We numerically study the static configurations of pancake vortices in layered superconductors. We analyze how disordering induced by random pinning centers competes with inter-layer ordering and in-layer ordering. In general, for inter-layer ordering, 3D states composed of coupled vortex lines are formed for strong inter-layer coupling strength and weak pinning strength. In contrast, 2D states composed of decoupled individual pancake vortices are formed for weak inter-layer coupling strength and strong pinning strength. For in-layer ordering, with increasing pinning forces, the in-layer structure evolves from crystals to Bragg glasses (BGs), vortex glasses (VGs), and liquid-like structures. Changing the vortex density, an initially fast disordering, then slow ordering procedure is found for both in-layer and inter-layer ordering, which is a possible clue to second peak effect (SPE). The reason behind this non-monotonic behavior is discussed. Our results are summarized in a phase diagram in the plane of ‘inter-layer coupling strength sm versus pinning strength fp.

Magnetization relaxation, critical current density, and vortex dynamics in aBa0.66K0.32BiO3+δsingle crystal

We have conducted extensive investigations on the magnetization and its dynamical relaxation on a Ba$_{0.66}$K$_{0.32}$BiO$_{3+\delta}$ single crystal. It is found that the magnetization relaxation rate is rather weak compared with that in the cuprate superconductors, indicating a higher collective vortex pinning potential (or activation energy), although the intrinsic pinning potential $U_\mathrm{c}$ is weaker. Detailed analysis leads to the following discoveries: (1) A second-peak effect on the magnetization-hysteresis-loop was observed in a very wide temperature region, ranging from 2K to 24K. Its general behavior looks like that in YBa$_2$Cu$_3$O$_7$; (2) Associated with the second peak effect, the magnetization relaxation rate is inversely related to the transient superconducting current density $J_\mathrm{s}$ revealing a quite general and similar mechanism for the second peak effect in many high temperature superconductors; (3) A detailed analysis based on the collective creep model reveals a large glassy exponent $\mu$ and a small intrinsic pinning potential $U_\mathrm{c}$; (4) Investigation on the volume pinning force density shows that the data can be scaled to the formula $F_{p}\propto b^p(1-b)^q$ with $p=2.79$ and $q=3.14$, here $b$ is the reduced magnetic field to the irreversible magnetic field. The maximum normalized pinning force density appears near $b\approx0.47$. Finally, a vortex phase diagram is drawn for showing the phase transitions or crossovers between different vortex phases.

Analysis of peak effect in the critical current density and flux pinning properties in iron based superconducting Ca10(Pt4As8)(Fe2−xPtxAs2)5 single crystal

We present the magnetic properties of the iron based superconducting Ca10(Pt4As8)(Fe2-xPtxAs2)5 (x =0.01) single crystal as a function of field and time. The single crystals were grown by Bridgman method and showed Tc of 32 K. The results show that the well-known secondary peak effect becomes prominent in certain temperature range in the field dependent magnetic hysteresis. Moreover, the maximum of pinning force density Fmax versus Hmax follows a scaling law Fp,max ≈ 49.5Hmax3/2 indicating the single type pinning mechanism is involved in the sample within our measured temperature range. The time dependence of the magnetization indicates thermally activated flux motion and the relaxation rate reveals a plateau in the temperature region where secondary peak effect is more prominent after that a sudden increase is observed. We found that the magnetization dependence of activation energy U obtained by Maley's method follows a power law in low temperature region with exponent μ = 0.15 which is in good agreement with collective pinning theory.

Vortex phase diagram and temperature-dependent second-peak effect in overdopedBi2Sr2CuO6+δcrystals

We study the vortex phase diagram of the single-layer ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CuO}}_{6+\ensuremath{\delta}}$ (Bi2201) superconductor by means of bulk magnetization measurements on high-quality oxygen-overdoped crystals. In striking contrast with the results found in the moderately doped two- and three-layer Bi-based cuprates, Bi2201 exhibits a strong temperature-dependent second-peak effect. By means of measurements of the in-plane and out-of-plane first-penetration field we provide direct evidence that this phenomenon is mainly associated to an increase in the electromagnetic anisotropy on warming. The effect of oxygen-doping $\ensuremath{\delta}$ on the vortex phase diagram results in both the irreversibility and second-peak lines shifting to higher temperatures and fields. This enhanced stability of the Bragg glass phase suggests that the interlayer coupling between Cu-O layers increases with $\ensuremath{\delta}$. In addition, we found that the critical temperature follows the parabolic relation with the number of holes per Cu-O plane that holds for most single- and two-layer cuprates.

