Boundary Weak Links Research Articles

Hg(Tl)BaCaCuO high- Tc thin film rf-SQUID operating above 115 K

An rf-SQUID has been fabricated on Hg 0.8Tl 0.2Ba 2Ca 2Cu 3O 8+ δ thin film using natural grain boundary weak links. The film was prepared by the spray pyrolysis technique and had T c = 123 K. Voltage-flux modulation of the SQUID has been obsrved up to 117.5 K. The flux noise density, √S Ф of the rf-SQUID is ≈ 2.5 × 10 −3Ф 0/√ Hz at 1 Hz and 77 K. Th value of √S Ф remains almost the same when the operating temperature is increased from 77 to 112 K, and its value becomes ≈ 4 × 10 −3Ф 0/√ Hz at 114 K.

Development of high- superconducting thick-film rf-SQUID sensors and their application in non-destructive testing

Radiofrequency-SQUID (superconducting quantum interference device) sensors have been fabricated using natural grain boundary weak-links in high- Br - Sr - Ca - Cu - O screen printed thick films. The films have predominantly high- 2223 phase and are c-axis oriented with large size grains ( - ) and of 104 K. These sensors showed good voltage - flux (V - ) characteristics at 77 K. The rf-SQUIDs have been operated in the flux-locked-loop mode at 77 K and showed a flux noise density in the white noise region. The potential of these SQUIDs in non-destructive detection of defects in steel plates has been demonstrated.

Well-defined grain boundary weak-links in silver-sheathed ( tape conductors

The key factors that make the thallium-based 1223 phase conductors attractive candidates for high-field applications at 77 K are a high transition temperature (about 115 K) and an irreversibility line comparable to that of . However, a third important factor is the grain boundary behaviour. We have performed detailed current - voltage (I - V) studies of a silver-sheathed ( (Tl-1223/Ag) tape. Although the tape has a fairly high self-field critical current density at 77 K (about ), the I - V characteristics display the `weak-link' signatures well known for : linear dissipation above a sharply defined critical current, a pronounced decrease of in a magnetic field H and hysteresis with respect to field cycling. Broader implications for the superconductive behaviour of HTS grain boundaries are discussed.

Hysteresis of the current-voltage characteristics and electric field effects in bulk YBCO superconductors

Some YBCO ceramics have been found to exhibit hysteresis of the current/voltage, I/V, characteristics, which reveals at a certain current If greater than critical current Ic a transition with switch from one branch of the I/V curve to another. In present work, the influence of an electric field at 77 K on hysteresis of I/V curves of ceramic YBCO/Ag and YBCO after hydrogen treatment has been studied. It was established that in strong electric field (120 MV/m) the conductivity increases at current I>Ic, the field effect being observed only in the upper branch of hysteresis loop. The electric field effect in YBCO after hydrogen treatment is reversible. The observed effects are assumed to be associated with a change of grain boundary weak links from S/I/S-to S/N/S-type in the course of hydrogen treatment or addition of silver into ceramics.

Radio frequency-SQUID effect in YNi2B2C due to natural grain boundary weak links

We report the observation of radio frequency-superconducting quantum interference device (rf-SQUID) effect due to natural grain boundary weak links in a bulk sample of a YNi2B2C superconductor at 4.2 K, confirming that the superconducting grains are coupled at the grain boundaries through Josephson effect. Periodic oscillations in voltage-flux characteristic due to rf-SQUID effect have been clearly observed at 4.2 K when the driving rf frequency was kept slightly away from the resonance frequency of the tank circuit. Phase reversal in the voltage-flux characteristics on changing the rf frequency has been observed. Noise spectrum of the YNi2B2C bulk rf-SQUID is also reported.

Biaxial alignment of high-Tc superconductor polycrystals VIII: Phi-circle scan analysis of EuBa2Cu3O7−δ (Eu-123)

To reduce grain boundary weak links due to large-angle grain orientation mismatch, bulk (thick film) EuBa2Cu3O7−δ high-Tc superconductor specimens were biaxially aligned by a mechanical force such that their [001] axes were perpendicular to the specimen surfaces and by a magnetic force so as to orient their [010] axes to lie parallel to the magnetic field direction in the specimen plane. We describe here the characterization of such EuBa2Cu3O7−δ thick films by φ-circle XRD scans of (013) reflections, demonstrating a high degree (33%) of c-axis alignment with FWHM of <5° and approximately Gaussian distributions of a, b-axis alignment with a FWHM of 47°. The formation of “granular single crystals”, toward which this work is a further advance, can be seen as establishing a new paradigm in ceramics science.

