Properties Of YBa2Cu3O7 Research Articles

Influence of ramp shape and morphology on the properties of YBa 2Cu 3O 7− δ-ramp-type junctions

The correlation between the shape and the morphology of ramps prepared by ion-beam etching of YBa 2Cu 3O 7− δ thin films, and the properties of ramp-type junctions were investigated in detail. We examined the influence of different fabrication parameters on the YBa 2Cu 3O 7− δ ramps by atomic force microscopy. Ramp-type junctions were fabricated using PrBa 2Cu 2.9Ga 0.1O 7− δ as barrier material. We observed a strong influence of the shape of the ramp on the homogeneity and, thus, on the transport properties of the junctions. Furthermore, we observed that the roughness of the ramps is strongly influenced by the voltage of the ion-beam during etching. Best results are achieved when an additional wet cleaning step by bromine solution in ethanol is introduced prior to the deposition of the barrier and the top electrode. As a result from our optimization, the on-chip spread of the junction critical current was reduced to 11%.

Erratum to “Thermal and mechanical effects on the properties of YBa 2Cu 3O 7− δ deposits obtained by electrodeposition of precursors in presence of cyanide” [Physica C 276 (1997) 251–258

Erratum to “Thermal and mechanical effects on the properties of YBa 2Cu 3O 7− δ deposits obtained by electrodeposition of precursors in presence of cyanide” [Physica C 276 (1997) 251–258

Binder effect on microstructure and properties of YBa 2Cu 3O 7− x extruded wires

YBa 2Cu 3O 7− x wires have been extruded with 2 and 5 wt.% of hydroxy propyl methylcellulose (HPMC) as binder. Both sets of wires sintered below 930°C have equiaxed grains while the wires sintered above this temperature have elongated grains. In the temperature range which gives equiaxed grains, the wires extruded with 5 wt.% HPMC have higher grain size and density. Cracks along the grain boundaries are often observed in the wires having elongated grains. Critical current density, J c, increases initially, reaches a peak and then decreases with the sintering temperature. The sintering temperature giving a peak in J c strongly depends on the heat treatment scheme for the wires extruded with 5 wt.% HPMC. TEM studies show that defective layers are formed along grain boundaries for the wires extruded with 5 wt.% HPMC after 5 h oxygenation. After 55 h oxygenation, the defective layers become more localised and grain boundaries adopt an overall cleaner appearance. Densification with equiaxed grains and clean grain boundaries produces the highest J c's for polycrystalline YBa 2Cu 3O 7 wires.

Interaction of process parameters in the laser deposition of YBa 2Cu 3O 7 films

The electrical and morphological properties of YBa 2Cu 3O 7 films grown on SrTiO 3(100) substrates in a laser deposition process using a pulsed KrF excimer laser are investigated. For the first time, a quantitative order of importance of the relevant deposition parameters and their interactions that determine the quality of such films can be given. This is achieved by using statistical methods such as analysis of variance and multiple regression analysis for the design and evaluation of the experiments. The films prepared under optimum conditions grow c-axis oriented with low outgrowth densities and reveal transition temperatures T c≅89 K, resistivities ρ(100 K)≤60 μΩ cm, and resistance ratios R(300 K)/ R(100 K)≅3. The surface of the targets used for the deposition turned out to be crucial. Fresh targets yield significantly better electrical properties with a much lower spread of the data. Thus, a region in the process parameter space of YBa 2Cu 3O 7 film deposition has been identified where electrical and morphological properties are at optimum, simultaneously. This is prerequisite for the preparation of multilayer devices.

