δTc Pinning Research Articles

Role of electron–phonon coupling in the superconducting state of MgB2 polycrystals fabricated at different conditions

In this study, the superconducting properties of MgB2 bulk samples fabricated at different conditions were investigated. Ex situ MgB2 samples were further mixed with Mg and B at different amounts and sintered at different times and temperatures. Using Raman spectroscopy, the bearing of electron-phonon (e-ph) coupling (EPC) has been investigated. The dependence of temperature on magnetization, M(T), along with the dependence of magnetic field on magnetization, M(H), were evaluated to analyze the superconducting characteristics. Using the McMillan formular, which is modified by Allen–Dynes, and the frequency of phonon located in the mid peak centre ω2 (E2g), the contribution of EPC, whose value is closely proportional to the variation of critical temperature was reckoned. The lowest EPC value was determined by Raman spectrum analysis to be 1.094 for the MgB2 sample added with 0.5 Mg and sintered at 1000 °C for 1 h. In addition, the sample showed the lowest critical temperature along with the highest critical current density. The small bundle field (Bsb) as well as the large bundle field (Blb) were obtained using the collective pinning theory, demonstrating the expansion of the small also the large bundle regimes under suitable sample fabrication conditions. Based on the dependence of temperature on normalized critical current density (j) and normalized small bundle field (Bsb), δTc pinning is the predominant flux pinning mechanism across totally MgB2 bulk samples. The model of Dew–Hughes confirmed that the addition of Mg along with B creates surface pinning in the matrix by analyzing the pinning properties.

Coexistence of different pinning mechanisms in Bi-2223 superconductor and its implications for using the material for high current applications

We investigate the pinning mechanism in polycrystalline samples of Bi-2223. Using the differential magneto-optical (DMO) imaging technique, we track the magnetic field penetration in our sample. DMO imaging studies show circular regions with an average diameter of 20 μm with dark contrast appearing at temperatures near Tc. We identify these as strong-pinning regions with a substantially higher local penetration field than the surrounding regions. A unique feature of these strong-pinning centers is that they survive high temperatures (near Tc) and produce a non-Gaussian distribution of the penetration field strength. Analysis of the magnetic field dependence of the pinning force shows two distinct pinning mechanisms: a predominantly surface pinning mechanism is active at low temperatures well below Tc, while at higher temperatures near Tc, there is a crossover into a purely δTc pinning. Our studies show that surface pinning effects are most likely related to grain alignment, grain boundary, and voids in the sample. The strong δTc pinning is related to local stoichiometric fluctuations in the sample. One can potentially exploit this for enhancing the high T and Jc values of superconductors. We investigate the impact of these pinning centers on the current distribution in a macroscopic Bi-2223 superconducting cylindrical tube. We map the current distribution using an array of hall sensors distributed around the cylinder. The map reveals a non-uniform current distribution across the tube at high currents. This study suggests an inhomogeneous distribution of strong-pinning centers across large length scales in superconductors which are used for current lead applications.

From δl- to δT c-pinning in CaKFe4As4 single crystals obtained by adjusting their defect structures

We have investigated the vortices pinning behaviors of CaKFe4As4 with two types of defect structures by alternating current and direct current susceptibility measurements under the Ginzburg-Landau theory and collective pinning model. We observed a new dissipative peak at temperature T J in each out-of-phase () curve even at low magnetic field in the samples with thick CaFe2As2 intercalations, which has been ascribed to the weak Josephson-like link. The unrelaxed current J c and the normalized pinning energy g(t) have been investigated for both types of the CaKFe4As4 crystals based on the dynamic relaxation rate Q and transient current J s by means of generalized inversion scheme. For the samples with monolayer-KFe2As2 intercalations, the temperature dependence of J c and g(t) is found to be in agreement well with the model of single vortex δT c-pinning. But for the samples with thick CaFe2As2 intergrowths, both the δT c- and δl-pinning are in functioning at low magnetic field, but the δl pinning will play a leading role at magnetic field above 3.0 T. Differential charge analysis shows that the intergrowth of KFe2As2 should be normal phase and the CaFe2As2 will become superconducting one due to the charge transfer. The non-superconducting KFe2As2 will induce an abruptly spatial change of T c resulting to strong δT c-pinning, and the superconducting CaFe2As2 phase can mainly cause to fluctuate of mean free path leading to δl-pinning. The present work illustrates how the defect structure affects the flux pinning behaviors and can provide a new clue to design high-performance superconducting wire/tapes and films for strong magnetic field applications.

