Enhancement of the critical current density in YBCO/Ag composites

YBa₂Cu₃O_7 + xAg?(x = 0.0, ?5.0, ?15.0 and 20 wt%) composite samples have been prepared by the solid state reaction method. The changes in structure are confirmed from the X-ray diffraction analysis and SEM measurements. The critical current density is calculated using Bean's formula from the magnetization measurement. We find that the addition of silver in YBCO enhances the critical current density (?J_C) by a factor of nearly six (for 15% Ag) in comparison to pure YBCO. Enhancement of the pinning force (F_P) by a factor of ten is also reported. The enhancement in ?J_C is investigated over a wide range of magnetic fields. These significant changes in ?J_C and F_P are attributed to the presence of Ag particles as efficient artifical pinning centers in YBCO.     

Effect of air humidity on the microstructure and properties of large-area YBa2Cu3O7−x films by TFA-MOD process

Large-area YBa₂Cu₃O_7?x (YBCO) films had been successfully fabricated on LaAlO₃ (LAO) substrate by metalorganic deposition using trifluoroacetates (TFA-MOD) and the effect of air humidity on the structure and properties of YBCO films was systematically investigated. The precursor coating process was performed in the air humidity range of 70–40%. According to the X-ray diffraction (XRD) and scanning electron microscopy (SEM) images, the films prepared at high air humidity showed poor electrical performance due to large pores and impurity phases. In contrast, dense, homogeneous and epitaxially grown YBCO films prepared at low air humidity with the critical current densities of 3.8–5.0 MA cm^?2 at 77 K had been obtained.     

Modified Lanthanum Zirconium Oxide buffer layers for low-cost,high performance YBCO coated conductors

The pyrochlore Lanthanum Zirconium Oxide, La₂Zr₂O₇ (LZO), has been developed as a potential replacement barrier layer in the standard RABiTS three-layer architecture of physical vapor deposited CeO₂ cap/YSZ barrier/Y₂O₃ seed on Ni–5%W metal tape. The main focus of this research is to ascertain whether: (i) we can further improve the barrier properties of LZO; (ii) we can modify the LZO cation ratio and still achieve a high level of performance; and (iii) it is possible to reduce the number of buffer layers. We report a systematic investigation of the LZO film growth with varying compositions of La:Zr ratio in the La₂O₃–ZrO₂ system. Using a metal–organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of La_xZr_1?xO_y (x = 0.2–0.6) on standard Y₂O₃ buffered Ni–5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial LZO phase with only (0 0 1) texture can be achieved in a broad compositional range of x = 0.2–0.6 in La_xZr_1?xO_y. Both CeO₂ cap layers and MOD–YBCO films were grown epitaxially on these modified LZO barriers. High critical currents per unit width, I_c of 274–292 A/cm at 77 K and self-field were achieved for MOD–YBCO films grown on La_xZr_1?xO_y (x = 0.4–0.6) films. These results indicate that LZO films can be grown with a broad compositional range and still support high performance YBCO coated conductors. In addition, epitaxial MOD La_xZr_1?xO_y (x = 0.25) films were grown directly on biaxially textured Ni–3W substrates. About 3 μm thick YBCO films grown on a single MOD–LZO buffered Ni–3W substrates using pulsed laser deposition show a critical current density, J_c, of 0.55 MA/cm² (I_c of 169 A/cm) at 77 K and 0.01 T. This work holds promise for a route for producing simplified buffer architecture for RABiTS based YBCO coated conductors.     

Temperature behavior of electron transport in normal-metal-HTSC heterojunctions

Current transport in micron-sized normal-metal-high-temperature superconductor heterojunctions (Au/YBa₂Cu₃O_6+x ) was studied for two crystallographic orientations of YBCO films. It is shown that depending on the transport-current flow direction relative to the crystallographic axes of the YBCO film, the electronic transport properties of Au/YBCO heterojunctions with highly transparent boundaries change from quasi-tunneling (along the YBCO c axis) to close-to-Ohmic (in the directions lying in the YBCO basal plane).     