Effect of Ultrafine Gd211 on Processing and Superconducting Properties of Gdbacuo Bulk Superconductors

GdBaCuO bulk superconductors from precursors of Gd123 and ultrafine Gd211 powder have been fabricated by melt-textured-growth method in air. The ultrafine Gd211 powder was synthesized by an improved Pechini method. It was found that the lower temperature of synthesizing ultrafine Gd211 powder, the higher peritectic reaction temperature of GdBaCuO bulk superconductors. Jc reaches 4.7×104 A/cm2 at 77 K and self-field for ultrafine Gd211 starting particles with 0.2μm in diameter, and higher compared to larger Gd211 starting particles fabricated by solid state reaction. However, a large secondary peak effect did not appeared at Jc-B curves. ,

A new aspect of superconductivity observed (Bi, Pb)2(Sr, La)2CuO6+δ single crystals

Physical properties of superconductivity were studied with changing the doping level for the co-substitution of Pb for Bi and La for Sr in Bi-2201 phase. Zero-resistance Tc of 43K was obtained for as-grown crystals, and the anisotropy parameter γ was obtained from the resistivity ratio ρc/ρab. The second-peak effect was observed in the temperature range of 20–30K. In addition to the 40K-class superconductivity, a new superconductivity with the onset temperature of about 75K was observed in magnetic measurement. Irreversible behavior of the superconductivity was obtained in the magnetic field applied perpendicular to the sample surface.

Flux pinning at high magnetic field in melt-processedSmBa2Cu3O7 with nanocrystalline Sm211/Nd422 additives

Nano-sized Sm2BaCuO5 (‘Sm211’) and Nd4Ba2Cu2O10 (‘Nd422’) non-superconducting particles were prepared by a typical sol–gel process ofmixing poly-acrylic acid (PAA) with metal ion solutions. Small amounts (1 to1 × 10−2 wt%) of nano-sized Sm211 and Nd422 powders were mixed homogeneously with the precursors ofSm123+25 wt% Sm211 followed by the melt-texture processing in air. It is observed that superconductingproperties of SmBCO with nano-Sm211 or nano-Nd422 additives were greatly enhancedand a clear secondary peak effect was observed. The scale rule analysis shows that theδTc pinning mechanism may be responsible for theJc enhancement at high field. A systematic study of the relationship ofJc(H,T) with different amounts of nm-Sm211 and/or Nd422 additions was performed to determinethe optimum nano-particle doping contents and their pinning mechanisms respectively.

Growth, microstructures, and superconductivity of Bi2−xPbxSr2Ca1−yGdyCu2O8+z single crystals

Bi 2−x Pb x Sr 2 Ca 1−y Gd y Cu 2 O 8+δ (x=0.34 and y=0.18, 0.34) crystals were grown by the self-flux method. The crystals have a cleavage thickness of only half unit cell up to two unit cells with Tc only dropping 20 K as y is increased from 0.18 to 0.34 for as-grown crystals. However, Tc increased to almost the same value of about 80 K after annealing in air regardless of the Gd doping levels. The co-doping produced enhanced flux pinning compared to the sole Gd doping. A secondary peak effect presented in crystals with x=0.34 and y=0.34 was explained by phase segregations containing Gd-rich clusters.

Numerical simulation of a bulk superconductor during pulsed field magnetization

We have carried out numerical simulation for magnetization of bulk superconductors during pulse field magnetization based on the Bean's and Kim's models. The focus was placed on the simulation for the superconductors exhibiting the secondary peak effect. A temperature rise and trapped field profiles of the sample exhibiting the secondary peak effect in the J c– B curve were different from those without the secondary peak. We show that the temperature of the superconductor rose locally and the trapped magnetic field profile became concave at some point for the samples with the secondary peak effect.

Numerical simulation of a bulk superconductor during pulsed field magnetization

We have carried out numerical simulation for magnetization of bulk superconductors during pulse field magnetization based on the Bean's and Kim's models. The focus was placed on the simulation for the superconductors exhibiting the secondary peak effect. A temperature rise and trapped field profiles of the sample exhibiting the secondary peak effect in the J c – B curve were different from those without the secondary peak. We show that the temperature of the superconductor rose locally and the trapped magnetic field profile became concave at some point for the samples with the secondary peak effect.

Melt processing and superconducting properties of single-domain GdBa 2Cu 3O y, (Sm 0.5Gd 0.5)Ba 2Cu 3O y and (Sm 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y superconductors fabricated in air

Single-domains of GdBa 2Cu 3O y , (Sm 0.5Gd 0.5)Ba 2Cu 3O y and (Sm 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y with and without BaCuO 2− δ doping were successfully prepared under the ambient atmosphere via a two-step cooling method. At 77 K, all samples show a well-developed secondary peak effect. The combination of two-step cooling method and BaCuO 2− δ addition is very effective to suppress the LRE–Ba substitution and hence improve superconducting properties. The Gd–Ba–Cu–O samples exhibit very high J c values, which are comparable to those of the oxygen-controlled melt-growth (OCMG) Gd–Ba–Cu–O samples.

