Stable barium compounds in YBa2Cu3Oy superconductors

The stability of various amounts of Ba₃Cu₃In₄O₁₂ (334) or BaTbO₃ (BTO) in a sintered YBa₂Cu₃O_y (YBCO) matrix was examined. Samples with added 334 or BTO exhibited critical temperatures (T_c) above 90 K for up to 20 vol.% addition and improved critical current densities (J_c) under a magnetic field. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis indicated that 334 and BTO did not react with the YBCO matrix under the sintering conditions used. The normalized J_c under a magnetic field of 1 T reached a maximum at 14 vol.% of 334 addition and 20 vol.% BTO addition. YBCO thin films with added BTO showed a gradual decrease in the T_c with increasing BTO content. YBCO films with added 334 showed a constant T_c of 87 K up to a 334 content of 4 vol.%.     

YBCO films buffered by pyrochlore Nd2Mo2O7 layer on YSZ substrates by chemical solution deposition

A chemical solution deposition process was used to grow epitaxial Nd₂Mo₂O₇ (NMO) buffer layers on YSZ substrates to produce YBa₂Cu₃O_7?δ (YBCO) coated conductors. The NMO precursor solution prepared using metal acetylacetonates was spin-coated onto single crystal YSZ substrate of 10 mm × 10 mm in size at 3000 rpm for 30 s and heat-treated at 1000 °C for 2 h in Ar after calcinated at 550 °C for 1 h. The YBCO film was deposited by TFA-MOD route on top of the NMO/YSZ architecture. The phase purity and the crystalline orientation of NMO and YBCO films were evaluated by X-ray diffraction (XRD). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to observe their microstructure and their surface roughness. The critical current density (J_c) of YBCO film on NMO/YSZ is 1.8 MA/cm² at 77 K in self-field, which indicates that the Nd₂Mo₂O₇ is a potential buffer for YBCO coated conductor.     

Flux pinning by discontinuous columnar defects in 74 MeV Ag-irradiated YBa2Cu3O7 coated conductors

Linear damage tracks are created in production-quality YBCO coated conductors by irradiation with 61–74 MeV Ag ions. The ion tracks are observed by transmission electron microscopy to be elongated but discontinuous. The in-field transport critical current (I_c) is enhanced significantly for fields applied parallel to the irradiation direction with a broad peak appearing in the magnetic field-angle dependence of the critical current, coinciding with the irradiation direction. The zero-field I_c is typically reduced somewhat, however annealing at 200 °C partially restores this and enhances the in-field I_c even for field parallel to the irradiation direction. Lower energies tend to produce a weaker peak, but also retain the zero-field I_c to a greater extent, consistent with a trend of greater discontinuity in the ion tracks.     

AC response of YBa2Cu3O7 thin film superconductor

Low-field ac measurements of magnetic susceptibility of YBa₂Cu₃O₇ high-temperature superconducting thin film were carried out over a wide range of temperatures and ac magnetic field amplitudes. A strong field dependence of the real χ′ and imaginary χ″ components was observed. The field dependence of the imaginary component is used to extract the temperature dependence of the critical current density in the sample using the critical state model. The exponent β of the power law relation J_c ～ (1 ? T/T_c)^β was determined from ac-susceptibility data and different values were found. Experimental results are compared with predictions of some existing theoretical models describing the ac response of superconducting thin film in perpendicular ac field.     

Reel-to-reel fabrication of meter-long YBCO coated conductor

YBa₂Cu₃O_7?δ (YBCO) superconductors were coated on the CeO₂/YSZ/Y₂O₃ buffered Ni-5at%W tapes by a reel-to-reel pulsed laser deposition (PLD). The process of a multi-layer deposition of YBCO film was explored. X-ray diffraction texture measurements showed good both in-plane and out of plane crystalline orientations in YBCO films. The average values calculated at a full width at half maximum (FWHM) of the peaks from phi-scans (φ) and omega (ω) scans for one meter-long YBCO tape were 7.49° and 4.71°, respectively. The critical current (I_c) was over 200 A/cm-width at 77 K and under self-field for meter-long YBCO tape. The critical transition temperature of the YBCO tape was typically as 90.1 K with 0.5 K transition widths.     