Fluctuation induced magneto-conductivity studies in YBa2Cu3O7−δ + xBaZrO3 composite high-Tc superconductors

Fluctuations on the electrical conductivity of polycrystalline YBa₂Cu₃O_7?δ + xBaZrO₃ (x = 1.0, 2.5, 5.0 and 10.0 wt.%) superconductors were investigated from the resistivity vs. temperature data for zero field and 8 T (Tesla) external magnetic fields. Attempts have been made to identify the optimum inclusion of BaZrO₃ (BZO) in YBa₂Cu₃O_7?δ (YBCO) superconductors. The phase formation, texture and grain alignments were analyzed by XRD and SEM techniques. Then the effects of superconducting fluctuations on the electrical conductivity of granular composite superconductors were studied for zero field and 8 T external magnetic fields. Though inclusions of BZO sub-micron particles are not expected to influence superconducting order-parameter fluctuation (SCOPF) much, the transition from 2D to 3D of the order parameter in the mean-field region depends on the BZO content in the composites. It has been observed that BZO residing at the grain boundary of YBCO matrix influences the tailing region without having significant change in the mean-field critical temperature. In the present work, attention has been focused mostly in the experimental domain relatively above the T_c. It reveals that, 1 wt.% composite exhibits a better superconducting property in comparison with pure YBCO.     

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.     

Superconductivity and ferromagnetism in the orthorhombic system (TbxLu1−x)RuB2

The pseudoternary orthorhombic system Tb_xLu_1?xRuB₂) (0 ? x ? 0.1 andx = 1.0) has been investigated by static magnetization, ac susceptibility and resistivity measurements down to 1.5 K. Superconductivity occurs for 0 ? x ? 0.1, while ferromagnetic order occurs for x = 1.0. For each sample studied, the Tb ion maintains a configurationally stable trivalent state. The initial linear depression of the superconducting transition temperature yields a coupling constant value N(0)Γ² between conduction electrons and magnetic Tb³⁺ moments of 3.5 x 10^-4 ev-atom-states/spin-direction.     

Inducing self-assembly of Y2BaCuO5 nanoparticles via Ca-doping for improved pinning in YBa2Cu3O7−x

Different mechanisms may exists as a means to provide additional or specialized enhancement of existing nanoparticulate pinning in YBa₂Cu₃O_7?x (YBCO) thin films. In the particular case of Y₂BaCuO₅ (Y211) nanoparticles, Ca-doping of these nanoparticles via addition to the Y211 target material provides an additional increase to the J_c(H). YBCO + Y211 samples were created by pulsed laser deposition with alternating targets of YBCO with Y211 and Y211 doped with Ca. Initial indications suggest that this improvement in pinning results from some scattered short-ranged self-assembly of the nanoparticles into short nanocolumns.     

Electronic conductivity in La1−xBaxF3−x crystals

The electronic conductivities of solid solutions La_1?xBa_xF_3?x (0 ? x ? 0.0952) were investigated up to 533 K using the Hebb-Wagner dc polarization technique. The electrochemical cell (-)La|La_1?xBa_xF_3?x|Pt(+) has been utilized with Pt as the ion-blocking electrode. Under steady-state conditions the La_1?xBa_xF_3?x solid solutions exhibit electronic conductivity. The electronic conductivity vanishes in pure LaF₃. Together with ac conductivity measurements it appears that the ionic transference number for La_1?xBa_xF_3?x (0 ? x ? 0.0952) is essentially unity over the temperature range studied.     

