Pure YBCO Research Articles

Effect of annealing time on the structure and properties of YBCO films by the TFA–MOD method

We systematically investigated the effect of high-temperature annealing time on the structure and properties of YBCO films prepared by the TFA–MOD method. Critical current density ( J c) was measured using j c-scan Leipzig system to compare the electrical properties of the films. SEM and XRD were used to determine the microstructure and orientation of the films. The partially crystallized films have rough surfaces and impurity phases which attribute to poor electrical performance. In contrast, completely crystallized films have smooth surfaces, pure (0 0 1) YBCO phase, and exhibit high electrical performance with J c values over 3 MA/cm 2 (77 K, 0 T) and Δ T c lower than 1 K.

Effects of artificial pinning centers on vortex pinning in high-temperature superconducting films

One-dimensional artificial pinning centers (APCs) were introduced perpendicular to the surface of the c-axis oriented YBa2Cu3O7−δ (YBCO) films by using Y2O3 nano-islands which were distributed on the substrates. In-field Jc of the films was remarkably enhanced by this method and reached to 0.12 MA/cm2 (77K, B∥c, 5T), which was 2–2.5 times higher than that of a pure YBCO film. The present films had a large Jc peak when the field was applied close to the c-axis. The results indicate that the strong APCs were introduced into the YBCO films. The vortex pinning by the present APCs is discussed based on the accommodation angle of the vortices.

Control of Y2O3 nanoislands deposition parameters in order to induce defects formation and its influence on the critical current density of YBCO films

We successfully introduced high-density linear defects as artificial pinning centers (APCs) of the quantized vortices into YBCO films, during the film deposition procedure. APCs were introduced perpendicular to the film surface by using the distributed nanosized Y2O3 islands prepared on SrTiO3(100) substrates. It is possible to induce strong changes on the shape and density of the Y2O3 islands by varying the deposition conditions. The highest Y2O3 islands density, 226/μm2, was obtained with fifteen laser pulses at 800°C of substrate temperature. Even if only 10% of the induced defects act as effective pinning centers, Jc of YBCO film grown on the Y2O3 nanoislands/SrTiO3 substrate increased to 1.8×107A/cm2 (20K, B∥c, 0T), which is 1.5 times higher than that of the pure YBCO film.

YBCO Films Prepared by PLD Using Nanocrystalline Targets Doped With<tex>$hboxBaZrO_3$</tex>or Y211

YBCO thin films were prepared by pulsed laser deposition from a pure nanocrystalline YBCO target and similar targets doped with BaZrO/sub 3/ or Y/sub 2/BaCuO/sub 5/ (Y211 phase) in order to investigate the effect of these impurities on magnetic flux pinning. The doped targets were pressed from nanopowders prepared by a sol-gel method from starting solutions whose nominal molar ratios corresponded to compositions YBCO+2.2 and 6.5 wt% BaZrO/sub 3/ and YBCO+3.0 wt% Y211 phase. The biggest changes in the superconducting properties relative to the films prepared from the pure target were observed in the films ablated from the BaZrO/sub 3/-doped targets. Although the doping reduces the critical temperature, the critical current density calculated from the hysteresis loops is significantly increased in external magnetic fields above 0.15 T. Addition of Y211 to the target is found not to be as effective for improving the flux pinning of the films in high magnetic fields as addition of the BaZrO/sub 3/. J/sub c/'s in zero field and 77 K were 1.2 MA/cm/sup 2/ in the pure YBCO film, 1.1 MA/cm/sup 2/ in the 2.2 wt% BaZrO/sub 3/ doped film and 3.2 MA/cm/sup 2/ in the 3.0 wt% Y211 doped film.