Biaxial alignment of high-Tc superconductor polycrystals VII: Demonstration of biaxial alignment of EuBa2Cu3Ox (Eu-123) by pole figure measurement

Abstract Bulk EuBa 2 Cu 3 O x high- T c superconductor thick film specimens of 350 μ thickness were fabricated using a combination of magnetic field and mechanical force (acting normal to each other) to form a “granular single crystal” to reduce the grain boundary weak links due to large-angle grain orientation mismatch. The biaxially aligned (and hence, triaxially aligned) specimens exhibited textures in which the grains of the superconductor ceramics were aligned by a mechanical force such that their [001] axes were perpendicular to the surfaces of the thick film specimens, whereas their [010] axes were oriented by a magnetic force to lie parallel to the magnetic field direction in the specimen plane. The top and bottom surfaces of the double aligned thick films have been characterized by reflection XRD (2θ-scan and χ-circle scan) and XRD pole figure determination. We describe here the biaxial alignment process for EuBa 2 Cu 3 O c , as currently practiced by this group and report the first semiquantitative results of an X-ray pole figure analysis of material prepared by this method.

Transport properties of submicron YBaCuO low angle grain boundary weak links

Submicron weak links in YBaCuO thin films fabricated on low angle grain boundary bi-crystals were studied. Their transport properties may be explained in terms of an S-S'-S weak link model.

Weak link induced longitudinal flux motion: A new look at the J c of granular high T c superconductors

It is argued that current flow in granular superconductors leads to generation of current loops near the grain boundaries due to the very flow of current along the magnetic flux density gradient near grain boundary weak links. By applying Faraday's law of induction, it has been shown that longitudinal flux motion plays a prominent role in determining the behaviour of bulk high T c superconductors in low magnetic field. Our model provides a simple explanation for many current and magnetic field limited features of granular high T c superconductors.

HTSC grain boundary weak links employing high-quality ICM sputtered YBCO films

Bicrystal grain boundary junctions of YBCO have been fabricated by using the Inverted Cylindrical Magnetron d.c. sputtering technique. The main characteristics of the junction behaviour are reported. Some aspects of the experimental results are discussed in the framework of existing models developed for describing weak-link structures.

Effects of photoinduced hole doping on transport properties of YBa2Cu3Oy grain boundary junctions

The effects of photoinduced hole doping on the electrical characteristics of grain boundary weak link junctions based on high Tc YBa2Cu3Oy thin films are investigated. Visible light illumination induces a substantial enhancement in the critical current Ic of approximately 20%–40% even at temperatures well below Tc. The Ic enhancement is accompanied by a clear decrease in the junction resistance RN which can be explained by the photoinduced hole doping of the degraded regions with oxygen deficiency and/or disorder near the grain boundary. It is also demonstrated that photoinduced hole doping provides a way of improving the IcRN product and also a way of controlling the Ic value in situ in these weak link junctions.

Influence of grain boundary weak links on the nonequilibrium response of YBaCuO thin films to short laser pulses

The transient voltage response in both epitaxial and granular YBaCuO thin films to 80 ps pulses of YAG∶Nd laser radiation of wavelength 0.63 and 1.54 μm was studied. In the normal and resistive states both types of films demonstrate two components: a nonequilibrium picosecond component and a bolometric nanosecond one. The normalized amplitudes are almost the same for all films. In the superconducting state we observed a kinetic inductive response and two-component shape after integration. The normalized amplitude of the response in granular films is up to five orders of magnitude larger than in epitaxial films. We interpret the nonequilibrium response in terms of a suppression of the order parameter by the excess of quasiparticles followed by the change of resistance in the normal and resistive states or kinetic inductance in the superconducting state. The sharp rise of inductive response in granular films is explained both by a diminishing of the cross section for current percolation through the disordered network of Josephson weak links and by a decrease of condensate density in neighboring regions.

A method to improve grain boundary current-carrying capability in melt-textured YBa2Cu3O7- delta

A major impediment for many applications of bulk high-Tc superconductors is the extremely low current-carrying capability due to grain boundary weak links. A method has been developed to eliminate secondary phases and reduce microcracks at the grain boundaries in YBCO superconductor by removing a fraction of the liquid phase formed during melt texturing. Samples fabricated using this method consist of large domains containing several grains of oriented platelets with grain boundaries free of secondary phases and microcracks. The processing conditions have been tailored to obtain domains extending over a length of 15 to 20 mm, a width of 8 to 10 mm and a thickness of up to 5 mm. The grain boundary misorientation is characterized by optical microscopy as well as X-ray pole figure analysis. These grain boundaries, though having large misorientation angles up to 27.80 degrees , are found to carry high currents at 77 K, both in self-field and in an applied magnetic field of 1.5 T. These results give promise of the manufacture of polycrystalline YBCO superconductors with high current-carrying capability.