Transverse resistance of YBa2Cu3O7−δ single crystals

Measured is the transverse electrical resistance of YBa₂Cu₃O_7−δ single crystals with different oxygen deficiency values (δ) in the temperature range T_c − 300 K. The experimental data are approximated by an empiric expression accounting for the fluctuation conductivity near T_c and the semiconductor-like resistance regime. Our analysis of the concentration dependences of the fitting parameters, in particular, reveals that the resistance temperature dependence is largely affected by the sample's non-homogeneity. The latter, in turn, causes a T_c anisotropy and variable-range hopping conductivity between different phases. The deduced maximal values of the basal-plane coherence length, ξ_xy(0), are comparable with those for low-temperature superconductors.     

Doping dependence of irreversibility line in Y1−xCaxBa2Cu3O7−δ

The irreversibility lines (IL) for series of Y_1?xCa_xBa₂Cu₃O_7?δ (x = 0.025; 0.10 and 0.20) polycrystalline samples with different overdoping were investigated. The irreversibility fields were determined from measurements of third harmonics AC susceptibility as a function of DC field at constant temperature. For the weakly overdoped sample (with x = 0.025) H_irr(77 K) is about 7 T, which is higher than the previously reported for the non-substituted one. The irreversibility line behavior is typical for glass–liquid phase transition and this is confirmed by transport measurements. On increasing the overdoping the irreversibility fields were shifted towards lower temperatures. The behavior of H_irr(T) for the highly overdoped sample (with x = 0.20) is influenced by the surface barrier effect. It is supposed that in highly overdoped specimen the process of phase separation is enhanced and the Fermi clusters grow in size. This leads to a suppression of the bulk pinning and to a domination of the surface barrier effects and flux creep as well. As a confirmation, the obtained quadratic J_c(T) dependences were presented demonstrating the existence of S–N–S type inter-grain joints in the highly overdoped samples.     

Electronic structure of PrBa2Cu3O7 after high-pressure compression from first principles

The influence of the high-pressure compression (P ? 9 GPa) on the electronic band structure, charge distribution and magnetic moments of the fully oxygenated, ideally stoichiometric PrBa₂Cu₃O₇ system was investigated. The following changes were observed as a result of the simulated compression: (i) the reduced total density of states (DOS) at the Fermi level E_F, (ii) the increased difference between spin-up and spin-down total DOS in the nearest vicinity of E_F (spin splitting), (iii) the lowered occupation of Pr 4f band crossing E_F, (iv) the stronger localization of partial charges in all atomic spheres, (v) the reduced amount of holes in the CuO₂ planes and (vi) the enhanced magnetic moments in the system, especially in the cupric-oxide planes. Our results indicate the possibility of negative pressure effect on the superconductivity in the optimally doped PrBa₂Cu₃O₇ system at the sufficiently high hydrostatic pressure.     

Effect of La2Zr2O7-buffered YSZ substrate on YBa2Cu3Oy thin films by chemical solution deposition

Epitaxial YBa₂Cu₃O_y (YBCO) thin films have been fabricated by chemical solution deposition (CSD) on La₂Zr₂O₇-buffered YSZ single crystal substrate, where the buffer layer has three kinds of morphology – flat surface, rough surface and pore surface. The effect of LZO buffer layer’s roughness on the YBCO films was evaluated by X-ray diffraction, scanning electron microscopy and temperature-dependent resistivity measurements. The flat surface of LZO layer is beneficial to highly epitaxial YBCO films and high critical current density.     

Pinning potential in thick PrBa2Cu3Ox/YBa2Cu3O7−δ quasi-multilayers

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa₂Cu₃O_x (PrBCO) nano-dots and layers of YBa₂Cu₃O_7?δ (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 nm, and total thickness between 1.13 and 5.31 μm. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J_c at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J_c only in high fields. We have also investigated the frequency dependence of J_c from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nano-dots, both isotropic and c-axis correlated.     