Calcium and oxygen doping in Y Ba2Cu3Oy

Both oxygen and calcium play important roles in inducing superconductivity in Y Ba₂Cu₃O_y (YBCO), which is an antiferromagnetic insulator at low O and Ca content. O induces superconductivity in Ca-free YBCO, while Ca does similarly in oxygen-deficient YBCO. For doping oxygen HgO was used as it decomposes at 476 ^°C into Hg, which escapes from the matrix leaving the crystal unaltered, and O, which provide a way to dope O in YBCO. Considering these facts, polycrystalline samples of Y _1−xCa_xBa₂Cu₃O_y with x=0, 0.1 and 0.2 with and without HgO addition were prepared through a solid-state reaction method. The samples were sintered at 950 ^°C in open atmosphere. These synthesized samples were characterized through using the X-ray diffraction technique (XRD) for phase evaluation, scanning electron microscopy (SEM) for grain morphology, energy dispersive X-ray analysis (EDX) for compositional analysis and the four-contact measurement technique for determining the superconducting transition temperature.     

Etude De La Conductivite Electrique Des Ferrites Oxyfluores A Structure Spinelle

An investigation of the electrical conductivity of some oxyfluoride spinels of formula Zn_x²⁺Fe_1?x³⁺[M²⁺ Fe³⁺]O_4?xF_x (M = Fe, Co, Ni) and Fe³⁺[N_x²⁺Fe²⁺Fe_1?x³⁺]O_4?xF_x (N = Fe, Ni) shows that the conduction depends on the composition of the B sites: the activation energy increases, the conductivity and the Fe₃O₄ transition temperature decrease as the substitution rate of Fe³⁺ by N²⁺ in the B sites increases. The authors conclude to a hopping mechanism between the B cations; the anionic sublattice and the cationic A sublattice do not participate in the conduction.     

Features of the twin structure of YBa2Cu3O7−x epitaxial films

X-ray diffraction is used to investigate YBa₂Cu₃O_7?x (YBCO) films on NdGaO₃(110) and a ${{(100)CeO_2 } \mathord{\left/ {\vphantom {{(100)CeO_2 } {(1\bar 102)}}} \right. \kern-0em} {(1\bar 102)}}Al_2 O_3 $ heterostructure. Symmetric, asymmetric, and axial geometries for θ and θ/2θ scans are used to obtain diffraction spectra from different crystallographic planes. The orientational and quantitative twinning characteristics of the films are determined. While the crystallographic parameters of these two types of film (the films are c-axis oriented with c=11.67 Å) are similar, there are differences in the twin structure. In particular, the features of the NdGaO₃ structure lead to the appearance of an angle differing from 90° (90.20°) between the possible (110) and θ/2θ twin planes in a YBCO film and a different number of twin components in each system of twins. It is concluded from an analysis of the broadening of reflections, which are sensitive to twinning, that there is not twinning in a 60% film of YBCO on Al₂O₃ with a CeO₂ buffer layer.     

Polarization contributions to the vacancy binding energy in doped oxides

An effective medium theory due to Onsager is generalized to yield the macroscopic dielectric constant of a random assembly of associated vacancy-dopant bound pairs embedded in a dielectric host. The model is then further developed to give the vacancy-dopant binding energy, employing concepts long-established in electrolyte theory. The results of this analysis are used to compute the activation enthalpy for electrical conduction in the ionic conduction regime. The values so found are in reasonable accord with experimental data from various sources on Ce_1?xCa_xO_2?x, with x?0.02, assuming a vacancy migration enthalpy of 0.71 eV. The latter is the only adjustable parameter in the theory here developed. With this same value, the predicted variation of the low-temperature conduction activation enthalpy in Ce_1?2xY_2xO_2?x and Ce_1?2xGd_2xO_2?x (again at small x) are acceptably reproduced, although more experimental data would be desirable. The dielectric constant of the yttria-doped material is also described by the present model, again with no adjustable parameters. Several different features of the theory lead to its loss of validity in more concentrated mixtures. These are examined in detail.     

