Grain Boundary Weak Links Research Articles

Weak-link behavior of grain boundaries in superconducting Ba(Fe1−xCox)2As2 bicrystals

We show that despite the low anisotropy, strong vortex pinning, and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1−xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe1−xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry.

How Grain-Boundaries Influence the Intergranular Critical Current Density of Cu1−x Tl x Ba2Ca3Cu4O12−δ Superconductor Thin Films?

The insulating and metallic behavior of the grain-boundary weak links has been studied in thallium rich and the samples with small amount of thallium in the charge reservoir layer of Cu1−x Tl x Ba2Ca3Cu4O12−δ superconductor thin films. The influence of the nature of grain boundaries on the inter-granular critical current density (J c) has also been investigated. From the power law dependence of H ac∼(1−T p/T c) n , it was observed that n=1 gives a best fit for the J c of thallium rich samples and n=2 provides a best fit for the J c of the samples with small amount of thallium. The polycrystalline thin film samples showing the power law dependence of J c as n=1 make superconductor-insulator-superconductor (SIS) type while the samples with n=2 follow superconductor-normal metal-superconductor (SNS) types of Josephson junctions. The insulating grain boundaries decrease the inter-granular Josephson coupling and hence the transport properties are suppressed.

Anisotropic Characteristics of In-Plane Aligned a-Axis YBa2Cu3O7−δ Thin Films

Highly in-plane aligned a-axis oriented YBa 2 Cu 3 O 7-δ (YBCO) thin films with unprecedented transition temperature (T c ∼ 91.3 K) have been deposited on (100) LaSrGaO 4 substrates using a continuously grown PrBa 2 Cu 3 O 7-δ (PBCO) template layer by pulsed laser. The anisotropies between b- and c-axis were investigated by transport measurements and atomic force microscopy (AFM). Although, the onset T c 's along b- and c-axis appeared approximately the same, a significant filing was observed in the resistive transition along c-axis. The temperature-dependent of the tailing effect, however, cannot be explained by thermally activated phase-slippage (TAPS) commonly encountered in grain-boundary weak links.

Magnetic field and current distributions at an artificial grainboundary in YBa2Cu3O7-x

Using a magneto-optical technique, magnetic field distributions havebeen measured in melt-textured YBa2Cu3O7-x, joined toform an artificial grain boundary, in an external magnetic field perpendicularto the sample surface. An inversion scheme using the inverse-matrix method hasbeen applied to obtain the current distribution from the magnetic fielddistribution. Both magnetic field and current distributions around the grainboundary show different behaviour between the field increasing up to 150 mTand decreasing down to 0 T after zero-field cooling. The magnetic field at aweakly coupled section of the grain boundary in the increasing field is higherthan that in the decreasing field, even in the same external field. This studysupports a model in which such differences in the magnetic field at the weak-linkgrain boundaries give rise to the hysteresis behaviour in the field dependenceof transport critical current density in polycrystalline samples.

Picosecond response of optically driven Y-Ba-Cu-O microbridge and Josephson-junction integrated structures

We report our studies on single-picosecond electrical pulse excitation and detection in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) transmission lines containing microbridges and grain-boundary Josephson junctions. The structures were patterned in YBCO films grown by laser ablation on MgO bicrystals and consisted of 20-/spl mu/m-wide coplanar lines, separated by a 20-/spl mu/m-wide gap. Each transmission line contained a 5-/spl mu/m-wide and 10-/spl mu/m-long microbridge and a 5-/spl mu/m-wide grain-boundary weak link, and was overlaid with 50 nm of Au to improve its high frequency properties. Using a Ti:sapphire laser, we excited the microbridge with 100-fs-wide 400-nm-wavelength optical pulses and studied response of our test structures, utilizing a cryogenic electro-optic sampling system. We directly observed 2.1-ps-wide pulses generated by YBCO microbridges, as well as the single-picosecond Josephson junction response. The junction response depended directly on the current bias and its polarity with respect to the excitation electrical pulse, and exhibited single-flux-quantum-like characteristics. Our test structures can be regarded as examples of all-YBCO ultrafast optoelectronic circuits.