Effect of pinning on the flux motion of Gd–Ba–Cu–O bulk superconductors

We have observed the flux motion using Hall probe sensors that are placed on the surface of high quality Y–Ba–Cu–O and Gd–Ba–Cu–O disks 15 mm in diameter with 0.95 mm thickness with applying pulsed magnetic field. The peak value of the field was varied from 0.1 to 0.8 T. The effects of static bias fields was also studied in the field range of 0–3 T at 77 K. Gd–Ba–Cu–O shows the clear secondary peak effect on the J c– B curve so that the pinning property was different from Y–Ba–Cu–O for which J c monotonically decreases with field. The flux motion was enhanced in Y–Ba–Cu–O with increasing static bias field, while that was suppressed in Gd–Ba–Cu–O, reflecting the secondary peak effect.

Superconducting properties of melt-textured RE–Ba–Cu–O (RE: Ho, Er and Y) bulk superconductors with various RE211 contents

We investigated the effect of RE 2BaCuO 5 (RE211) contents on the superconducting properties of melt-textured RE–Ba–Cu–O bulk superconductors (RE: Ho, Er and Y). REBa 2Cu 3O y (RE123) samples containing 5–40 mol% of RE211 were fabricated with the cold-seeding method in air using an Nd123 bulk seed crystal. The superconducting properties of the Ho–Ba–Cu–O samples were strongly affected by the Ho211 content and the oxygen-annealing temperature. In the J c– B curves of the Ho–Ba–Cu–O samples with low-Ho211 contents, the secondary peak effect was clearly observed. The peak position shifted to lower fields with increasing oxygen-annealing temperature, presumably due to an increase in oxygen-vacancies. The secondary peak effect was also found in the Er– and Y–Ba–Cu–O samples with 5 mol% of RE211.

The influence of BaCuO2−δ addition in Gd–Ba–Cu–O single-grain superconductors fabricated in air

High quality Gd–Ba–Cu–O single-grain bulks with different BaCuO2−δ adding contents were grown in air via a hot-seeding melt process. At 77 K, all samples show a well-developed secondary peak effect with high Jc values, which are comparable to that of OCMG-processed Ga–Ba–Cu–O samples. The BaCuO2−δ addition can effectively suppress the Gd–Ba substitution and therefore enhance the superconducting properties. For the sample with a composition of Gd123:BaCuO2−δ = 1:0.3 in a molar ratio, the Jc(B = 0) and Jc,peak(B = 1.3 T) reach 6.45 × 104 A cm−2 and 6.04 × 104 A cm−2, respectively, which are the record high Jc values at 77 K for Gd–Ba–Cu–O samples melt processed in air.

E-J characteristics and n value of melt-textured REBa/sub 2/Cu/sub 3/O/sub x/ (RE: Nd, Y)

Melt-textured NdBa/sub 2/Cu/sub 3/O/sub x/ (Nd123) exhibits the secondary peak effect on the magnetic field dependence of the critical current density (J/sub c/), which is attributed to the field-induced pinning. The electric field (E)-the current density (J) characteristics will be affected by the field-induced pinning. With the aim of clarifying this effect, we have investigated the E-J characteristics for melt-textured Nd123 with the secondary peak and YBa/sub 2/Cu/sub 3/O/sub x/ (Y123) with smaller secondary peak through dc magnetization and ac susceptibility. The n value was estimated on the assumption that E follows the power law: E=E/sub c/(J/J/sub c/)/sup n/. The n value was about 30 at 77 K for Y123. In the case of Nd123, the peak structure was observed on the magnetic field dependence of the n value, which exceeded 60 at 77 K. This suggests that the n value is increased by the field-induced pinning.

Variation of pinning mechanism in Bi 1.6Pb 0.4Sr 2CaCu 2O 8+ δ single crystal

The pinning mechanism of Bi 1.6Pb 0.4Sr 2CaCu 2O 8+ δ single crystal was investigated by analyzing the normalized pinning forces. We found that our single crystal showed a very strong secondary peak effect in a wide temperature range, from 20 to 80 K ( T c=92 K). We observed that the pinning mechanism was changed from δ l pinning to δ T c pinning at low fields and from the catastrophic instability to a combination of the catastrophic instability and the avalanche instability at high fields with temperature.