Tuning the contact resistance in organic thin-film transistors with an organic-inorganic hybrid interlayer

We demonstrated the tunable contact resistance in pentacene thin film transistor (TFT) by inserting an organic-inorganic hybrid interlayer between Au electrode and pentacene layer. The contact resistance of pentacene-TFT varies with concentration of pentacene-TFT varies with concentration of MoO_x in organic-inorganic hybrid interlayer. MoO_x in organic-inorganic hybrid interlayer. The contact resistance of the device with 55 wt% MoO_x doped pentacene interlayer is about 7.8 times smaller than that of device without interlayer at the gate voltage of −20 V. Comparing the properties of pentacene-TFT without interlayer, the performance of the pentacene-TFT with 55 wt% MoO_x doped pentacene was significantly improved: saturation mobility increased from 0.39 to 0.87 cm²/V s, threshold voltage reduced from −21.3 to −7.2 V, and threshold swing varied from 3.75 to 1.39 V/dec. Our results indicated that the organic-inorganic hybrid interlayer is an effective way to improve the performance of p-channel OTFTs.     

On the 3d electron contribution to the electronic structure of tetrahedral I–III–VI2 compounds

The reflectivity of single crystals of (CuInSe₂)_1?x(2ZnSe)_x which have the chalcopyrite structure for x ? 0.43 and the sphalerite structure for x ? 0.48 was measured. A band near 3 eV was observed for all compositions in the chalcopyrite structure but was not seen in the sphalerite phase. Its abrupt disappearance from x = 0.43 to x = 0.48 indicates that it may not be due to transitions from the copper d-levels to the lowest conduction band but may be associated with interband pseudo-direct transitions which become allowed by zone folding.     

Effect of electron irradiation on the galvanomagnetic properties of In<Subscript>x</Subscript>Bi<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>3</Subscript> semiconductor single crystals

The effect of 5-MeV electron irradiation of p-In_xBi_2?xTe₃ single crystals (x=0, 0.04, 0.07), performed at 250 K, on the galvanomagnetic properties of the crystals was studied. The irradiation was shown to change the conduction from the p to the n type. Annealing at temperatures of 310–390 K restores the conduction to the p type. The reversal of the conduction type and variation of the carrier concentration can be accounted for by an increase in the concentration of charged point radiation defects produced in In_xBi_2?xTe₃ by irradiation. Electron irradiation of p-type Te single crystals reduces the electrical resistivity without reversing the conduction type. Annealing restores the original properties almost completely.     

Thermal diffusivity measurement of YBa2Cu3O7−x thin film with a picosecond thermoreflectance technique

The thermal diffusivity of YBa₂Cu₃O_7?x (YBCO) film was measured using the optical pump–probe method. A theoretical finite-difference model was employed to calculate the diffusivity value, and the best fit for the c-axis oriented YBCO film showed an average thermal diffusivity of 0.25 ± 0.05 mm² s^?1. The obtained result is compared to previous reports measured using various methods.     

Photoelectrochromic properties of NiO film deposited on an N-doped TiO2 photocatalytical layer

N-doped TiO₂ film was synthesized on indium–tin oxide (ITO) conducting glass substrate by the hydrolysis method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Then high porous NiO was deposited onto the TiO_2?xN_x layer by chemical bath deposition (CBD) to prepare a double-layer TiO_2?xN_x/NiO electrode. The photoelectrochromic properties of the TiO_2?xN_x/NiO electrode were discussed through the results of UV–vis transmittance spectra, cyclic voltammogram and photocurrent transient measurements. It was found that the TiO_2?xN_x/NiO electrode was sensitive to light and exhibited a noticeable photoelectrochromism. The NiO film changed its color from colorless to brown, and the transmittance varied from 86.8% to 14.5% at 500 nm after 1 h irradiation.     

A neutron powder diffraction study of the helimagnetic structure of TlCo2Se2−xSx

The magnetic layer structure of TlCo₂Se_2−xS_x has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition (x): the spins in TlCo₂Se₂ form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co-Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.     

Non-adiabatic polaron hopping conduction in CaMn1−xCrxO3 (0?x?0.3)

DC electrical conductivity (σ_dc) of electron-doped antiferromagnetic CaMn_1−xCr_xO₃ (0?x?0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ_dc data shows that small polarons are formed at lower concentrations (?5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (?10%) concentrations. This is consistent with the fact that doped Cr³⁺ has larger ionic size compared to that of Mn⁴⁺. Again, strong electron-phonon (e-ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott's variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO₃ manganites.