C. Heiden

Microstructure of epitaxial YBa2Cu3O7 films on step-edge SrTiO3 substrates

Abstract The microstructure of YBa 2 Cu 3 O 7 films epitaxially grown on step-edge (100) SrTiO 3 substrates has been characterized by means of high resolution transmission electron microscopy. The results indicate a relationship between the microstructure of the film acreoss a step and the angle step makes with the substrate plane. On a steep, high-angle step, the film grows with its c -axis perpendicular to that of the film on the substrate surface so that two grain boundaries are formed. On a low-angle step, the film grows without any change in c -axis orientation across the step and without grain boundaries. Epitaxial second phases intergrowths across the steps have been found in some cases which may act as barrier layers when they cut through th YBa 2 Cu 3 O 7 film.

Sensitive RF-SQUIDs and magnetometers operating at 77 K

Large 6-mm*6-mm and 8-mm*8-mm flux-focusing washer structures with inductances L/sub S/ between 25 pH and 500 pH were fabricated from epitaxial, c-axis YBa/sub 2/Cu/sub 3/O/sub 7/ films. Double step-edge junctions having a low 1/f noise were incorporated in these washers to form RF-SQUIDs (superconducting quantum interference devices). A high tank circuit frequency near 150 MHz was chosen to reduce noise and improve the sensitivity of the SQUIDs. At 77 K, a transfer function exceeding 100 mu V/ Phi /sub 0/ and a white flux noise of 3*10/sup -5/ Phi /sub 0// square root Hz down to below 0.3 Hz were attained with L/sub S/=25 pH. A magnetometer with L/sub S/=190 pH was demonstrated. The best magnetic field and energy resolutions were 170 fT/ square root Hz and 5*10/sup -29/ J/Hz respectively, over a signal frequency range to below 1 Hz. The magnetometer was used to record human magnetocardiograms and auditory evoked human brain responses.<<ETX>>

YBa2Cu3O7 thin film SQUID gradiometer for biomagnetic measurements

Low‐noise rf SQUID washers fabricated from YBa2Cu3O7 epitaxial thin films have been used to construct a first‐order electronic gradiometer operating at 77 K and suitable for biomagnetic measurements. Mechanical adjustment of the two‐SQUID gradiometric setup made it possible to attenuate signals due to far magnetic field sources by three orders of magnitude. A magnetic field resolution of ≤280 fT/Hz1/2 above 2 Hz was attained through the use of large flux focusers. The fine structure of human heart magnetocardiograms was recorded in unshielded space. In a shielded room, magnetoencephalograms were obtained. The system was used to obtain new data on auditory evoked cortical response.

Fully textured growth of Y1Ba2Cu3O7−δ films by sputtering on LiNbO3 substrates

We present details about the deposition of thin Y1Ba2Cu3O7−δ films on single‐crystal LiNbO3 substrates by planar dc magnetron sputtering. Among other data, results of transition temperature (Tc(R=0) =88 K) and critical current density (jc >2×109 A/m2 at 77 K) plus some details about film‐to‐substrate orientation which is present in all three crystallographic directions are reported.

Microwave rf SQUID integrated into a planar YBa2Cu3O7 resonator

We fabricated and characterized microwave rf SQUIDs integrated into a planar, S‐shaped λ/2 microstrip resonator. This 3 GHz resonator was fabricated from a pulsed‐laser‐deposited YBa2Cu3O7 epitaxial film. The SQUID structures incorporated double step‐edge junctions and had a loop inductance of 120 pH. Such unoptimized SQUIDs operated between 4.2 and 85 K with dV/dΦ=18–20 μV/Φ0 at 77 K. At that temperature, the energy resolution of (8±2)×10−29 J/Hz above 0.1 Hz (in the best samples) was limited by the white noise, SΦ1/2=(7±1)×10−5 Φ0/Hz1/2. Optimization may increase dV/dΦ and improve the energy resolution by up to an order of magnitude.

