Properties Of YBa2Cu3O7 Research Articles

Generation of harmonics by means of YBa2Cu3O7 − x high-temperature superconducting polycrystals

Based on the nonlinear magnetic properties of YBa2Cu3O7 − x high-temperature superconducting polycrystals, a harmonic generator offering a wide spectrum of harmonics is devised. It is shown that, as the number of the harmonic increases, its amplitude decreases gradually. The device has a wide dynamic range, and its design is simpler and more reliable compared with currently available analogues.

Transport properties of YBa 2Cu 3O 7−δ thin films near the critical state with no applied magnetic field

Transport measurements carried out on twinned YBa 2Cu 3O 7−δ films are compared to the predictions of a previously proposed model suggesting that the vortices move along the films twin boundaries that behave as rows of Josephson weak links [P. Bernstein, J.F. Hamet, J. Appl. Phys. 95 (2004) 2569]. The obtained results suggest that, except if the films are very thin, the twin boundaries consist of superimposed rows of weak links with mean height, d ¯ s , whose mean length along the TBs is an universal function of T/ T c, the reduced temperature. This conclusion yields a general expression for the critical surface current density of the films as a function of T/ T c and of the number of superimposed weak links rows, while the critical current density depends on d ¯ s . A comparison of the measurements reported by various authors shows that the nature of the substrate and the growth technique have both a strong effect on d ¯ s . The existence of superimposed weak links rows is attributed to extended defects generated by Y 2O 3 inclusions.

Thin films of YBaCuO high-temperature superconductor grown in a simplified magnetron sputterer and their microwave application

The physicochemical properties of YBa2Cu3O7−δ high-temperature superconductor exclude designing standard magnetron sputterers and adopting standard film growth conditions that would ensure good and highly reproducible results. A simple and flexible magnetron sputterer making it possible to grow high-quality films (with a critical temperature of 90 K, a critical current density of 4 MA/cm2, and a surface resistance of ≤0.5 mΘ at a frequency of 10 GHz and 77 K) is described. The high quality of the films is proved by successfully applying them in a microwave circuit demonstrating an intrinsic Q factor of 58 000 at a frequency of 70 MHz and 77 K, which is higher than the results obtained by other teams of researchers.

Nonlinear microwave properties of YBa2Cu3O7−x block epitaxial films near the superconducting transition temperature

The structure and local properties of a nonlinear microwave response of YBa{sub 2}Cu{sub 3}O{sub 7-x} block films, as well as microbridges of these films, are studied using low-temperature scanning microscopy. Correlation between the halfwidth of the temperature dependence of the third harmonic power W{sub TH} and the curve of the electron-beam induced voltage W{sub EBIV} is observed for the medium size of the block. A theoretical model is proposed, which demonstrates that the nonlinear microwave response for large blocks is determined by intrablock pinning of Abrikosov vortices; for blocks of smaller size, the response acquires an additional contribution associated with pinning of hypervortices on the Josephson network of the boundaries between the blocks.

Influence of Ag-doping and thickness on superconducting properties of YBa 2Cu 3O 7 films

Pure and silver doped epitaxial high quality YBa 2Cu 3O 7 films of various thicknesses have been grown by pulsed laser deposition. Ag-doping procedures have demonstrated that the critical current density ( J c) is possible to enhance by a careful adjustment of the doping. Comparing thickness dependences of J c for doped and undoped samples, it turns out that the thickness strongly affects the doping influence. Applied magnetic field and temperature dependences of J c for doped and undoped films of different thicknesses are discussed in order to explain the observed results.

Tuning the superconducting properties of YBa 2Cu 3O 7 tapes grown by chemical methods

Two innovative strategies for the growth of artificial pinning centers by using chemical methods are presented. First one consists of the controlled generation of artificial nanostructured templates by using diluted metalorganic solutions. We show the role of the different growth parameters as well as lattice mismatch in the final template (either BaZrO 3 nanodots on SrTiO 3 or CeO 2 nanodots on LaAlO 3). Second approach is the growth of secondary phase oxide nanoparticles (BaZrO 3 and Y 2O 3) inside the YBCO matrix grown by the trifluoroacetates route. The relevance of lattice mismatch is also investigated in this case. We show that both strategies lead to a significant improvement of vortex pinning properties of the YBCO films suggesting that artificial pinning centers has been effectively generated. These results prove that chemical methods are a promising technique for the nanostructuration of coated conductors.

