Epitaxial Superconducting Research Articles

Epitaxial Growth of YBa2Cu3O7 Films onto LaAlO3 (100) by Using Oxalates

Abstract Due to the current necessity to obtain epitaxial superconductor films at low cost, we report the growth of YBa2Cu3O7 (Y123) films by chemical deposition. The procedure involved simple steps such as precipitation of stoichiometric amounts of yttrium, barium and copper acetates in oxalic acid (H2C2O4). The precursor solution was dripped onto LaAlO3 (100) substrates with the help of a Fisher pipette. The films were annealed in oxygen atmosphere during 12 h at three different temperatures: 820 °C, 840 °C and 860 °C. After 820 °C and 860 °C annealing, X-ray diffraction (XRD) analysis revealed high intensity of the ( 00l ) reflections denoting that most of the Y123 grains were c-axis oriented. In addition, we also observed a-axis oriented grains (( h00 ) reflexion), minor randomly oriented grains and other phases (such as Y2BaCuO5 and CuO). In contrast, the sample treated at 840 °C, we noticed c – and a-axis oriented grains, very small amounts of randomly oriented grains without formation of other phases. From the magnetization versus temperature measurements, the critical temperatures were estimated at 70K and 90K for the samples annealed at 820 °C and 860 °C respectively.

Epitaxial Superconducting δ-MoN Films Grown by a Chemical Solution Method

The synthesis of pure δ-MoN with desired superconducting properties usually requires extreme conditions, such as high temperature and high pressure, which hinders its fundamental studies and applications. Herein, by using a chemical solution method, epitaxial δ-MoN thin films have been grown on c-cut Al(2)O(3) substrates at a temperature lower than 900 °C and an ambient pressure. The films are phase pure and show a T(c) of 13.0 K with a sharp transition. In addition, the films show a high critical field and excellent current carrying capabilities, which further prove the superior quality of these chemically prepared epitaxial thin films.

Microwave nonlinearity measurements of epitaxial YBa 2Cu 3O 7−x thin film by bridged resonator

A simple bridged resonator method is presented for microwave nonlinearity measurement of epitaxial superconductor thin film. By this method, microwave current is concentrated in local bridged part on the resonator, and local inductive and resistive nonlinearity in this part are measured. At the same time, undesired nonlinearity are eliminated from the tested results by the special current distribution of the resonator, which enhances the accuracy of the measured nonlinearity.

Extraction of Misorientation Components from the Total Misorientation at Grain Boundaries using Electron Diffraction in a Y0.9Sm0.1Ba2Cu3O7 Film

Grain boundary (GB) misorientation determines the transport properties of REBa2Cu3O7 (REBCO) superconducting films (rare earth (RE)). In this study, the misorientation angles between the adjacent grains in a Y0.9Sm0.1Ba2Cu3O7 (YSBCO) epitaxial superconducting film on buffers performed via ion‐beam‐assisted deposition have been characterized using transmission electron microscopy. The in‐plane and out‐of‐plane misorientation at GBs and sub‐GBs has been separated from the total misorientation. Most of the GB misorientation angles are smaller than 5°, with the in‐plane component γ<3° and out‐of‐plane components α≤1° and β<2°. This study provided a relatively simplified, practical, and useful method for determination of GB misorientation angles and separation of the in‐plane and out‐of‐plane misorientation from the total misorientation for epitaxial REBCO superconducting films.

Ultrathin epitaxial superconducting niobium nitride films grown by a chemical solution technique

Ultrathin epitaxial superconducting NbN (18 nm) films, exhibiting a superconducting transition temperature of 14 K and a critical current density as high as 5.2 MA cm(-2) at 5 K under zero magnetic field, were grown on SrTiO(3) (STO) by a chemical solution technique, polymer assisted deposition (PAD).

Systematic enhancement of in-field critical current density with rare-earth ion size variance in superconducting rare-earth barium cuprate films

Enhanced in-field critical current densities (Jc’s) have been obtained in epitaxial superconducting (RE1,RE2)Ba2Cu3O7−x (RE1=rare-earthion 1, and RE2=rare earth ion 2) films grown on both single crystal and buffered metallic substrates. For a constant average RE ionic radius (equal to that of yttrium), there is a systematic dependence of the in-field Jc on the RE ion size variance, with a small, but nonzero, variance being optimum. Compared to the standard YBa2Cu3O7−x composition, a factor of two improvement in Jc(75.5K) is reproducibly observed at 0.2T(‖c) for the composition Dy1∕3Ho2∕3Ba2Cu3O7−x on both single crystal and buffered-metallic substrates. Angular dependent magnetic field studies and transmission electron microscopy indicate the presence of additional pointlike random defects.