<tex>$J_c$</tex>Enhancement in<tex>$rm YBa_2rm Cu_3rm O_rm x$</tex>Thin Films by Introduction of One-Dimensional Artificial Pinning Centers

A novel technology to introduce artificial pinning centers (APCs) into YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films prepared by pulsed laser deposition (PLD) was investigated for a drastic improvement of J/sub c/ in the films. Linear-like defects (one-dimensional APCs) were introduced perpendicular to the surface of the c-axis oriented films during the deposition process by using distributed Y/sub 2/O/sub 3/ nano-islands on substrates. The density of nano-islands was varied within 0.8-1.2/spl times/10/sup 10//cm/sup 2/ by PLD. A normalized resistivity as a function of temperature in magnetic fields shows a sharper resistivity drop for the film with APC, compared to pure YBCO film. J/sub c/ of the film with APC was also increased to 0.12 MA/cm/sup 2/ (77 K, B//c, 5 T), which was about two times higher than that of the pure YBCO film. The film had a very large J/sub c/ peak when the field was applied close to the c-axis. The peak J/sub c/ increased with the number density of Y/sub 2/O/sub 3/ nano-islands. This indicates that strong APCs parallel to the c-axis were incorporated into the YBCO film.

Phase separation in fully oxygenatedY1−yCayBa2Cu3Oxcompounds

A micro-Raman spectroscopic study has been carried out on fully oxygenated ${\mathrm{Y}}_{1\ensuremath{-}y}{\mathrm{Ca}}_{y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ (YBCO) compounds with $y=0.0\char21{}0.2$ and oxygen concentration varying with Ca in the range ($x\ensuremath{\sim}6.960$ and 6.995). The spectral profiles of the $4{A}_{g}$ and $1{B}_{{1}_{g}}$-like symmetry phonons show minor modifications with Ca, but two modes that appear close in energy to the in-phase and the out-of-phase phonons gain intensity with the Ca substitution. The changes in the intensity of these two satellite modes with the amount of Ca are correlated with modifications in the dimpling of the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes, as detected by extended x-ray absorption fine structure, and can be attributed to the formation of superstructures and to phase separation phenomena. The evolution of two bands in the in-phase phonon, associated in pure YBCO with local structural distortions and the coexistence of optimal and oxygen overdoped phases, remains completely independent of the Ca concentration. It appears that the Ca atoms have the tendency to be surrounded by Y atoms forming an independent nanophase of the type ${\mathrm{Y}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ with the additional carriers from Ca not contributing to an excess doping of the system.

Control of Y2O3 nano-islands density to introduce artificial pinning centers in YBCO thin films

AbstractWe successfully introduced high-density linear defects as artificial pinning centers (APCs) of the quantized vortices into YBCO films, during the film deposition procedure. APCs were introduced perpendicular to the film surface by using the distributed nano-sized Y2O3 islands prepared on SrTiO3(100) substrates. It is possible to induce strong changes on the shape and density of the Y2O3 islands by varying the deposition and annealing conditions. The highest Y2O3 islands density, 226/μm2, was obtained with fifteen laser pulses at 800°C of substrate temperature, and applying an annealing temperature of 800°C. Even if only a small fraction of the induced defects act as effective pinning centers, Jc of YBCO film grown on the Y2O3 nanoislands/SrTiO3 substrate increased to 1.8 107 A/cm2 (20 K, B//c, 0 T), which is 1.5 times higher than that of the pure YBCO film.

Electrical and magnetic properties of Si ion implanted YBa 2Cu 3O 7− δ thin films and microbridges

Fabrication of superconducting bilayer YBa 2Cu 3O 7− δ (YBCO) thin film structure by Si ion implantation and properties of microbridge patterned on that are presented. YBCO thin film of 150 nm thickness was grown on single crystal (100) SrTiO 3 substrate by inverted cylindrical magnetron sputtering. The sample was implanted with 100 keV, 1×10 16 Si ions/cm 2. Upon implantation with Si, the sample lost its electrical conductivity and diamagnetism while its crystalline structure was preserved after the annealing of the sample. The implanted ions do not alter the overall crystal structure of high temperature superconductor film. This allows the growth of epitaxial superconducting second layer YBCO film on top of the implanted area without using any buffer layer, thus providing an effective method of fabricating multilayer structures. The second layer film and the microbridge patterned by laser writing technique, showed the superconducting properties similar to those of pure YBCO base layer with a reduced critical current density.