Weak links and flux pinning in Ag/BiPbSrCaCuO tapes

Weak links and pinning properties of Ag-sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+y (2223) wires have been investigated through measurements of the transport and magnetisation J c and of the irreversibility lines (IL). The IL for both the tape and the scraped powder from the tape are nearly equally shifted to higher temperatures in comparison with that for the normal powder, suggesting that the enchanced IL is attributable to intragrain pinning. Results on the transport and magnetisation J c of these tapes suggest that the critical current is controlled by flux pinning. Grain boundary weak links are largerly reduced for high quality tapes at high temperatures, while the weak link behaviour becomes pronounced at low temperatures.

Planar superconducting YBCO microwave mixer employing an engineered grain boundary weak link junction

A microstrip transmission line was patterned from a superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ film which was laser deposited on a SrTiO/sub 3/ bicrystal substrate. A microbridge in the microstrip was then patterned at the bicrystal interface to fabricate a superconducting grain boundary junction. Microwave mixing was observed with an input local oscillator frequency of 9.000 GHz, and an input signal frequency of 9.941 GHz. The output signal at 941 MHz was detected as a transmitted signal. The junction was current biased slightly above the critical current I/sub c/. The measured conversion loss was compared to the calculated value based upon a numerical solution of the equation for a superconducting weak link junction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Detection of small magnetic fields using thick BSCCO film RF squid operating at 77 K

A high T c RF SQUID fabricated from thick BSCCO film using naturally present grain boundary weak links has been used for measuring small magnetic fields originating from an analogue quartz wrist watch and magnetic rock samples. For these measurements, the samples were kept at ambient temperature at a distance of ≈4 cm from the SQUID, which was operated in the ‘locked mode’ at 77 K. The flux noise density of the SQUID in the white noise region was 5 × 10 −4 Φ 0 Hz − 1 2 at 77 K.

Microstructure of an artificial grain boundary weak link in an YBa 2Cu 3O 7−σ thin film grown on a (100)(110), [001]-tilt Y-ZrO 2 bicrystal

The microstructure of an artificial grain boundary in an YBa 2Cu 3O 7-σ(YBCO) thin film grown on a (100)(110), [001]-tilt yttria-stabilized-zirconia (YSZ) bicrystal substrate has been studied using transmission electron microscopy (TEM). The orientation relationship between the YBCO film and the YSZ substrate was [001] YBCO‖[001] YSZ and [110] YBCO‖[100] YSZ for each half of the bicrystal film. However, the exact boundary geometry of the bicrystal substrate was not transferred to the film. The substrate boundary was straight while the film boundary was wavy. In several cases there was bending of the lattice confined within a distance of a few basal-plane lattice spacings from the boundary plane and microfaceting. No intergranular secondary phase was observed but about 25% of the boundary was covered by c-axis-tilted YBCO grains and a-axis-oriented grains, both of which were typically adjacent to CuO grains or surrounded by a thin Cu-rich amorphous layer.

Improved microwave performance of Ag-doped Y1Ba2Cu3O7−δ thin film microstrip resonators

Microwave transmission properties of undoped and Ag-doped laser ablated thin film Y1Ba2Cu3O7−δ microstrip resonators have been studied both on &amp;lt;100≳ MgO and &amp;lt;100≳ LaAlO3 substrates at X-band frequencies. While the Q factor and microwave surface resistance, Rs, of undoped films showed better performance on LaAlO3 as compared to that on MgO, Ag-doped films on LaAlO3 showed far greater improvement as reflected not only by a decrease in Rs but also by a total absence of its microwave power dependence up to 13 dBm at 77 K. These results are explained as due to the influence of Ag in increasing the grain size and grain alignment and thus, significantly decreasing the density of grain boundary weak links which are known to affect the microwave transmission in high temperature superconductor films.

Bi-epitaxial template grain boundaries with different in-plane angles on (100) MgO substrates

Using laser ablation CeO 2 and YBa 2Cu 3O 7−d layers have been deposited on (100) oriented MgO substrates for the creation of bi-epitaxial grain boundary weak links. X-Ray Diffraction measurements were used to measure the (in-plane) orientations of the layers. The CeO 2 layer has been structured using Ar ion beam etching or CaO lift off. Using these two structuring techniques it is possible to obtain two different grain boundary weak link angles: 18 and 27 degrees. Up till now only 45 degrees weak link angles have been reported in literature, using templates for the creation of these grain boundaries.

A BSCCO(2223) thin film DC SQUID utilising natural grain boundary weak-links

We report the first low noise thin film BSCCO(2223) SQUID operating at 77 K and above. The natural grain boundary weak links combine with a highly textured film structure to allow noise levels comparable to those achieved in YBCO or TBCCO devices.