The effect of Gd diffusion-doped on structural and superconducting properties of YBa 2Cu 3O 7− x superconductors

The effect of Gd diffusion on structural evolution and superconducting properties of YBa 2Cu 3O 7− x (Y123) prepared in the bulk polycrystalline form were studied. The gadolinium (Gd) diffusion in superconducting Y123 ceramic has been studied over the temperature range of 600–900 °C by energy dispersive X-ray fluorescence technique (EDXRF). The temperature dependence of Gd diffusion coefficient in grains ( D 1) and over the grain boundaries ( D 2) are described by the relations: D 1 = 8.9 × 10 −6 exp(−1.25/ kT) and D 2 = 2.3 × 10 −5 exp(−1.09/ kT). For the Gd diffused-doped samples, magnetization and resistivity measurements show that the critical transition temperature, T c, increased from 88 to 91 K and the critical current density, J c, which was calculated from M-H loops taken at 77 K, increased from 55 to 122 A cm −2 in comparison with those of undoped Y123. The possible reasons for the observed increases in T c and J c due to Gd diffusion were discussed. In addition, magnetic hysteresis curves of the Gd900, Gd800, Gd700 and undoped Y123 are presented for temperature of 5 K. It was found that the hysteresis is enhanced by the Gd diffusion. Such enhancement, which is considered to represent a characteristic strength of intergrain coupling, is more clearly recognized when critical current densities are compared.

Correlation between structural disorder and electric properties in YBa2Cu3O7−δ (YBCO)

The degradation of the surface resistance Rs of YBCO material by ion milling is usually supposed to be linked to oxygen loss and crystallographic defects. To study this effect, the structural and physical analysis of the milled YBCO thin film has been carried out using Ion Beam Analyses and SQUID measurements. The cationic composition and the oxygen quantity measured in the disordered layer on the top of the film are the same on milled and reference samples. An increase of the bulk defect concentration, especially in the oxygen sublattice, is observed in the layer underlying the disordered layer. From SQUID measurements, the milled sample shows a degradation of susceptibility vs temperature X(T) compared to the reference sample. This observation is in a good agreement with the structural analysis and the Rs measurements. After an oxygen annealing at 450°C on the milled samples, the cationic disorder decreased and the bulk defect concentration has not been affected. The X(T) characteristic has improved for the milled film showing its correlation with the evolution of the disordered layer. However the Rs value of the annealed material is not restored.

Simple high-temperature superconducting yttrium cuprate-based device for measuring weak magnetic fields

The earlier data for the magnetic properties of YBa2Cu3O7 − x polycrystals are used to design a highly sensitive device measuring weak magnetic fields. The sensitivity of the device is close to that of SQUIDs and much higher than the sensitivity of flux-gate meters. At the same time, the device is simpler in design and more reliable in operation than SQUIDs. Unlike SQUIDs, the device makes it possible to directly measure the magnetic field strength, has a wide measuring range, and exhibits a well-defined directional pattern.

Stripe order and pseudogap state in YBa 2Cu 3O 6+ x as seen from magnon-assisted interband absorption

Charge and spin inhomogeneities are believed to be fundamental for understanding the normal and superconducting properties of YBa 2Cu 3O 6+ x and other cuprates. In the present paper, this problem is studied by optical interband absorption spectroscopy, which probes a quasi-static pattern of dynamical inhomogeneities. We investigated the temperature evolution of the exciton–bimagnon and exciton–two-magnon bands in metallic YBa 2Cu 3O 6+ x films in the pseudogap and superconducting states. Our results are indicative of a stripe structure which develops upon cooling in the pseudogap state and persists in the superconducting state. A correlation is found between the temperature behavior of the optical magnon-assisted features and the published data on temperature evolution of the neutron (π, π) resonance and muon spin depolarization rate. The continuum interband absorption is shown to be sensitive to the appearance of superconductivity in YBa 2Cu 3O 6+ x and NdBa 2Cu 3O 6+ x films. This effect is discussed in the framework of interrelation between the interband absorption and the kinetic energy.