Experiments on eddy current NDE with HTS rf SQUIDS

HTS SQUIDs are promising sensors for applications in eddy current NDE. Due to their high field sensitivity at low frequencies, they are especially suitable for applications, where a large penetration depth is required. We have set up two SQUID systems for low frequency eddy current NDE. 3 GHz rf SQUIDs made from YBCO are used as sensors with a field resolution of about 1pT//spl radic/(Hz). One system, where the SQUIDs are cooled by immersing them in liquid nitrogen, is optimised for testing fellows of aircraft wheels. The fixed cryostat is mounted close to the outer surface of the rotating fellow. The second system uses a cryocooler for SQUID cooling. A motorised x-y stage is used to move the SQUIDs above the plane specimen to be tested. Due to the high slewrate of the SQUID electronics, both systems can easily be operated in an unshielded environment. A double-D shaped coil is used to reduce the excitation field at the location of the SQUID. We report on first results of measurements on aircraft parts.

Inspection of aircraft parts with high remanent magnetization by eddy current SQUID NDE

We have developed an eddy current NDE system based on HTS rf SQUIDs as magnetic field sensors. Due to their high field sensitivity even at very low frequencies, SQUIDs are especially suitable for applications where a large eddy current penetration depth is required. We have used 3 GHz rf SQUIDs in our system made from YBCO thin films with a field resolution of about 1 pT//spl radic/(Hz) and a high dynamic range, more than 140 dB//spl radic/(Hz) in unshielded environment. With this system we could detect deep lying (up to 26 mm) cracks in test samples and original aircraft parts. Measurements made on the same sample showed an improvement in the signal to noise ratio of up to 3 orders of magnitude for cracks deeper than 13 mm, when comparing a conventional eddy current NDE unit with our SQUID system. Test objects containing ferromagnetic structures with a high remanent magnetization (often more than 1 mT), like aircraft wheels or steel bolts in part of the aircraft wing, very often cause instabilities of the flux locked loop operation of the SQUID. To prevent unlocking, we have developed a new background field compensation scheme. Special compensation electronics takes care of slowly varying magnetic fields of up to 1 mT/s and enabled us to perform eddy current measurements in presence of slow (<30 Hz) background field variations of up to 5 mT.

Effect of Zn doping on the superconductivity of RBa2Cu3−xZnxO7−y (R = Yb, Er, Y, Dy, Gd, Eu, SmandNd)

Abstract Zn, having a definite valence state and filled 3d levels, is used for substituting Cu sites in RBa 2 Cu 3− x Zn x O 7⇔ y ( R = Yb , Er , Y , Dy , Gd , Eu , Sm and Nd ). Substitution of Zn for Cu in these systems causes a rapid nearly linear suppression of the superconducting transition temperature T c . All the RBa 2 Cu 3− x Zn x O 7− y samples are essentially single phase and retain the orthorhombic structure in the range of substitution (0≤ x ≤0.1). The reduction of T c by Zn doping in RBa 2 Cu 3− x Zn x O 7− y was found to be strongly dependent on the ionic radius of the rare-earth element R. The superconducting-nonsuperconducting transition accompanied by a transition into the semiconducting phase in these systems is interpreted in terms of a Mott transition.

A YBa/sub 2/Cu/sub 3/O/sub 7/ thin film SQUID gradiometer for measurements in unshielded space

Low-noise RF superconducting quantum interference device (SQUID) washers fabricated from YBa/sub 2/Cu/sub 3/O/sub 7/ epitaxial thin films have been used to construct a first-order electronic gradiometer operating at 77 K and suitable for biomagnetic measurements. Mechanical adjustment of one SQUIDs plane in a gradiometric setup, which consisted of two SQUIDs separated by a vertical 6-cm base, made it possible to attenuate signals due to far DC magnetic field sources by three orders of magnitude. A magnetic field resolution of 280 fT/ square root Hz above 2 Hz was attained through the use of large thin-film and bulk flux focusers. The fine structure of human-heart magnetocardiograms could be recorded in unshielded space. In a shielded room, magnetoencephalograms were obtained. The system was used to obtain new data on the acoustically evoked cortical response.<<ETX>>

Magnetic hysteresis and critical current densities of polycrystalline REBa2Cu3O7−y☆

Abstract Magnetization curves of bulk REBa 2 Cu 3 O 7−y samples (RE = Gd, Dy, Er, Eu, Y) with T c( R = 0) ⩾ 90 K have been measured at various temperatures for different sweep amplitudes of the magnetic field up to H max = 4.7 T . Using the critical state model, intergranular and intragranular critical current densities were derived from the hysteresis loops at low and high fields, respectively. There are indications of enhanced flux pinning by a quenched second superconducting phase, since the field dependence of intragranular critical current density was found to exhibit a maximum at intermediate temperatures.