Enhancing the superconducting performance of melt grown bulk YBa2Cu3Oy via ultrasonically refined Y2BaCuO5 without PtO2 and CeO2

The electromagnetic properties of YBa2Cu3O7-δ (Y-123) were improved by reducing size of the secondary phase Y2BaCuO5 (Y-211) via ultra-sonication and optimization of the top seeded melt growth (TSMG). Ultra-sonication a clean and low-cost technique, which excels over other techniques in producing refined Y-211 particles with sharp edges, uniform size, and contamination-free surfaces. The Y-211 powder was ultra-sonicated for various intervals ranging from 0 to 100 min, with a constant power (300 W) and frequency (20 kHz). Single grain Y-123 bulks were prepared from these powders and their electromagnetic and superconducting performance was compared. The superconducting transition temperature of all the bulk samples was around 91 K and the transition width was less than 1.5 K. At 77 K, the highest critical current density (Jc) and trapped field was found in the sample with Y-211 ultra-sonicated for 100 min. Flux pinning studies revealed a strong contribution from δTc pinning. The present results demonstrate that the superconducting performance can be improved by ultra-sonication process producing nanometer-sized Y- 211 particles in bulk Y-123. This technique can help to produce large bulk superconductors for commercial exploitation.

The second peak effect and vortex pinning mechanisms in Ba(Fe,Ni)2As2 superconductors

Vortex pinning mechanisms have been studied systematically in BaFe1.9Ni0.1As2 single crystal as a function of temperature and magnetic field. The obtained shielding current density, Js, showed a second peak in the intermediate magnetic field range at high temperatures. The temperature dependence of the shielding current density, Js(T), was analysed within the collective pinning model at different magnetic fields. It was found that the second peak reflects the coexistence of both δl pinning, reflecting spatial variation in the mean free path (l), and δTc pinning, reflecting spatial variation in the superconducting critical temperature (Tc) at low temperature and low magnetic fields in BaFe1.9Ni0.1As2 single crystal. The results clearly show that pinning mechanism effects are strongly temperature and magnetic field dependent, and the second peak effect is more powerful at higher temperatures and magnetic fields. It was also found that the magnetic field mainly controls the pinning mechanism effect.

Dwell Time Influence on Spark Plasma-Sintered MgB2

Samples of MgB2 with relative density above 95% were obtained by spark plasma sintering (SPS) at 1150 °C for a heating time of 100 °C/min and a maximum pressure of 95 MPa. Dwell time was of 1, 4, 7, 10, and 20 min. Samples show that dwell time has a low influence on superconducting characteristics. However, small differences were observed and they are discussed from the viewpoint of density, structural, microstructural, critical current density, irreversibility field, and pinning type. The dominant contribution for all investigated samples is given by point and δ T c pinning.

Role of nano and micron-sized inclusions on the oxygen controlled preform optimized infiltration growth processed YBCO superconductors

In the present work, with the aim of improving the local flux pinning at the unit cell level in the YBa2Cu3O7-δ (YBCO) bulk superconductors, 20wt% of nanoscale Sm2O3 and micron sized (Nd, Sm, Gd)2BaCuO5 secondary phase particles were added to YBCO and processed in oxygen controlled preform optimized infiltration growth process. Nano Dispersive Sol Casting method is employed to homogeneously distribute the nano Sm2O3 particles of 30–50nm without any agglomeration in the precursor powder. Microstructural investigations on doped samples show the chemical fluctuations as annuli cores in the 211 phase particles. The introduction of mixed rare earth elements at Y-site resulted in compositional fluctuations in the superconducting matrix. The associated lattice mismatch defects have provided flux pinning up to large magnetic fields. Magnetic field dependence of current density (Jc(H)) at different temperatures revealed that the dominant pinning mechanism is caused by spatial variations of critical temperatures, due to the spatial fluctuations in the matrix composition. As the number of rare earth elements increased in the YBCO, the peak field position in the scaling of the normalized pinning force density (Fp/Fp max) significantly gets shifted towards the higher fields. The curves of Jc(H) and Fp/Fp max at different temperatures clearly indicate the LRE substitution for LRE' or Ba-sites for δTc pinning.