THz mixing properties of YBa 2Cu 3O 7−δ grain boundary Josephson junctions on bicrystal substrates

Using high- T c grain boundary Josephson junctions (GBJJs) made of YBa 2Cu 3O 7−δ (YBCO) deposited across MgO bicrystal boundary, at TeraHertz (THz) waveband we successfully demonstrated both fundamental and harmonic frequency mixing. Radiation from a far-infrared (FIR) laser was coupled to the junction, which was integrated with a planar bow-tie antenna, via an extended hyperhemispherical silicon lens. Fundamental mixing manifested itself in the junction's DC current-voltage ( I- V) curve as a third Shapiro step in addition to those two induced by the THz laser lines from a slightly misaligned resonator. In harmonic mixing between a THz laser line and a microwave local oscillator, the highest harmonic number we could get was 490 with a signal-to-noise-ratio of 9 dB at the intermediate frequency (IF). The IF output ( P IF) depended strongly on the bias voltage, the local oscillator power level, the harmonic number ( N), etc.

Microwave dynamics and electromagnetic properties of YBa 2Cu 3O 7 step-edge junctions on MgO substrates

In this paper, we report the successful fabrication and current-voltage characteristics of the epitaxial c-axis YBa 2Cu 3O 7 step-edge junctions (SEJs) on MgO substrates. The junctions were fabricated by patterning the laser deposited YBa 2Cu 3O 7 thin films on MgO substrates with ionmilled steps. Integer Shapiro steps were observed in the current-voltage characteristics up to 1 THz under 72 GHz external microwave irradiation. Additional subharmonic Shapiro steps have also been found in some junctions with small step height and film thickness ratio, which may be explained by the inhomogeneous YBa 2Cu 3O 7 grain boundary interface formed at the edges of the substrate step. Self-induced electromagnetic resonance (Fiske mode) peaks have been observed in the current-voltage curves of the SEJs with large step height and film thickness ratios. The Swihart velocity and the effective shunting capacitance have been estimated to be 6.9 × 10 6 m s −1 and 55 fF μm −2 at 4.2 K, respectively, from the voltage of the observed Fiske resonance. The shunting capacitance obtained from the electromagnetic resonance compares well with that estimated from the McCumber constant β c .

First-principles band-structure calculations as a tool for the quantitative interpretation of Raman spectra of high temperature superconductors

We have performed first-principles band structure calculations in order to investigate vibronic and optical properties of YBa2Cu3O7. A formalism describing temperature dependent Raman spectra from such ab-initio results has been applied to the 500 cm−1 apex oxygen mode and its overtone in good agreement with experimental results. The dynamical matrix of the five A1g modes established by atomic-force calculations is studied in detail showing rather good agreement with experimental eigen-frequencies and normal coordinates. The effect of isotope substitutions on the phonon frequencies is investigated. We demonstrate that the calculated vibronic properties of high Tc materials are improved by applying a generalized gradient correction scheme for the treatment of exchange and correlation effects instead of the local-density approximation.

Geometry effects on the electronic properties of YBa2Cu3O7

We have performed electronic structure calculations for the high T c compound YBa2Cu3O7 with a 3% reduced unit-cell volume. The effect on band structure and Fermi surface is investigated. The predominant features are van Hoove singularities at the Fermi level. Keeping the experimental lattice parameters, nearly the same effects occur when replacing the local-density approximation by a generalized gradient correction scheme.

Ba2RESbO6 (RE = Nd, Eu and Dy); Characterization and Potential Use as Substrate for Superconducting YBCO Films

Perovskite compounds of the general formula, Ba2RESbO6 (where RE = Nd, Eu and Dy) were sintered as single phase materials by standard solid state techniques. These compounds are found to be isostructural and to have a complex perovskite structure as observed by powder X-ray diffraction measurement. No chemical reaction between YBa2Cu3O 7-δ and Ba2RESbO6 compounds, was observed even under severe heat treatment at 950°C, and the addition of Ba2RESbO6 up to 20 vol% in YBa2Cu3O 7-δ, shows no detrimental effect on the superconducting properties of YBa2Cu3O 7-δ. The dielectric constant and loss factor values of sintered specimens, are in a range suitable for their use as substrates for microwave applications. Thick films of YBCO fabricated on highly polished Ba2RESbO6 specimens, gave a superconducting zero resistance transition T c(o) of 92 K, with a critical current density, J c of 2×104 A/cm2 at 77 K.