The approximate analytic expressions for field dependence of surface impedance of high-Tc superconducting thin film in low- and intermediate-field region

Assuming that a high-Tc superconducting thin film is composed of a network of superconducting grains connected by grain-boundary weak links, the equation describing its response under microwave current irf is solved using a perturbation method to the third order of irf. Based on the obtained results, analytic expressions for film’s microwave surface impedance are derived, revealing its dependence on a rf magnetic field in low and intermediate field limit. Good qualitative agreement with experiments is observed.

Photodoping effect in Y-Ba-Cu-O Josephson junctions and thin films

We report our studies on the photo doping effect in YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) grain-boundary weak links and partially oxygen-depleted (T/sub c,mid/ /spl ap/52 K) thin films, We focus our attention on the erasable process, in which persistent photoconductivity (PPC) introduced by light at low temperatures relaxes back above 260 K. In the tested 2-/spl mu/m-wide step-edge grain-boundary junctions, PPC manifested itself as above 30% increase of the critical current, while the illuminated films exhibited up to 2.5 K enhancement of T/sub c/'s. In the case of films, we also observed upon illumination a continuous decrease (no saturation effect) of both the longitudinal and transversal (Hall) resistivities. In-situ Hall effect band model, PPC at low temperatures results in the increase of both the hole mobility and concentration, while at temperatures above 250 K, the mobility is reduced after prolonged illumination, whereas the carrier concentration is enhanced even more than allow temperatures. We have also demonstrated that the T/sub c/ enhancement due to photodoping is a function of the carrier concentration rather than the mobility, Our experimental results indicate that the physical origin of PPC is same in both YBCO grain-boundary junctions and partially oxygen-depleted films, and can be understood as the coexistence of photoassisted oxygen ordering and photoinduced charge transfer.

Critical current properties of screen-printed Ag-Bi2223 tapes

Critical current densities J/sub c/ (T,B,/spl theta/) were investigated on screen-printed, c-axis-oriented Ag-Bi2223 mono-layer tape, at temperatures between 55 and 77 K in fields up to 8T as a function of field angle /spl theta/ between B and c axis. At temperatures T/spl les/30 K, a shift of a peak angle from /spl theta/=90/spl deg/ (i.e., B/spl perp/c axis) occurs in the J/sub c/ vs. /spl theta/ curve accompanied by an additional asymmetry. Such anomalies are explained by the history effect in the field dependence of J/sub c/ for B/spl par/c axis ascribed to the grain-boundary weak links, on the assumption that J/sub c/ is dominated by the field component B/spl par/=Bcos/spl theta/ parallel to the c axis. At elevated temperatures, disappearance of the history effect results in a perfectly symmetrical angular dependence of J/sub c/ with a peak at /spl theta/=90/spl deg/. An increase in the number of ceramic layers for the screen-printed tape does not cause significant influence on critical current properties.

History effect and anisotropy of critical current density in screen-printed (Bi, Pb) 2Sr 2Ca 2Cu 3O x/Ag tape

Transport critical current density J C( T, B, θ) of screen-printed, c-axis-oriented (Bi,Pb) 2Sr 2Ca 2Cu 3O x /Ag composite tape with a ceramics mono-layer core was investigated between 4.5 and 77 K in magnetic fields up to 8 T as a function of the field angle θ between B and the c axis. At low temperatures T ⪕ 30 K , a shift of a peak angle from θ = 90° (i.e., B ⊥ c axis) occurs in the J C versus θ curve, accompanied by an additional asymmetry. Such anomalies are explained by the history effect in the J C values on magnetic field for B∥ c axis ascribed to the grain-boundary weak links, on the assumption that J C is dominated by the field component B ∥ = Bcos θ parallel to the c axis. At elevated temperatures, disappearance of the history effect results in a perfectly symmetrical angular dependence of J C with a peak at θ = 90°.