Flux pinning mechanism in SiC and nano-C doped MgB2: evidence for transformation from δTc to δℓ pinning

Magnetic and transport properties of 10 wt% SiC doped MgB2 and 5 wt% nano-C doped MgB2 were studied by resistance and critical current density measurements. The results showed improvement of the critical current density for the MgB2 superconductor doped with SiC in comparison with the nano-C doped sample. The flux pinning mechanisms of both doped MgB2 superconductors have been investigated based on the collective theory. It was found that the pinning mechanism in MgB2 was transformed by SiC doping from transition temperature fluctuation induced pinning, δTc pinning, to mean free path fluctuation induced pinning, δℓ pinning, while in the MgB2 doped with nano-C, δTc and δℓ pinning coexist. Their contributions are strongly temperature dependent, however. The δℓ pinning is dominant at low temperature, decreases with increasing temperature, and is suppressed completely at temperatures close to Tc. The δTc pinning mechanism shows the opposite trend.

A comprehensive study of the pinning mechanisms of MgB2 wires treated with malic acid and their relationships with lattice defects

The effects of sintering temperature on the lattice parameters, structural strain, critical temperature (Tc), critical current density (Jc), irreversibility field (Birr), upper critical field (Bc2), and resistivity (ρ) of MgB2 wires treated with 10 wt.% malic acid (C4H6O5) are investigated in this paper. The a-lattice parameter of the sample treated with malic acid was drastically reduced, to 3.0745 Å, as compared to those for the undoped samples. Reduction in the a-lattice parameter is related to crystalline imperfections arising from carbon substitution—as confirmed by x-ray diffraction and Raman spectra—which play a vital role in enhancing Jc, Bc2 and Birr. We have also analyzed the pinning mechanisms, and concluded that flux pinning is dominated by point and correlated pinning at lower and higher magnetic fields, respectively, for the carbon-doped samples sintered at both 700 and 900 ° C. The degree of flux pinning enhancement and the ratio RHH (Bc2/Birr) have been estimated to guide us towards further enhancement of Jc. We argue that δℓ and δTc pinning mechanisms, based on variation of the mean free path (ℓ) and the critical temperature, respectively, coexist in the MgB2 wires treated with malic acid, regardless of the sintering temperature. The δℓ pinning is predominant at lower operating temperatures, and δTc pinning starts close to Tc, which means that spatial variation in the charge carrier mean free path is the main mechanism responsible for the flux pinning in the MgB2 wires treated with malic acid that were sintered at 700 and 900 ° C.

Vortex flux pinning mechanism and enhancement of in-field Jc in succinic acid doped MgB2

The field dependence of the resistivity and the critical current density, Jc(B), of MgB2 doped with 10wt% wet and dry succinic acid have been investigated by magnetic measurements. The dry succinic acid significantly enhanced the upper critical field, the irreversibility field, and the Jc(B) compared to the wet succinic acid doped MgB2 and the pure MgB2. The field dependence of Jc(B) was analyzed within the collective pinning model. The observed temperature dependence of the crossover field, Bsb(T), from the single vortex to the small vortex bundle pinning regime shows that flux pinning arising from variation in the critical temperature, δTc, is the dominant mechanism for the wet sample over the whole studied temperature range, while there is a competition between δTc pinning and the pinning from variation in the mean free path, δl, for the dry sample.

Evidence for transformation from δTc to δl pinning in MgB2 by graphene oxide doping with improved low and high field Jc and pinning potential

Flux pinning mechanism of graphene oxide (GO) doped MgB2 has been systematically studied. In the framework of the collective pinning theory, a B-T phase diagram has been constructed. By adjusting the GO doping level, the pinning mechanism in MgB2 transformed from transition temperature fluctuation induced pinning, δTc pinning, to mean free path fluctuation induced pinning, δl pinning, is observed. Furthermore, in terms of the thermally activated flux flow model, the pinning potential in high field (B > 5 T) is enhanced by GO doping. The unique feature of GO is the significant improvement of both low field Jc and high field Jc.

Fluctuation of mean free path and transition temperature induced vortex pinning in (Ba,K)Fe2As2 superconductors

The vortex pinning mechanisms of Ba0.72K0.28Fe2As2 single crystal have been studied systematically as a function of temperature and magnetic field. The temperature dependence of the critical current density, Jc(T), was analysed within the collective pinning model at different magnetic fields. It was found that both the δl pinning mechanism, i.e., pinning associated with charge-carrier mean free path fluctuation, and the δTc pinning mechanism, which is associated with spatial fluctuations of the transition temperature, coexist in the Ba0.72K0.28Fe2As2 single crystal in fields smaller than 4 T. Their contributions are strongly temperature and magnetic field dependent. At lower temperature and B ≤ 4 T, the δl pinning is the dominant mechanism, and its contributions decrease with increasing temperature. At temperatures close to the critical temperature, however, there is evidence for δTc pinning. At magnetic fields larger than 4 T, the δl pinning mechanism is the only effect.