Microstructural studies of K 2CO 3 and Rb 2CO 3 doped YBCO HTSC

The influence of Me 2CO 3 (Me=K or Rb) additions on the microstructural morphology of YBa 2Cu 3O x (YBCO) HTSC with nominal composition Y (1−0.2 x) Ba (2−0.2 x) M x Cu 3O y ( x=0–2.0) were investigated by means of orientation imaging microscopy which provides a method for measuring a large number of individual grain orientations and relating them directly to the microstructural features by means of evaluating electron backscatter Kikuchi patterns in scanning electron microscopy. We investigated the influence of the alkali additions on the grain orientation distributions of YBCO. The samples are characterized by grain orientation maps, and pole and inverse pole figures. Finally, the grain orientation distribution functions are obtained from the measured data. Within a certain range of doping (up to 5 wt.% in the initial batch), the grain sizes are found to increase as compared to pure YBCO, accompanied by an improvement of the superconducting properties ( T c). It is shown that the additions of alkali carbonates do not introduce any preferred grain orientations in YBCO HTSC in the entire doping range.

Magnetostriction and magnetoacoustic measurements on pure and Zn-doped YBCO crystals

Magnetostriction hysteresis data reveal the presence of the fishtail effect in isothermal scans between 72 and 80 K. Thermal cycles in a constant magnetic field show the behaviour of pinning of flux lines in the different phases. Ultrasonic attenuation and the relative change of the sound velocity for various magnetic fields were measured. The aim is to examine the elastic behaviour of the flux line lattice. In the framework of the thermally assisted flux flow model the depinning temperature TD(B) and the activation energy U(B) were been deduced. The measurements on YBCO crystals are discussed and compared with investigations on YBCO/Zn+Ag composites. The magnetostrictive effects are verified by supplementary DC-magnetization measurements.

Temperature dependence of the critical current of YBCO–STO–LCMO heterostructures near Tc

The complex differential susceptibility of a YBCO film and YBCO–STO–LCMO structures with different thicknesses of the STO layer (0, 2, and 7 nm) is investigated. It is shown that for a YBCO–LCMO structure the temperature of the superconducting transition Tonset and the critical current density jc are observed to decrease in comparison with a pure YBCO film. In a YBCO–STO–LCMO structure, on the contrary, Tonset increases, while jc increases for the structure with the 2 nm thickness of STO and decreases for that with 7 nm of STO. It is conjectured that there is a change of the vortex pinning mechanism as the temperature is lowered.

Phase stability of the mixed rare-earth yttrium/erbium and yttrium/ytterbium barium cuprates, RE 0.5RE ′0.5Ba 2Cu 3O 7− x (RE=rare-earth), and ‘YBa 2Cu 3O 7− x (YBCO)+BaF 2’

A thermodynamic study of the phase stability of the mixed rare-earth barium cuprates, RE 0.5RE ′ 0.5Ba 2Cu 3O 7− x (RE/RE ′BCO, RE=Y, RE ′=Er, Yb) and ‘YBCO+25 wt.% BaF 2’ was carried out over the temperature range 800–960 °C and pO 2 range 3.0×10 −4 to 2.1×10 −1 atm O 2, using a coulometric titration technique. At temperatures, T, less than the peritectic temperature, T p, the phase stabilities of the mixed rare-earth compounds lie between the two single rare-earth parent systems. However, at T> T p the phase stabilities of the mixed rare-earth compounds depend on the solubilities of the rare-earths in the melt. The stability of ‘YBCO+BaF 2’ is reduced compared to pure YBCO and is very close to pure ErBa 2Cu 3O 7− x (ErBCO).