The Nature of Grain Boundaries in the High-Tc Superconductors

Grain-boundary weak links limit the critical current density of poly­ crystalline high-Tc superconductors. Themes that have evolved from research on these grain boundaries include (a) an apparent general and dramatic distinction between the behavior of low-angle and high-angle grain boundaries, (b) electrical heterogeneity along as well as among boundaries, (c) indications that the macroscopic current path may or may not cross certain high-angle grain boundaries, (d) a key role for TEM­ EELS in studies of the electronic structure near grain boundaries, and (e) evidence that current models for grain boundary structure pertain to the high-Tc superconductors and may eventually help us to understand their electromagnetic character. Results regarding each of these topics are reviewed in this paper.

Modelling of the resistive transition of textured YBaCuO thin films sputtered on various application oriented types of substrates

The resistive transition of YBaCuO granular films has been phenomenologically interpreted in terms of a resistance network representing superconducting grains linked by grain-boundary weak links, and taking into account the possible existence of parasitic phases via a distribution of critical temperatures. The model relevance has been tested for various 0.5 to 1.5 μm thick films, rf sputtered on substrates of industrial interest, such as silicon wafers with SiO 2/Si 3N 4 diffusion barrier layers or yttria-doped polycrystalline zirconia slabs.

Mechanisms of optical response in superconducting YBa2Cu3O7−δ thin films and grain-boundary weak links

The optical response of single grain-boundary weak links in superconducting YBCO thin films has been investigated. At temperatures well below the transition temperature, the grain boundary exhibits an optical response which is qualitatively different from transition edge response. Using a resistively shunted-junction model for the grain-boundary weak link, we find that the optical response below the transition temperature can be described by radiation induced thermal modulation of the critical currents of the weak links. The dependence on the bias current and the temperature distinguishes the weak-link optical response from the transition edge optical response. We discuss novel aspects of a potential radiation detector based on grain-boundary weak-link junctions.

High- Tc edge-geometry SNS weak links on silicon-on-sapphire substrates

High-quality superconductor/normal-metal/superconductor (SNS) edge-geometry weak links have been produced on silicon-on-sapphire (SOS) substrates using a new SrTiO 3/“seed-layer”/cubic-zirconia (YSZ) buffer system. The seed layer is a thin YBa 2Cu 3O 7- x (YBCO) or PrBa 2Cu 3O 7- x (PBCO) film, which provides a template for growth of the SrTiO 3. This multilayer buffer system eliminates problems with series grain-boundary weak links seen in edge junctions on single YSZ buffer layers on SOS, while the use of moderate-dielectric-constant SOS substrates should benefit high-frequency applications and enable integration with silicon circuitry. SNS weak links fabricated on SOS with PBCO and Co doped YBCO normal-metal layers exhibit current-voltage characteristics qualitatively consistent with the resistively shunted junction model, with modulating AC Josephson steps and operation to temperatures above 77 K. These are the first reported epitaxial edge-geometry SNS devices on SOS substrates.

Optical response of an ultrathin film and a large-angle grain-boundary in superconducting YBa2Cu3O7−δ

We have investigated the optical response of epitaxial ultrathin (3 nm) YBa2Cu3O7−δ film on NdGaO3 substrate and thin (50 nm) film of YBa2Cu3O7−δ on a bicrystal SrTiO3 substrate. While the current-voltage characteristics of the ultrathin film indicate a Kosterlitz–Thouless transition (associated with vortex–antivortex unbinding) at a temperature TKT, we do not observe any optical response at or below TKT. An enhanced optical response with nonbolometric characteristics was observed from the grain-boundary as compared to the bolometric optical response of the epitaxial film at the resistive transition. These results indicate that the grain-boundary weak links play a significant role in the enhanced optical response near Tc.