Electron backscattered Kikuchi diffraction technique: for a better understanding of epitaxial superconducting film growth on buffered Ni (RABiTS) tapes

A reel-to-reel MOCVD system has been designed to synthesize high quality YBCO coated tapes on Ni RABiTS (Rolling Assisted Biaxially Textured Substrates). Special buffer layer stackings have been deposited to improve the quality of the superconductor. Several buffer layers architectures rose in dense YBCO films with a similar texture, but fluctuations in critical current densities (Jc) were measured. Therefore, a more accurate and detailed measurement method such as Electron Backscattered Kikuchi Diffraction (BKD) has been used to assess the crystalline quality of the stackings. The studied sequence here was NiW /NiO /YSZ /CeO2 /YBCO. The grain and the subgrain structure as well as their size and disorientation have been analysed for each layer. First on Ni tape, secondly on the protecting NiO (200) buffer layer, then on the following CeO2 (200) film. Finally, unpublished BKD diagrams of the subsequent YBCO (001) films have been reported here. The grain evolution has been successfully studied showing the buffer layer effect on the grain growth. In this work we highlight the correlation between the misorientation of the grains (inducing NiO (111) growth and cracks) and the grain boundary morphology (size, grooves). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Variable temperature scanning laser microscopy of wider width high temperature superconducting films

We have investigated the spatial distribution of resistive properties in 2 mm wide and 10 mm long epitaxial superconducting films using a variable temperature scanning laser microscopy (VTSLM). This technique measures ac voltage of bolometric response created by a laser beam. We have observed the spatial nonuniformity of superconducting transition temperature in the resistive region, which has never been reported in samples wider than 300 /spl mu/m using scanning laser techniques. This result is a significant step toward developing VTSLM for coated conductor diagnosis.

The study of critical current density of superconducting thallium oxide films from spray-deposited precursors

Spray deposition is a promising and economic approach for fabricating high-temperature superconducting films for the Tl-oxide superconductors. The Tl-oxides provide the potential for operation at 77 K in practical magnetic fields of 3– 5 T , which has been demonstrated by measurements on the irreversibility behavior of the Tl-1223 single layer compound with Bi and Sr substitution. In this report, we present the preparation and characterization of Tl 0.78Bi 0.22Sr 1.6Ba 0.4Ca 2Cu 3O 9 superconducting films fabricated from spray-deposited Tl-free, Ag-containing and Cu-rich precursors and reacted with thallium vapor using a two-step crucible process at 650°C/815°C in flowing argon for only 10/10 min . An epitaxial superconducting film was synthesized successfully and critical current densities of 8×10 5 A/ cm 2 at 77 K in zero-field were obtained with excellent in-field performance.

Microstructural relationships between 45° [0 0 1] tilt bicrystal substrates and epitaxial superconducting films

Microstructures of superconducting thin films epitaxially grown on both symmetric and asymmetric 45° cubic ceramic bicrystal substrates have been investigated by transmission electron microscopy. The grain boundary (GB) of the asymmetrically tilted SrTiO 3 (STO) substrate is straight with a single (1 1 0)/(1 0 0) facet, while that of the symmetric 45° bicrystal substrate is wavy and contains many asymmetric step-like (1 1 0)/(1 0 0) nanofacets. GBs of the superconducting films grown on such symmetric 45° bicrystal STO substrates are composed of only (1 1 0)/(1 0 0) facets, hence meander. All the results indicate that the interfacial energy of asymmetrical (1 1 0)/(1 0 0) is lower than the symmetric configuration (2 5 0)/(5 2 0) in the STO. A combination using asymmetric 45° bicrystal MgO substrates (another kind of cubic ceramic that has the same crystal geometric microstructure with that of STO and larger thermal expansion coefficient than that of YBa 2Cu 3O 7− δ (YBCO)) and enhanced lateral growth method, such as liquid phase epitaxy, enables us to obtain long single facets (about 10 μm long) in YBCO films.