Epitaxial growth of Ce2Y2O7 buffer layers for YBa2Cu3O7−δ coated conductors using reel-to-reel DC reactive sputtering

Biaxially textured Ce₂Y₂O₇ (CYO) films were deposited on Ni–5at.%W (Ni–5W) tapes by a DC reactive sputtering technique in a reel-to-reel system. Subsequent YBa₂Cu₃O_7?δ (YBCO) films were prepared using pulsed laser deposition leading to a simplified coated conductor architecture of YBCO/CYO/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CYO buffer layer with a texture spread down to 2.2° and 4.7° for the out-of-plane and in-plane alignment, respectively. Microstructural investigations showed a dense, smooth and crack-free surface morphology for CYO film up to a thickness of 350 nm, implying an effective suppression of cracks due to the incorporation of Y in CeO₂. The superconducting transition temperature T_c of about 90 K with a narrow transition of 0.8 K and the inductively measured critical current density J_c of about 0.7 MA/cm² indicate the potential of the single CYO buffer layer.     

Reactivity of the Cu2O(1 0 0) surface: Insights from first principles calculations

We present a summary of results of systematic first principles calculations of the electronic and geometric structures of the Cu₂O(1 0 0) surface and the process of CO oxidation on this surface (energetics and pathways of adsorption, diffusion and reactions of CO and O₂ on the surface). The (p, T) phase diagram of the Cu₂O(1 0 0) in equilibrium of with gas phase O₂ built using the ab initio thermodynamics approach suggests that the O-terminated surface is preferred over the Cu-terminated one within the entire ranges of pressures and temperatures in which the compound exists. Metastable Cu-terminated Cu₂O(1 0 0) is found to undergo a surface reconstruction in agreement with experiment. We find CO to oxidize spontaneously on the O-terminated Cu₂O(1 0 0) surface by consuming surface O atoms. Our calculations also show that the surface O-vacancies left in the course of the CO oxidation can be easily filled with dissociative adsorption of the gas phase O₂ molecules, which are usually present in reaction environment.     

Attenuation of interfacial pinning enhancement in YBCO using a PrBCO buffer layer

Numerous experimental results have suggested that the J_c of YBa₂Cu₃O₇ (YBCO) films is significantly higher near the film–substrate interface than in the remainder of the film. We previously proposed that this effect is due to interfacial pinning enhancement caused by stress and the resulting misfit dislocations at the heteroepitaxial interface. To test this hypothesis we have used a non-superconducting PrBa₂Cu₃O_7?δ (PrBCO) buffer layer to minimize the lattice mismatch with YBCO. We find that the PrBCO layers lower J_c of the 0.4 μm YBCO films in a predictable way, and that, if sufficiently thick (～0.5 μm), they eliminate interfacial enhancement altogether. Our interpretation of this result is that the defects responsible for interfacial enhancement of flux pinning originate at the bottom of the non-superconducting PrBCO layer, which screens the pinning centers from vortices in YBCO. This result demonstrates that the pinning enhancement arises from stress at the film–substrate interface.     

Direct injection of spin-polarized carriers across YBa2Cu3O7-δ-La0.3Ca0.7MnO3 interface at 77 K

We report here injection of spin-polarized carriers from a half-metallic La_0.3Ca_0.7MnO₃ (LCMO) colossal magnetoresistive (CMR) thin film into a high-temperature superconducting YBa₂Cu₃O_7-δ (YBCO) thin film studied using a micro-bridge. The LCMO and YBCO films were grown on 〈100〉 LaAlO₃ (LAO) substrate sequentially using pulsed laser deposition (PLD). I-V measurements carried out at 77 K show that while normal critical current, I _c ⁿ , of the micro-bridge is 80 mA, the critical current, I _c ^p , through the micro-bridge when injected from the CMR layer is 38 mA. This clearly shows that spin-polarized quasiparticles injected from the the CMR layer into the YBCO layer suppress the critical current of the superconductor via the pair-breaking phenomena.     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

High performance GdBa2Cu3O7−z film preparation by non-fluorine chemical solution deposition approach