Thickness effects of SrTiO 3 buffer layers on superconducting properties of YBa 2Cu 3O 7− δ coated conductors

A thin layer of SrTiO 3 has been successfully used as a buffer layer to grow high quality superconducting YBa 2Cu 3O 7− δ (YBCO) thick films on polycrystalline metal substrates with a biaxially oriented MgO template produced by ion-beam-assisted deposition (IBAD). Using this architecture, 1.5 μm-thick-YBCO films with an in-plane mosaic spread in the range of 2–3° in full width at half maximum, and a critical current density over 2 MA/cm 2 in self-field at 75 K have been achieved routinely. More interestingly, we found the thickness of SrTiO 3 buffer layers strongly affects the properties of YBCO films. The critical current density of YBCO films increases dramatically when the thickness of SrTiO 3 buffer layer reaches an optimum range of 40–120 nm. Microstructure studies, including transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and ion-milling experiments suggest that the YBCO texture evolution, STO outgrowths and STO surface dents are strongly related to STO thickness. They are the key factors for YBCO self-field J c variation.

Morphology, structure, and electrical properties of YBa 2Cu 3O x thin films on tilted NdGaO 3 substrates, deposited by DC-sputtering

Thin YBa 2Cu 3O x (YBCO) films were deposited using DC-sputtering technique on NdGaO 3 substrates, tilted from (1 1 0) orientation by 0–26°. The structure and surface quality of the substrates were carefully characterized to obtain reliable results of thin films deposition. Structural, morphological and electrical properties of the YBCO thin films show three different ranges of inclination angle: vicinal, intermediate and high. In the vicinal range the properties of the film are generally the same as of the standard films deposited on (1 1 0) NdGaO 3 substrate. An increase of the inclination angle to the intermediate range results in a significant improvement of morphology and structural quality of the film. Best electrical parameters are measured for the films of the intermediate range also. Probable reason for such behavior is simultaneous and regular seeding of the film in the joints of facets on the substrate surface. Further increase of inclination angle leads to step bunching and oxygen out-diffusion, destroying both structural and electrical perfection of the tilted-axes YBCO film.

The effect of Ag diffusion on properties of YBa 2Cu 3O 7− x thin films produced by electron beam deposition techniques

Superconducting YBa 2Cu 3O 7− x thin films were prepared on pure MgO and Ag/MgO substrates (without and with Ag buffer layer) using an electron beam evaporation technique. The effects of isothermal annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties were investigated by X-ray diffraction, scanning electron microscopy, critical temperature, critical current density and room temperature resistivity measurements. The optimum annealing conditions causing a high degree of preferential orientation with the c-axis perpendicular to the substrates were found to be the isothermal annealing at 930 °C for 5 h. Annealing of films on Ag/MgO substrates is accompanied by Ag diffusion from the buffer layer into YBCO films. The higher rate of crystallization of the YBCO films, the higher degree of c-axis orientation, the higher dense surface morphology, the increased lattice parameter c (by ≈0.1%), the reduced room temperature resistivity (2–3 times), the slightly enchanced critical temperature ( T c = 92 K at R = 0) and the critical current density ( J c = 4.2 × 10 5 A/cm 2 at 77 K) were observed for the Ag-doped films (on Ag/MgO substrates) in comparison with those for the undoped films (on MgO substrates). The temperature dependence of the Ag diffusion coefficient in YBCO films in the range 600–800 °C was described by the relation D = 1.9 × 10 −6 exp(−0.73 eV/kT).

The structure and superconductivity of Y 1− xCa xBa 2Cu 3O 7− δ coatings on YSZ single crystal substrate

It is reported that Ca-doping can improve the weak links between grain boundaries and increase the critical current density J c of YBa 2Cu 3O 7− δ polycrystalline films [Phys. Rev. B 58 (1998) 9543; Nature 407 (2000) 162]. In this paper, we investigated the preparation and superconducting properties of YBa 2Cu 3O 7− δ (YBCO) and Y 1− x Ca x Ba 2Cu 3O 7− δ ( x = 0.05, 0.1, and 0.2) (YCBCO) coatings on yttrium stabilized zirconia (YSZ) single crystal substrates by powder melting growth process. The X-ray diffraction (XRD) indicated that: (1) the coatings were of YBa 2Cu 3O 7− δ (Y123) phase with c-orientation; (2) the Y 2BaCuO 5 (Y211) phase remained clearly in the YBCO coating, but much less in the YCBCO coatings. The effect of Ca-doping on the formation of the Y211 phase was analyzed with the help of the differential thermal analysis (DTA). The results of R– T and I– V relationship at 77 K demonstrated that the zero resistance temperature T c0 decreased with the Ca content, while the critical current density J c (77 K) was improved for x = 0.05.