The preparation of YBCO epitaxial superconducting films by a chemical solution deposition process

The utility of a chemical precursor solution for the production of YBa2Cu3O7-x superconducting films has been investigated. The homogeneous precursor solution made of metal acetates, 1,3-bis(dimethylamino) -2-propanol, and acetic acid was applied to [1 0 0] MgO substrates via spin-coating. Subsequent heating and sintering removed the organic components. The best epitaxial film obtained by this chemical solution deposition method has a Tc of 78 K.

The Edge Effect of a Microstrip Resonator on the Field Dependence of the Surface Resistance of a High-Tc Superconducting Thin Film

A new behavior of the field dependence of microwave surface resistance (Rs), which was observed on a microstrip resonator and may be caused by the edge of the center strip, is reported in this paper for epitaxial high-Tc superconducting (HTSC) thin films. The exhibited behavior is that Rs remains almost unchanged below a certain rf magnetic field Hrf, and then increases abruptly at this field, after which it increases in proportion to Hrf. To explain the behavior, the morphology of the microstrip resonator was examined by atomic force microscopy (AFM), which showed that the edge of the resonator was damaged in some regions because of the acid etching. If the damaged edge is considered as a weakened granular superconductor, the observed Rs behavior could be explained well in terms of the high-frequency critical state model. This implies that the edge condition should be considered in studying the field dependence of Rs when the planar resonator technique is used.

Conductors with controlled grain boundaries: An approach to the next generation, high temperature superconducting wire

Much of the conductor development effort in the last decade has focused on optimizing the processing of (Bi, Pb)2Sr2Ca2Cu3Ox oxide-powder-in-tube conductors and (Bi, Pb)2Sr2CaCu2O8 (Bi-2212) and TlBa2Ca2Cu3Ox thick film conductors. It is demonstrated that in each of these conductors, critical current densities are dictated by the grain boundary misorientation distributions (GBMD's). Percolative networks of low-angle boundaries with fractions consistent with the active cross-sectional area of the conductor exist in each of these conductors. Further enhancements in the properties require increased numbers of small-angle grain boundaries. Given the processing methods used to fabricate these materials, no clear route employing a simple modification of the established processing method is apparent. To address this need, conductors with controlled or predetermined GBMD's are necessary. Development of biaxial texture appears to be the only possible way to increase the number of small-angle boundaries in a practical and controllable manner. We summarize in this paper recent results obtained on epitaxial superconducting films on rolling-assisted-biaxially-textured-substrates (RABiTS). This technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield structurally and chemically compatible surfaces. Epitaxial YBa2Cu3O7–δ films grown using laser ablation on such substrates have critical current densities exceeding 106 A/cm2 at 77 K in zero field and have a field dependence similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of the next generation high temperature superconducting (HTS) wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

Epitaxial superconductors on rolling-assisted biaxially-textured substrates (RABiTS): a route towards high critical current density wire

Abstract Fabrication of epitaxial, high- J c , biaxially aligned YBCO thick films on rolling-assisted biaxially-textured substrates (RABiTS) is summarized. The RABiTS technique utilizes standard thermomechanical processing to obtain long lengths of flexible, biaxially oriented substrates with smooth surfaces (rms ∽50 nm). The strong biaxial texture of the metal (in-plane 6–7° FWHM) is conferred to the superconductor by deposition of intermediate metal and/or oxide layers which serve both as a chemical as well as a structural buffer. Epitaxial YBCO films grown using laser ablation on RABiTS™ have critical current densities exceeding 10 6 A cm −2 at 77 K in zero-field and have field dependences similar to epitaxial films on single crystal ceramic substrates. The texture of the base metal has been achieved in lengths over 1 m and scaleable techniques are being pursued to deposit epitaxial multilayers. 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.

Thermal model for the bolometric response of high- superconducting films to chopped optical radiation

Thermal detectors which convert the incident radiation to heat are frequently used to give a periodic response to the external signal modulated at a fixed low-frequency chopping frequency. Many workers have ascertained the bolometric response of high-quality epitaxial superconducting films on different substrates to chopped laser radiation. Based on these observations we made an a priori assumption that the voltage response of thin film is entirely due to a thermal mechanism, i.e. in the proposed one-dimensional heat conduction analysis any possible non-equilibrium processes are neglected. A new analytical formula is derived for the mean temperature increase in the superconducting film. This formula generalizes the previously well known analytical approximations. Further we explore a specific numerical example based on experimental data for the optical response of a thin () high- Bi-based film on an substrate irradiated by chopped He - Ne laser radiation ( nm). On the basis of the theory and the chopping frequency dependence of the peak voltage response an implementation of a new diagnostic method for evaluation of the thermal Kapitza conductance at the superconducting film - insulator interface is demonstrated.