Effective activation energy and phase diagram in the Er-doping MTG-YBa 2Cu 3O 7− δ crystal

The resistivity of melted textured growth (MTG) Y 0.8Er 0.2Ba 2Cu 3O 7− δ (YErBCO) crystal has been measured as a function of temperature T, applied magnetic field H and the angle θ between the applied magnetic field and ab-plane. Based on the Yeshurun–Malozemoff model [Phys. Rev. Lett. 60 (1988) 2202], we investigated the vortex dynamic behavior from the Arrhenius form ρ( T, H)= ρ 0exp(− U 0/ T) and found that the model can explain the dissipation in the vortex liquid state for the Er-doped YBa 2Cu 3O 7− δ (YBCO) crystal. The effective activation energies for H∥ c in the Er-doped MTG-YBCO are about 2.7×10 3 K (7.5 T) and 5.6×10 4 K (0.5 T) less than that for H∥ c in pure YBCO. The anisotropy factor γ=10 obtained from the angular dependencies of effective activation energy U( θ) and the zero resistance temperature T c0( θ) for Er-doped YBCO crystal are larger those that for YBCO crystal. The field dependence of the effective activation energy U e follows a power law, U e∝ H − α , with α=1.0 for H∥ c and for various angles θ at a fixed magnetic field of 4 T. The phase diagram of the Er-doped YBCO crystal is presented and discussed.

A study of grain orientation of Alkali doped polycrystalline YBCO ceramics using an EBSD technique

Polycrystalline KClO 3-, K 2CO 3-, Rb 2CO 3-doped and undoped YBa 2Cu 3O 7− x (YBCO) ceramic high- T c superconductors were examined using electron backscatter diffraction (EBSD) technique. We present orientation maps and pole figures of the samples. It is shown that the pure YBCO samples and the ones with Rb 2CO 3 and K 2CO 3 additives do not exhibit a very pronounced texture (i.e. preferred orientation). On the polycrystalline KClO 3-doped YBCO samples we find two major grain orientations (0 0 1) and (1 0 0) normal to the sample surface while the pure YBCO sample does not exhibit any orientation maxima. This observation is ascribed to the effect of a liquid phase due to the KClO 3-additive during the preparation process.

Influence of Ho substitution for Y on flux pinning in melt-processed YBCO superconductors

The Y 1− x Ho x Ba 2Cu 3O y ( x=0, 0.2, 0.4 and 0.6) bulk samples were prepared by a `powder melting process' method. Their microstructure, critical current density and flux pinning behavior were investigated by TEM and SQUID magnetometer. The results indicate that J c and flux pinning can be significantly improved by Ho substitution for Y. The normalized pinning force is found to be well scaled with the reduced magnetic field ( h= H/ H irr) and shows a peak at h max=0.4 in the Y 0.4Ho 0.6Ba 2Cu 3O y sample, which is much higher than that in pure YBCO specimen. Many submicron binary RE211 particles and high density of stacking faults are observed in the Ho-doped samples, which may be responsible for the enhancement of J c. Also, the paramagnetic moment of Ho has a large contribution to the increase in J c.

A simple and new 45° homo-biepitaxial Y 1Ba 2Cu 3O y junction on yttrium-stabilized zirconia substrates

We have developed a new bi-epitaxial structure, namely YBCO (780°C)/YBCO (600°C)/YSZ (in-plane rotation 45°) and YBCO (780°C)/YSZ (in-plane rotation 0°) boundary. In comparison to the YBCO/CeO 2/YSZ/MgO (in-plane rotation 45°) and YBCO/MgO (in-plane rotation 0°) boundary previously developed in our laboratory, the new preparation is much simpler and has many advantages. For one, this method avoids the difficulty of lattice mismatch between the pure YBCO film and the MgO substrate. A clean 45° rotation can be achieved without growing different kinds of buffer layers. Furthermore, the first YBCO layer can be used as an endpoint indication during dry etching for device fabrication. Most importantly, the I–V curves taken at low temperatures show the RSJ behavior, with a high J C value (∼10 5 A/cm 2) and I C R N product (∼1 mV) at 4.2 K.

