Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Submicrometer epitaxial films of YBa₂Cu₃O₇(YBCO) on (100) LaAlO₃ were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (T _c) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (J _c) at 77 K has an average value of 2×10⁵ A cm^?2, with a tendency for decreasingJ _c with increasing film thickness (0.2 to 0.8μm). Variations inJ _c are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJ _c. If the film composition is enriched in Y relative to Ba and Cu,T _c decreases by several degrees.     

Vapor pressures of Y, Ba, Cu precursors for the growth of YBa2Cu3O7 by MOVPE

The vapor pressures of the-diketonates Y(THD), Ba(THD), and Cu(THD), commonly used as precursors for MOVPE of YBa₂Cu₃O₇, and Y(MCP) were measured at different temperatures. A time-resolved static method recording the pressure vs. time at constant temperature was used, permitting us to deduce the vapor pressure even if the materials tend to decompose. The values of the constants of the Clausius-Clapeyron equation log₁₀p(T)/p ₀=A-B/T are,A=11.7, 8.7, 8.27, 16.6 andB=4359, 2654, 3602, and 6453 K for Y(THD), Y(MCP), Ba(THD), and Cu(THD), respectively withp ₀= 1 Pa and temperatureT in K. The thermal stability of the sources was measured and are discussed.     

The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-Tc Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 m (77 K, 87 GHz) than the undoped films.     

Superconducting behavior of high-Tc YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB500 G, in the case ofB perpendicular to thec-axis of the film.     

Charge-transfer in YBa2Cu3Ox

The charge-transfer hypothesis is shown to be inconsistent with data for YBa₂Cu₃O_x: (i) The two-step behavior ofT _c(x) (with jumps from zero to 60 K and then to 90 K) is not reflected as a similar, statistically significant two-step behavior in the bond-valence-sum charge of cuprate-plane Cu ions (as once believed), (ii) as a consequence of the law of conservation of charge, the derivatives of the layer charges with respect to oxygen contentx for both the Ba-O layers and the charge-reservoir Cu-O chains have the opposite signs to those predicted, and (iii) the charge-transfer observed for superconducting YBa₂Cu₃O_x is not sufficient to produce superconductivity, as demonstrated by insulating PrBa₂Cu₃O_x, which has virtually the same layer charges.     

Electronic Inhomogeneity and Possible Pseudogap Behavior of Spin Susceptibility in the Electron-Doped Superconductor Sr0.93La0.07CuO2: 63Cu NMR Study

The magnetic susceptibility, NMR spectra, nuclear spin-lattice relaxation rate (T ₁ ^–1) and the echo-decay rate (T ₂ ^–1) of ⁶³Cu were measured for the electron-doped infinite-layer superconductor Sr_0.93La_0.07CuO₂/T _c ^onset = 42.4 K). The results obtained revealed a clear tendency toward frustrated phase separation in this nominally underdoped high-T _c material. Above T _c the ⁶³Cu Knight shift is found to decrease upon cooling giving an evidence for a pseudogap-like decrease of the spin susceptibility. It is shown that unusual anisotropy of the ⁶³Cu Knight shift in the electron-doped CuO₂ layer can be understood as a compensation effect between the isotropic hyperfine coupling, mediated by the 4s Fermi-contact and 3d core-polarization exchange interactions, and the anisotropic on-site spin-dipolar hyperfine interaction of the Cu nuclei with the itinerant carriers, whose states near the Fermi energy have a sizeable admixture of Cu(4p_z) and/or Cu(3d_z ²) orbitals.     

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7Oz

The structural and superconducting properties of single-phase La_2.5–y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0–1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0–0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.     

A New Method for the Estimation of Hc2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the (T ) = H ^ab _c2/H^c2 with (T _c) 3.     

Film thickness dependence of critical current density for YBa2Cu3O7 films post-annealed at a low oxygen partial pressure

The variation of critical current density at 77 K as a function of film thickness was studied for YBa₂Cu₃O₇ films on (100) LaAlO₃ substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm^–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen.     

Chemical control of superconducting properties of (Y, Ca)(Sr, Ba)2Cu2.7Ga0.3O7?δ

The partial substitution of Ga at the Cu(1) (chain) sites of the YBa₂Cu₃O₇ structure allows synthesis at ambient pressure of Ba-free analogs, e.g., YSr₂Cu_2.7Ga_0.3O_7– . Materials with this composition have been found to be nonsuperconducting, but superconductivity has been induced by one or more of the following methods: Ca substitutions at the Y site; Ba substitutions at the Sr site; annealing in high-pressure oxygen. The influence of these chemical manipulations onT _c has been monitored and all methods have been found to enhanceT _c . The electronic effects of Ba substitutions have been deduced indirectly using powder neutron diffraction, and such substitutions appear to result in a redistribution of hole density into the Cu(1) sites from the superconducting CuO₂ planes.     

Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm^–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Growth and Characterization of YbBCO Films on Textured Gd2O3 Buffer-Layered Ni Tapes: All Sol–Gel Process

YbBa₂Cu₃O_7–x (YbBCO) thick films were grown on buffered, cube-textured Nickel tapes by sol–gel dip-coating method. Yb-123 films were prepared using solutions of Yb, Ba, and Cu organometallic compounds. A solution-based Gd₂O₃ buffer layer was deposited by dip-coating process with excellent texture and uniformity. The texture development and surface morphology of the buffer-layer films were examined by X-ray diffraction, pole figures, and ESEM analysis. Microstructure and characterization of Yb-123 films were done by ESEM, EDS, and XRD analysis. T _c and J _c were conducted by four-wire measurement method     

Transistor Doping Experiments on Superconducting CaCuO2 and C60 Interpreted on a Bond Ordering Model

The curve shape of T _c vs. holes injected by fields (transistor doping) of CaCuO₂ and C₆₀ are interpreted on a charge or bond ordering [BO] model. For CaCuO₂ both the magnitude of optimal T _c (89 K vs. a calculated 83 K) and the linear portions around a sharp T _c peak at h 0.17 = 1/6 are in accord with a universal algorithm for cuprates, in which T _c scales linearly with radical bond density to an optimal BO. These linear regions extrapolate to h = 0 and 1/3 respectively. Small trends to electronic freezing are observed at h = 1/8. A second linear region extends from h 1/12 to 1/16 with 4 times the slope of the first linear region, indicating a second mode of BO filling. A surprisingly similar behavior is observed for transistor doped C₆₀ type materials with characteristic sequence of BO dictated hole concentrations for T _c onset, rise, a linear region extrapolating to T _c = 0 at h = 0, indications for electronic freezing, a sharp peak and linear decrease thereafter. This suggests that a common phenomenology obtains for all high T _c materials, representing generally an ordering phenomenon of doped covalent radical bonds.     

Current progress in ternary LREBa2Cu3Oy materials and their application

The J_c and H_irr values at 77 K of ternary light-rare-earth compounds, LREBa₂Cu₃O_y “LRE-123”, are usually high enough to serve in various applications. Several sources of vortex pinning can be in these composites tailored to fit the needs of the particular application. The list comprises LRE/Ba solid solution, oxygen vacancies, large particles of secondary phases, twin planes, nanoscale lamellas, etc. By means of the latter defects one can achieve a very high irreversibility field. Refinement of secondary phase particles and the optimal choice of their amount enhance the electromagnetic performance in a broad temperature range, up vicinity of T_c, allowing levitation at liquid oxygen, 90.2 K. An optimum content of MoO₃ doubles the self-field super-current at 77 K, H||c-axis. Altogether, the pinning tailoring in ternary LRE-123 materials provides a flexible and reliable way to fit the electromagnetic performance with the needs of sophisticated high-temperature and high-magnetic-field applications.     

High Critical Current Density YBa2Cu3Ox Tapes Using the RABiTs Approach

Progress in the fabrication of epitaxial, high-J _c, biaxially aligned YBCO thick films on Rolling-assisted biaxially textured substrates (RABiTs) is reported. RABiT substrates comprise a biaxially textured metal substrate with epitaxial oxide buffer layers suitable for growth of superconductors. Oxide buffer layers have been deposited using three techniques: laser ablation, electron-beam evaporation, and sputtering. Epitaxial YBCO films grown using laser ablation on such substrates have critical current densities approaching 3 × 10⁶ A/cm² at 77 K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Critical current densities in excess of 0.2 MA/cm² have been obtained on stronger, nonmagnetic substrates. In addition, samples with J _e of 12.5 kA/cm² at 77 K have been fabricated. The highest strain tolerence obtained so far is 0.7% in compression and 0.25% in tension. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-J _c wire capable of carrying high currents in high magnetic fields and at elevated temperatures.     

Zero resistance at 117 K in Bi1.6Pb0.4Sr2Ca2Cu2+xOy compounds

In the Pb-doped Bi-Sr-Ca-Cu-O system, optimization of the composition and heat treatment conditions at which a greater amount of the high-T _c phase forms has been reported in our early paper [1], where the temperature of zero resistance was 107K. Recently, we have achieved zero resistance at 117 K and observed an a.c. susceptibility step at around 150 K by changing the Cu composition in the Bi^1.6Pb^0.4Sr²Ca²Cu^2+x O ^y system (x=0, 0.4, 0.8, 1.2, and 1.6).     

Microstructural and superconductive studies of Ag-YBa2Cu3O7?x composites

Composites of Ag-YBa₂Cu₃O_7–x were synthesized and carefully characterized for crystal structure and microstructure by X-ray diffraction, optical microscopy, SEM, TEM, and EDAX techniques in order to investigate the effect of Ag additions on the superconducting properties of 123 compounds. The a.c. susceptibility data show thatT _c (onset) of 123+Ag composites vary between 90.2 and 91.8 K. TheJ _c values we measured for 123 material without silver were in the range of earlier reported values for the pure 123 material. Whereas there is a relatively small increase in the critical current for the YBCO/Ag₂O ratio of 3, the variation of the grain size of the composites shows that theseJ _c changes are due to slight variations in the grain size rather than any dramatic effect of Ag inclusions as speculated earlier.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

Degradation of Superconducting Properties in MgB2 by Formation of the MgB4 Phase

Superconducting MgB₂ polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB₄ phase. A series of samples was placed in an -alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an -alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB₂ phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB₄ phase were formed and the T _zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J _cmag for the first series of samples was 1.9 × 10⁵ A/cm² at 10 K and 0 T, and for the second series of samples was 0.7 × 10⁴ A/cm² at 10 K and 0 T.