Electromigration studies of the role of oxygen defects in YBa/sub 2/Cu/sub 3/O/sub 7- delta / grain boundary weak links

Electromigration experiments were used to help establish the role of local basal-plane oxygen disorder in determining the properties of grain-boundary (GB) weak links in YBa/sub 2/Cu/sub 3/O/sub 7- delta / thin films. The response to electromigration currents allows an estimate of the atomic force required to promote chain oxygen disorder in this material. In addition, it was determined that a lesser force is sufficient to promote long-range motion (1-2 mu m) of oxygen defects once they have been created. The activation energy for this process is approximately 0.8 eV. The creation of localized regions of disorder by electromigration in microbridges without GBs shows that these regions have superconductive properties indicative of a filamentary superconductive system shunted by nonsuperconductive ohmic paths. Parallels are drawn between the bulk thin film behavior and the effects due to the presence of GBs. These boundaries can be viewed as limiting the degree of connection between two disordered regions created by the elastic strain gradients caused by the GB. >

Transport critical currents, I-V characteristics and magnetisation of Ag-sheathed BiSrCaCuO (2223) tapes

The grain-boundary weak-links in pressed Ag-sheathed BiSrCaCuO (2223) tapes have been studied with magnetic and transport techniques. Compared with unsheathed BiSrCaCuO, there is significant grain alignment, and about two orders of magnitude reduction in the weak-link resistance.

Transport critical currents, I–V characteristics and magnetisation of Ag-sheathed BiSrCaCuO (2223) tapes

Abstract The grain-boundary weak-links in pressed Ag-sheathed BiSrCaCuO (2223) tapes have been studied with magnetic and transport techniques. Compared with unsheathed BiSrCaCuO, there is significant grain alignment, and about two orders of magnitude reduction in the weak-link resistance.

Millimeter wave mixing from deliberate grain-boundary weak links in epitaxial Tl2CaBa2Cu2O8 films

Millimeter mixers have been fabricated from epitaxial films of Tl2CaBa2Cu2O8, using a bowtie antenna structure integrated with grain-boundary weak links that were deliberately formed during film growth. The bowtie antenna was utilized for broadband millimeter wave coupling of the radiation into the weak links. Heterodyne mixing of signals at both V band (55 GHz) and W band (94 GHz) frequencies, and subharmonic mixing at V has been observed at liquid nitrogen temperature for zero and finite bias voltage conditions. The dependence of the IF output on temperature, bias voltage, and millimeter wave signal power has been studied. Efficient mixing with bias voltage exhibited strong signals right up to Tc (101 K).

Mechanism of picosecond response of granular YBaCuO films to electromagnetic radiation

Ultrafast mechanisms of radiation detection in granular YBaCuO films are studied in the wide wavelength range from millimeter waves to near infrared. With an increase in radiation frequency, the Josephson detection at the grain-boundary weak links is replaced by electron heating into the grains. This change occurs in the submillimeter wavelength range. The electron-phonon relaxation time tau /sub eph/ is determined from direct measurements, quasi-stationary electron heating measurements, and the frequency dependence of the current at which maximum voltage shift is observed. The temperature dependence of tau /sub eph/ at T<or=40 K was found to be tau /sub eph/ approximately T/sup -1/. The results show that detectors with a response time of a few picoseconds at nitrogen temperature are attainable.

Granular YBaCuO films prepared by metalorganic chemical aerosol deposition technology

Fine-grain thin superconducting films can be prepared by metalorganic Chemical Aerosol Deposition Technology (CADT). In this paper, we present the preparation and properties of YBa 2Cu 3O 7-x films on the different substrates, Si and SrTiO 3 (100). It is shown that the zero-resistance temperature (T c,0) of the films on SrTiO 3 substrates is about 90 K, and the critical current density (J c) at 77 K is above 10 4 A/cm 2. In addition, these films exhibit significant grain-boundary weak link behaviour, which is very promising for applications in electronic devices.