Enhanced pinning effect in air-processed Gd-123 bulk superconductors with BaTiO3 addition

Introducing additives into the superconducting matrix is an effective way to enhance the flux pinning performance of superconductors. In this article, we introduce a typical ferroelectric material, namely BaTiO3, into the GdBa2Cu3O7−δ (Gd123)/Ag matrix to increase the inhomogeneous state with stronger pinning effect via polarities or ion substitutions. The top-seeded melt growth processed Gd123/Ag bulk superconductors with different doping amount of BaTiO3 ranging from 0.0mol.% to 0.4mol.% were fabricated with a cold seeding method. The effect of BaTiO3 addition on microstructure and superconducting properties was investigated systematically. Critical current density, Jc, shows that there are two optimized doping levels. One is 0.1mol.% with BaTiO3, in the bulk, Jc was enhanced both around zero field and middle field of 1.5T. The enhanced peak effect (δTc pinning) is rising from the Gd3+/Ba2+ solid solutions, which was further strengthened by a little amount of entrant of Ba2+ and caused a reduction of Tc. While the enhanced Jc around zero field (δl pinning) is coming from the atomic scale pinning centers formed in the CuO plane due to part of Cu2+/3+ ions were substituted by Ti4+. The other optimized doping level is 0.3mol.%, in this doping amount, Jc value is close to that in 0.1mol.% doped bulk at self field, but has a lower peak. Two reasons are involved here. The first is that the Gd3+/Ba2+ solid solutions were suppressed by the increased amount of Ba2+ and lead to a suppression of peak effect. The second is the non-superconducting phases (including the survived BaTiO3) formed in the melting process, which act as core pinning centers and contribute to the enhanced δl pinning. The novel doping effect of BaTiO3 is that it may enter as an interstitial among the crystal lattice of Gd-123. The properties of the final products depend on the doping amount. Microstructure analysis shows that Gd-211 particles are fined by proper doping amounts ranging from 0.1mol.% to 0.3mol.%.

Comparison of Flux Pinning Mechanism in Laser Ablated YBCO and YBCO:BaZrO3 Nanocomposite Thin Films

Thin films of YBCO and YBCO:BaZrO3 (BZO) nanocomposite have been deposited using the pulsed laser deposition technique. Substantial increase in critical current density (JC) and pinning force density (Fp) of the nanocomposite thin films was observed. The possible pinning mechanism in YBCO:BZO nanocomposite thin films has been explored and compared with the pinning mechanism in pure YBCO thin film by studying the variation of JC with magnetic field (B) and temperature. In the intermediate field regime (0.1–1 T), JC follows B−α with nearly similar values of α for YBCO and YBCO:BZO nanocomposite thin films indicating similar pinning mechanism in both thin films. The variation of JC with reduced temperature (t=T/TC) has been studied for both the films and it was observed that the mechanism of pinning in both YBCO and YBCO:BZO thin films is similar (δTC pinning). The observed enhanced values of JC and Fp of the nanocomposite thin film is attributed to the presence of BZO nanoparticles, which induces more defects due to lattice mismatch between YBCO and BZO leading to improved flux pinning properties of the nanocomposite thin film.

Flux pinning mechanisms in graphene-doped MgB 2 superconductors

The effects of graphene doping on the superconducting properties of MgB2 were studied. We found that small addition of graphene significantly improves the superconducting properties of MgB2, with only a small reduction in Tc. Low resistivity, high critical fields and enhanced flux-flow activation energy were observed for the optimally doped bulk sample. The spatial fluctuation in the transition temperature (δTc pinning) is the flux pinning mechanism in graphene-doped MgB2.

Effects of Nano scale Y2O3 Additions on Sm–Ba–Cu–O Materials

To enhance the critical current density (Jc) of Sm–Ba–Cu–O (SmBCO) superconductors, a small number of Y2O3 particles of nano scale (nmY2O3) were used. The Jc–H (the applied magnetic field) results show that the increase in Jc using nmY2O3 occurs mainly in high fields. In addition, by increasing the number of nmY2O3 particles, the flux pinning force-reduced magnetic field (Fp–h) curves shift to the right. The above results indicate the formation of δTc pinning centers. The microstructure results show that Y2O3 particles react with the 211 (Sm2BaCuO5), 123 (Sm1Ba2Cu3O7) and liquid phases at high temperatures. Both the high solubility of Sm in the liquid and the inter-diffusion of Sm and Y at high temperatures result in compositional fluctuations [(Sm,Y)–Ba–Cu–O]. It is considered that these compositional fluctuations contribute to δTc pinning. Finally, to determine the effect of compositional fluctuation caused by a different method, these results were also compared with those of the nano sized Y211 (nmY211)-doped samples.