Effects of substituting calcium for yttrium on the superconducting properties of YBa 2Cu 3O z bulk samples

We report systematic studies of AC magnetic susceptibility and transport properties of Y 1− x Ca x Ba 2Cu 3O z bulk samples with 0⩽ x⩽0.4. Single phase materials, reduction of carrier concentration and decrease of T c to 83 K were obtained at doping levels up to 20%. For Y 0.7Ca 0.3Ba 2Cu 3O z sample the improvement of grains boundary transport and screening capabilities has been observed as a result of the optimal ratio between carrier concentration and impurity phase BaCuO 2 presence. The appearance of bulk pinning and nonlinear effects starting at the highest temperature were detected also.

Effect of perovskite oxide cap layer on superconducting properties of YBa 2Cu 3O x films grown on mechanically polished SOE substrates

A new oxide cap layer, BaZrO 3 (BZO) with the perovskite structure, on the NiO buffer produced by surface-oxidation epitaxy is reported. BZO is the product as yttrium stabilized ZrO 2 buffer reacted with YBa 2Cu 3O x (YBCO), and it is chemically stable. BZO cap layer deposited on the polished NiO surface by pulsed laser deposition (PLD) was strongly (1 0 0) oriented, and in-plane texture, Δ φ for BZO was 10°. Δ φ for YBCO film, subsequently deposited on BZO/NiO/Ni substrate at 690 °C by PLD, is 9°. As the result, critical current density, J c, of YBCO/BZO/NiO/Ni reached 0.89 MA/cm 2 at 77 K and self-field.

Properties of YBa 2Cu 3O x and Bi 2Sr 2CaCu 2O x thin film microstrips patterned by argon ion beam

High temperature superconductor YBa 2Cu 3O x (YBCO) and Bi 2Sr 2CaCu 2O x (BSCCO) thin film microstrips were prepared by argon ion beam etching down to a geometrical width W g≈250 nm. A study of transport properties of the superconducting microstrips showed that due to the interaction of accelerated argon ions with the superconductor film the edges of microstrips are damaged to a depth Δ W≈85 nm. The effective width of YBCO microstrip W ef = W g−2Δ W≈80 nm is able to transmit a critical current density above 20 MA/cm 2 at a temperature of 30 K and a zero external magnetic field. The critical current density of similar BSCCO microstrips was several times smaller.

The effect of Pr- and Ca-doping on the flux pinning and superconducting properties in YBa2Cu3O7−δ

The effects of Pr- and Ca-doping on the critical current density Jc and the critical temperature Tc in (Y1−x−yPrxCay)Ba2Cu3O7−δ have been investigated. The Pr-doping has been found to enhance the pinning force and the Jc at 70 K on applying magnetic field; Jc takes a maximum at x=0.05 for μ0H=2.0 T (optimum concentration). This result indicates that the Pr ions in YBCO induce the nanometric inhomogeneities that act as effective pinning centers in the presence of magnetic field. The Ca-doping is expected to supply the mobile holes to the CuO2 planes. However, this doping does not contribute to further enhancement of Jc at the optimum x.

Electrical properties of YBa2Cu3O7−x films of various thicknesses

We have studied the electrical properties of high-Tc superconductor YBa2Cu3O7−x films obtained by the ion-plasma deposition technique on sapphire substrates. Dependences of the surface resistance and the critical current density on the film thickness are determined.

Non-universal glass scaling behavior in transport properties of YBa 2Cu 3O y films with columnar defects

Vortex phase diagrams have been studied on heavy-ion irradiated YBa 2Cu 3O y films by the measurements of current–voltage ( I– V) characteristics, resistance R and critical current density J c as a function of magnetic field B parallel to the columnar defects. In the scaling analyses of the I– V data, we find the non-universal nature of the glass transition. The dynamic critical exponent z, which is almost constant at high fields, shows an anomalous peak and the glass transition line T g( B) reveals an upturn behavior at B∼ B φ /3 with B φ the matching field, where we also observe step-like behavior both in R( B) and J c( B) curves. These results indicate the existence of a temperature-independent boundary in the B– T plane at which the dissipation mechanism changes.