Characterization and transport properties of YBa 2Cu 3O y/Y xPr 1− xBa 2Cu 3O y superlattices under magnetic fields using an off-axis rf magnetron sputtering technique

We report here on the growth and transport properties of YBa 2Cu 3O y /Y x Pr 1− x Ba 2O y superlattices. The YBa 2Cu 3O y /Y x Pr 1− x Ba 2Cu 3O y [(YBCO/PBCO) for x = 0 and (YBCO/YPBCO) for x = 0.5] superlattices were grown in situ in an off-axis radio-frequency magnetron sputtering systems. The superlattices structure of the samples was characterized by powder X-ray diffraction. Appearance of satellite diffraction peaks near the main (001) peaks confirms the superlattice structure. Resistivity measurements carried out under magnetic fields reveal thermal activated behavaiour. Hall coefficients, R H, in mixed state show sign reversal and a diminshing of R H is observed when the thickness of the YPBCO layers is increased or the thickness of the YBCO layer is decreased. Angular dependence of the critical current density, J c, in YBCO/PBCO superlattices shows a decoupled in the flux line dynamics as the coupling strength of the YBCO layers weakens.

Enhanced transport properties in YBa 2Cu 3O x films with crossed columnar defects of various inclination angles

We have studied the effect of heavy-ion irradiation on the transport properties of YBa 2Cu 3O x thin films. Columnar defects parallel to the c axis (0°) and crossed columnar defects with various inclination angles ( θ = ± 10°, ± 20°, and ± 40°) were introduced into films by 1.3 GeV U-ion irradiation with dose equivalent to 1 T vortex density. Transport properties including irreversibility fields, activation energies, and critical current densities have been measured in magnetic fields applied parallel to the c axis. The best transport properties were observed in samples of θ = ± 10° in most of the H-T plane we studied, consistent with the splay model. However, the relative enhancement of the critical current density in the θ = ± 10° compared to the 0° sample was not as large as in the case of a single crystal suggesting that in films the point defects also play an important part in flux pinning.

Theoretical investigations of structural and vibronic properties of YBa 2Cu 3O 7 by first-principles atomic-force calculations

We have determined the force-free geometry and the dynamical matrix for the five Raman-active A 1 g modes in YBa 2Cu 3O 7 by full-potential LAPW atomic-force calculations. Treating exchange and correlation effects by LDA leads to phonon frequencies in rather good agreement with experimental values. However, they are significantly improved by employing generalized gradient corrections to the local exchange-correlation potential.

Experimental study of quasiparticle tunneling in high Tc junctions

We have studied tunneling properties of YBa 2Cu 3O 7-insulator-normal metal junctions by measuring the current-voltage curves, the low-voltage junction resistance as a function of temperature, and the dynamic resistance as a function of the junction bias voltage. The insulating barrier was either SrTiO 3 or AlO x and the normal electrode was Au, Ag, or Nb. The current-voltage curves and the temperature dependent junction resistance showed tunneling characteristics with semiconducting background. The dynamic resistance versus bias voltage had fine structures in low voltages which might indicate the superconducting gap.

The ab-anisotrophy of twinfree YBa 2Cu 3O 7-δ films above and below T c

High quality twin-free c-axis oriented YBa 2Cu 3O 7-δ films were grown by pulsed laser deposition on NdGaO 3. We find resistive anisotropies between 1 < ϱ a/ϱ b < 1.6. The resistive anisotropy above T c appears to be correlated to the anisotropy in the critical current below T c. The normal state properties of the chains along b seem to have a significant influence on the superconducting properties of YBa 2Cu 3O 7-δ.