Magnetic properties of KClO3-doped Y(1-0.2x)Ba(2-0.2x)KxCu3Oy, (x = 0-0.40)HTSC

The influence of KClO3 addition on the magnetic propertiesand microstructural parameters (grain size, grain boundaries) ofhigh-temperature superconductors (HTSC) with nominal composition Y(1-0.2x)Ba(2-0.2x)KxCu3Oy (x = 0-0.40)were investigated. The ac complex susceptibility χac = χ'-iχ''was recorded as a function of temperature and concentration of KClO3at various ac magnetic fields up to 2 mT. The data were analysed using therelation h = a(1-Tm/Tc)n, where Tm represents the intergranular peaktemperature of the imaginary component χ''. The value of a is found toincrease for doped samples, which suggests enhanced flux pinning due to theKClO3 addition. The dc susceptibility as a function of temperaturewas measured by means of a SQUID magnetometer in field-cooled cooling (FCC)and field-cooled warming (FCW) modes in magnetic fields in the range 10 mT⩽µ0Ha⩽7 T on KClO3-doped and pure YBCO samples.The Meissner curves reveal that the samples are single phase in all appliedfields, but the low-temperature behaviour of the samples with KClO3additions is different from that of pure YBCO.

Investigation of glass behaviour of vortex arrays in superconducting submicron YBCO powders by magnetic relaxation measurements

Magnetic relaxation measurements are made in the remanent state of pure and Zn-doped submicron YBCO powders. A characteristic feature of these samples is dipolar interaction between single vortices trapped in different particles. Similar to other systems with dipolar interactions, such as powders of fine ferromagnetic particles, our samples show glassy behaviour: different metastable states with various pinning properties can be prepared by varying the strength of the magnetizing field, i.e. history of the sample. It is found also that the relation between the pinning energy and the magnetic moment can be described by an expression similar to the vortex glass interpolation formula.

Low temperature variation of YBa 2Cu 3O 7 − δ lattice parameters in Fe/YBa 2Cu 3O 7 − δ superconducting composites

In order to complement a previous study of the electrical properties of xFe/ (1 − x) YBa 2Cu 3O 7 − δ composites, the thermal dependence of lattice constants and orthorhombic strain (b − a) / (a+b) in the range 15 K < T < 300 K was investigated by high resolution X-ray diffraction. For low Fe contents ( x = 2%), the main differences with the pure YBa 2Cu 3O 7 − δ (YBCO) parameters are found at temperatures lower than 100 K only; moreover, for higher concentrations ( x = 5%) the orthorhombic strain is found to be about 10 % smaller than in the pure material in the whole temperature range. The quantitative differences of the a and b lattice constants with Fe content, compared with the pure YBCO value, and their temperature dependence suggest that the Cu(1) and Cu(2) positions are preferred for substitution. The substitution, however, is small enough to retain the orthorhombic structure, as the transition to the insulating tetragonal structure is not reached. At T < 100 K, the composite with x = 2% Fe exhibits an anomalous thermal behavior of lattice constants which may be caused by a magnetostrictive response to a magnetic ordering of Fe in the lattice.

63,65Cu nuclear quadrupole resonance study of Ni-doped YBa 2Cu 3O 7

We have prepared pure YBCO (YBa 2Cu 3O 7− δ ) and Ni-doped YBCO (YBa 2− x La x Cu 3− x Ni x O 7− δ , x=0.1) samples and performed 63,65Cu nuclear quadrupole resonance (NQR) measurements. La substitution introduced for charge compensation, as well as Ni doping, severely distorts the electric field gradient at the copper sites inducing extra 63,65Cu NQR peaks other than the main peaks, for both the plane and the chain sites. 63,65Cu NQR linewidths for the plane site increased significantly whereas those for the chain site remained similar. Furthermore, both the spin–lattice and the spin–spin relaxation times for the plane copper decreased whereas those for the chain copper increased. The Ni substitutional effects on linewidths and relaxation times confirm that Ni carrying a local moment substitutes for the plane copper.