Biaxially textured GdBa₂Cu₃O_7?z (GdBCO) films with T_c above 93 K have been prepared on (0 0 l) LaAlO₃ substrate by self-developed non-fluorine polymer-assisted chemical solution deposition (PA-CSD) approach. The GdBCO films show smooth and crack-free morphology. Many nanoscale particles with homogeneous distribution are observed in the GdBCO films, which have not been observed yet in the YBa₂Cu₃O_7?z (YBCO) films prepared by the same processing technique. Besides a high J_c (77 K, 0 T) of 2.28 MA/cm², the optimized GdBCO films show a better J_c–B behavior and an improved high-field J_c, compared to the YBCO films.     

Engineering of YBa2Cu3O7–δ grain boundary Josephson junctions by Au nanocrystals

We prepared and investigated grain boundary Josephson junctions based on SrTiO₃ bicrystal substrates. During the deposition of YBa₂Cu₃O_7–δ (YBCO) gold nanocrystals forming from an intermediate gold layer can modify the crystalline structure and thus the properties of the YBCO grain boundaries. The variation of the film thickness of the Au seed layer changes the growth conditions of the YBCO film and the Au nanocrystals. The values of the characteristic I_CR_N product do not change whereas the values of the critical current I_C decrease.

     

Influence of simultaneous doping of Sb and Pb on phase formation,superconducting and microstructural characteristics of HgBa2Ca2Cu3O8+δ

We report systematic studies of structural, microstructural and transport properties of (Hg_0.80Sb_0.2−xPb_x)Ba₂Ca₂Cu₃O_8+δ (where x = 0.0, 0.05, 0.1, 0.15, 0.2) compounds. Bulk polycrystalline samples have been prepared by two-step solid-state reaction route at ambient pressure. It has been observed that simultaneous substitution of Sb and Pb at Hg site in oxygen deficient HgO_δ layer of HgBa₂Ca₂Cu₃O_8+δ cuprate high-T_c superconductor leads to the formation of Hg-1223 as the dominant phase. Microstructural investigations of the as grown samples employing scanning electron microscopy reveal single crystal like large grains embodying spiral like features. Superconducting properties particularly transport current density (J_ct) have been found to be sensitive to these microstructural features. As for example (Hg_0.80Sb_0.05Pb_0.15)Ba₂Ca₂Cu₃O_8+δ compound which exhibits single crystal like large grains (∼50 μm) and appears to result through spiral growth mechanism, shows highest J_ct (∼1.85 × 10³ A/cm²) at 77 K. A possible mechanism for the generation of spiral like features and correlation between microstructural features and superconducting properties have been put forward.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

Conductivity of SnP2O7 and In-doped SnP2O7 prepared by an aqueous solution method

SnP₂O₇ and In-doped SnP₂O₇ have been prepared by an aqueous solution method using (NH₄)₂HPO₄ as phosphorous source. It was found that the solid solution limit in Sn_1 ? xIn_x(P₂O₇)_1 ? δ was at least x = 0.12. All pyrophosphates in the Sn_1 ? xIn_x(P₂O₇)_1 ? δ (x ≤ 0.12) series exhibit 3 × 3 × 3 superlattice structures. The conductivities of Sn_0.92In_0.08(P₂O₇)_1 ? δ in air are 6.5 × 10^? 6 and 8.0 × 10^? 9 S/cm at 900 and 400 °C, respectively, when prepared by an aqueous solution method and annealed at 1000 °C. The conductivity of undoped SnP₂O₇ is slightly lower. However, it was also found that the low-temperature conductivities of pyrophosphates annealed only at 650 °C are several orders of magnitude higher than those annealed at 1000 °C, which could be related to a trace amount of an amorphous secondary phase. The peak conductivity was in this case observed at around 250 °C, which is the same temperature as previously observed in In-doped SnP₂O₇ although the conductivity is still three orders of magnitude lower in the present study. These differences can be related to large differences in particle size and morphology, and all in all, the conductivities of SnP₂O₇-based materials are very sensitive to the synthetic history.