Double tunnel junction experiments with epitaxial niobium and tantalum films

We have used three-terminal double tunnel junction devices to investigate non-equilibrium quasiparticle and phonon transport in epitaxial superconducting films of niobium and tantalum. We show that the two junctions are coupled both by quasiparticle diffusion through the film and by the diffusion of recombination phonons through the sapphire substrate. By separating these two processes and making a random walk simulation of the quasiparticle diffusion we are able to estimate the effective quasiparticle lifetime in our niobium and tantalum films. This is not limited by recombination. Furthermore the quasiparticle lifetime in tantalum is less limited by the other (as yet unidentified) loss processes than that in niobium. This suggests that superconducting tunnel junction particle spectrometers utilising tantalum absorbers may display better energy resolution than those with niobium absorbers which are currently being investigated.

Epitaxial Thin Films And Heterostrcutures of Various Isotropic Metallic Oxides for Device Applications

AbstractWe have grown epitaxial thin films of various isotropic metallic oxides such as Sr1-xCaxRuO3 and La8-xSrxCu8O2Oin situ by 90° off-axis sputtering. These metallic oxides are pseudo-cubic perovskites with essentially isotropic properties, which could be ideal normal metals for SNS junctions in superconducting devices and for electrodes in ferroelectric devices. We have fabricated epitaxial ferroelectric heterostructures [SrRuO3/Pb(Zr0. 52 Ti0.4 8) O3 /SrRuO3] employing isotropic metallic oxide (SrRuO3) electrodes on substrates of (100) SrTiO3 and (100) Si with an yttria stabilized zirconia buffer layer. They exhibit superior fatigue characteristics over those made with metal electrodes, showing little degradation over 10 cycles, with a large remnant polarization (27 μC/cm2 ). We have also grown epitaxial superconducting heterostructures (YBa2Cu3O7 / La8-xSrxCu8O2O / YBa2Cu3O7 ) with a copper-oxide-based isotropic metallic oxide (La8-xSrxCu8O20) normal metal barrier. X-ray diffraction and cross-sectional transmission electron microscopy reveal these heterostructures to have high crystalline quality and clean interfaces. This material will facilitate fabrication of ideal SNS Josephson junctions with low boundary resistance due to its excellent chemical compatibility and lattice match with cuprate superconductors and will be useful for determining the source of interface resistance in such heterostructures.

Free-charge-carrier plasmons in Ba1-xKxBiO3: A close relation to cuprate superconductors.

Broad-range reflectance and ellipsometric spectra of high-quality epitaxial superconducting ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ thin films are reported. From these data, accurate spectral functions of ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ have been obtained. They are found to be quite similar to the in-plane spectra of the superconducting cuprates; in particular, the frequency dependence of the reflectance is nearly linear, while the dielectric loss function is nearly quadratic below the plasma edge in all of these compounds. It is therefore unlikely that acoustic plasmons or magnetic fluctuations are the cause of these anomalous spectral features.

Characterization of Epitaxial Superconducting YBacuO Thin Films with Three Different Orientations

AbstractEpitaxial thin films of YBaCuO were prepared by multilayer deposition from Y, Cu, and BaF2 sources with: (1) the a‐axis perpendicular to (100)SrTiO3; (2) the c‐axis perpendicular to (100)SrTiO3; and (3) the [110] axis perpendicular to (110)SrTiO3. XRD patterns as well as SEM and HREM images confirm that the films are highly oriented, essentially epitaxial. Both the a‐axis oriented and the c‐axis oriented films exhibit zero resistance at 91K. The [110] oriented film shows the sharpest transiton with a transition width of IK and zero resistance at 85K. The zero field critical current density, Jc, determined magnetically, is in excess of 107A/cm2 at 4.4K and 1.04 x 106A/cm2 at 77K for the c‐axis oriented film; for the a‐axis oriented film we obtained 6.7 x 106A/cm2 at 4.4K and 1.2 x 105A/cm2 at 77K. The orientation dependence of the critical current density in the basal plane of the a‐axis oriented film was studied. The largest Jc's occur along the in‐plane <100> axes of the substrate.