Fabrication techniques and electrical properties of YBa 2Cu 3O 7−x multilayers with rf sputtered amorphous SiO 2 interlayers

We have successfully fabricated YBa 2Cu 3O 7−x/YSZ/YBa 2/YSZ/YBa 2Cu 3O 7−x multilayer structures on single crystal LaAlO 3 (100), substrates. The YBa 2Cu 3O 7−x(YBCO) layers were deposited using pulsed laser ablation (PLD), the biaxially aligned YSZ (250 nm thick) layers were deposited using ion beam assisted PLD (IBAD-PLD), and an amorphous SiO 2 (1–2μm) layer fabricated via rf sputtering was sandwiched between the YSZ layers. Fabrication techniques and characterization results are reported for patterned layers in this work.

Properties of Y1Ba2Cu3O7−δ and Ba1−XKXBiO3 step-edge grain boundary junctions

A technique for the fabrication of sharp and straight step-edges on LaAlO3 (LAO) substrates by Ar-ion milling is reported here. Diamond-like carbon (DLC) thin films, deposited by pulsed laser ablation, were used as masks. Clear Shapiro steps were observed in the current-voltage (IV) characteristics of Y1Ba2Cu3O7−δ (YBCO) step-edge junctions with millimeter-wave irradiation. Also, the transport properties of epitaxial Ba1−XKXBiO3 (BKBO) films deposited on LaAlO3 step-edge substrates were studied.

Thin Films and Heterostructures of Layered Oxides: Preparation and Properties

A brief overview of the deposition and properties of YBa2Cu3O7 (YBCO) based heterostructures is presented. Emphasis is placed on the assessment of the deposition process and the properties of practical thin films and heterostructures of layered oxides. Besides a reference to the literature we present our recent results concerning deposition, structural and magnetic properties of YBa2Cu3OT/RE1- χ AxMnO3 (YBCO/REMO) heterostructures.

Influence of film growth conditions on the transport properties of YBa2Cu3O7−δ step-edge junctions

Abstract In order to investigate the influence of film growth conditions on the transport properties of step-edge junctions, YBa2Cu3O7−δ films were deposited on high-quality substrate steps by DC-sputtering at two different substrate temperatures and by off-axis laser ablation. RSJ-like junction properties and a low (1σ)-spread of the critical current density Jc (4.2 K) of 17% along one step were obtained for laserablated junctions. The transport properties of the differently prepared junctions correlated with the microstructure of the step region. This, in turn, might result from the large differences in surface diffusion during film growth.

Microstructural and physical properties of YBa 2Cu 3O &−δ superconductors prepared by the flame-quench-melt-growth (FQMG) method

The microstructural and physical properties of YBCO-123 superconductors prepared by the flame-quench-melt-growth (FQMG) method have been investigated in terms of XRD, optical, SEM and AC susceptibility measurements. Samples were prepared under various heat treatments in a flowing oxygen atmosphere. Optimum preparation conditions have been determined for good quality samples with a high degree of order in large grain alignment leading to high J c values for YBCO-123 at short heat treatments. T c and H c values have been determined by inductive methods. T c values are less sensitive to the heat treatment conditions while J c values increased considerably for samples grown at 1050°C for 20 h. High values of J c can be attributed to several various AC loss mechanisms related with flux pinning.

Thermal and mechanical effects on the properties of YBa 2Cu 3O 7−δ deposits obtained by electrodeposition of precursors in presence of cyanide

Electrodeposition of yttrium, barium and copper in metallic state from DMSO (dimethylsulfoxide) solutions containing cyanide as complexing agent for copper leads to the preparation of YBa 2Cu 3O 7−δ directly over metallic substrates. Superconducting properties on coatings obtained by this procedure showed the largest T c and J c obtained to date by techniques involving electrodeposition of precursors over metallic silver substrates. However, those properties can be further improved by slight modifications of the thermal treatments and by mechanically pressing of the deposit before the oxidation process. Values of J c near 3000 A/cm 2 have been obtained at liquid nitrogen temperatures in the absence of applied magnetic field. Notably, the initial pressing of the precursors coating leads to preferentially oriented crystals after the oxidizing process.