Concentration effect on high field pinning and transport behaviors in nano-Y2BaCuO5 doped Sm–Ba–Cu–O superconductor

Recent studies indicate that the addition of nanoscale RE2BaCuO5 (nmRE211) (RE=Y, Sm, and Nd) in melt-texture-grown Sm–Ba–Cu–O bulk material can enhance the critical current density (Jc), especially in high magnetic fields. It is known that the δTc pinning is the reason for high field peak effect in Jc-H curves. The Jc-H curves exhibit enhancement compared with those of undoped samples. The analysis of first derivations of R-T curves found two peaks in dR∕dT curves representing two characteristic temperatures in high magnetic fields, where the lower temperature is attributed to the weak superconducting phase. The relative strength of the two peaks varies with the concentration of nmY211 additions. This argument is consistent with microstructural observations and pinning analysis results.

Superimposition of Different Pinning Mechanisms by Combined Additives in Sm–Ba–Cu–O Superconductors

The critical current density (Jc) of melt-textured growth (MTG) RE–Ba–Cu–O (RE: rare earth elements) materials is strongly affected by the pinning mechanisms of various defects associated with microstructual variations. In this study, different additives were used, such as (a) 0.5 wt % Pt/1.0 wt % CeO2 (A1), (b) nano sized Sm2BaCuO5 particles (N1), and (c) co doping of A1 and N1 (C1). The influence of individual addition of A1 and N1 species was studied. Enhancement in the Jc–H curves was shown for both types of additives, however, at different magnetic fields. Microstructural studies showed that the size of 211-particle decreased and the number of particles increased significantly in the A1 samples, which resulted in increasing dislocations and stacking fault density (known as δl pinning) associated with the 211/123 interfaces. On the other hand, in the N1 sample, nano scale compositional fluctuation of Sm1+xBa2-xCu3Oy (where x as well as Tc varied on the nanometer scale) was proposed to result in δTc pinning. The study shows that two types of pinning centers (δl of the A1 sample and δTc of the N1 sample) can be superimposed by the combined addition of both additives. Thus, a high Jc at different fields can be achieved due to the activation of different pinning centers.

Matrix chemical ratio and its optimization for highest flux pinning in ternary (Nd–Eu–Gd)Ba2Cu3Oy

We have fabricated the ternary (Nd, Eu, Gd)Ba2Cu3Oy system with various Nd:Eu:Gd ratios with the aim of optimizing the pinning performance at 77 K. The magnetization measurements suggest that the three elements (Nd, Eu, Gd) contribute to pinning in different manners: Nd mainly enhances flux pinning at low magnetic fields, Eu controls the second peak position and the irreversibility field, while Gd slightly enhances intermediate and high-field Jc values. The scaling analysis of the pinning force density versus the reduced field h = Ha/Hirr showed a peak at h = 0.55. This value is higher than the theoretically predicted highest value of h = 0.5, corresponding to ΔTc pinning. We proved that an excellent flux pinning can be achieved in the entire magnetic field range when sub-micron secondary phase particles are dispersed in the NEG-123 matrix with an optimum Nd:Eu:Gd ratio.

H– T– J phase diagram and pinning mechanism in Ba 0.63K 0.37BiO 3− y single crystals

The H–T–J phase diagram of superconducting Ba 1− x K x BiO 3− y ( x=0.37, 0.41, 0.5) single crystals has been studied by means of SQUID and cantilever force magnetometry. Strong upward curvature of the H c2 ( T) dependence confirms at least intermediate strength of the electron–phonon coupling in this 3D-superconductor family. The normalized critical current deduced from a cantilever force response is well described by collective pinning theory in the temperature range 0.01 T c< T<0.8 T c. In contrast to the samples with x=0.37 and 0.41, where the crossover between δl- and δT c-pinning causes the peak effect, for x=0.5, the unified pinning mechanism related to the spatial fluctuations of the carrier mean free path does not allow pronounced anomalous magnetization over the whole temperature range studied.