The anomalous magnetization characteristic, intra-grain granularity and the magnitude of the flux pinning in single crystals of YBa2Cu307–δ are shown to be intimately linked. As the nominal oxygen deficiency δ decreases towards zero, the flux pinning declines and the crystals lose their explicitly granular signature. As δ is seldom specifically controlled at levels below ˜0.05 in even the most carefully made materials, small oxygen deficiencies may contribute significantly to both the flux pinning and the weak-link properties.

Oxygen-defect flux pinning, anomalous magnetization and intra-grain granularity in YBa 2 Cu 3 0 7–δ

Superconducting quantum interference devices (SQUIDs) are sensitive detectors of magnetic flux. A SQUID consists of a superconducting loop interrupted by either one or two Josephson junctions for the RF or dc SQUID, respectively. Low transition temperature (T/sub c/) SQUIDs are fabricated from thin films of niobium. Immersed in liquid helium at 4.2 K, their flux noise is typically 10/sup -6//spl Phi//sub 0/ Hz/sup -1/2/, where /spl Phi//sub 0//spl equiv/h/2e is the flux quantum. High-T/sub c/ SQUIDs are fabricated from thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/, and are generally operated in liquid nitrogen at 77 K. Inductively coupled to an appropriate input circuit, SQUIDs measure a variety of physical quantities, including magnetic field, magnetic field gradient, voltage, and magnetic susceptibility. Systems are available for detecting magnetic signals from the brain, measuring the magnetic susceptibility of materials and geophysical core samples, magnetocardiography and nondestructive evaluation. SQUID "microscopes" detect magnetic nanoparticles attached to pathogens in an immunoassay technique and locate faults in semiconductor packages. A SQUID amplifier with an integrated resonant microstrip is within a factor of two of the quantum limit at 0.5 GHz and will be used in a search for axions. High-resolution magnetic resonance images are obtained at frequencies of a few kilohertz with a SQUID-based detector.

Superconducting quantum interference devices: State of the art and applications

For two decades, academic and industrial researchers worldwide have used SQUID magnetometers to measure magnetic signals from the heart, brain, lungs, liver, nerves, skeletal muscle, stomach, intestines, eyes, and other organs, and have invested heavily in developing and promoting this technology. While there are as yet few accepted clinical applications of SQUIDs, various trends are encouraging. The introduction of SQUIDs to the nondestructive testing (NDT) of aircraft and other structural systems and materials is following a similar course: most of the effort is directed towards instrumentation development and demonstrations in simple systems, and instruments suitable for specific commercial applications are just now being prototyped. To assess the potential of either technology, it is useful to ask critical questions: why are we doing this, what have we learned so far, how easy is it, what does it cost, how might we best utilize advances in digital SQUIDs and high-temperature superconductivity, and what can competing technologies provide? Answers to such questions can help identify those specific technological niches for which SQUIDs are uniquely suited, and guide the optimization of SQUID systems that are targeted for particular NDT or biomagnetic measurements. >

SQUID magnetometers for biomagnetism and nondestructive testing: important questions and initial answers

The traditional distinction between tunnel and highly transmissive barriers does not currently hold for high critical temperature superconducting Josephson junctions, both because of complicated materials issues and the intrinsic properties of high temperature superconductors (HTS). An intermediate regime, typical of both artificial superconductor–barrier–superconductor structures and of grain boundaries, spans several orders of magnitude in the critical current density and specific resistivity. The physics taking place at HTS surfaces and interfaces is rich, primarily because of phenomena associated with d-wave order parameter (OP) symmetry. These phenomena include Andreev bound states, the presence of the second harmonic in the critical current versus phase relation, a doubly degenerate state, time reversal symmetry breaking and the possible presence of an imaginary component of the OP. All these effects are regulated by a series of transport mechanisms, whose rules of interplay and relative activation are unknown. Some transport mechanisms probably have common roots, which are not completely clear and possibly related to the intrinsic nature of high-TC superconductivity. The d-wave OP symmetry gives unique properties to HTS weak links, which do not have any analogy with systems based on other superconductors. Even if the HTS structures are not optimal, compared with low critical temperature superconductor Josephson junctions, the state of the art allows the realization of weak links with unexpectedly high quality quantum properties, which open interesting perspectives for the future. The observation of macroscopic quantum tunnelling and the qubit proposals represent significant achievements in this direction. In this review we attempt to encompass all the above aspects, attached to a solid experimental basis of junction concepts and basic properties, along with a flexible phenomenological background, which collects ideas on the Josephson effect in the presence of d-wave pairing for different types of barriers.

http://iopscience.iop.org/article/10.1088/0034-4885/68/11/R03/pdf

Weak links in high critical temperature superconductors

We attempt a comprehensive review of all published research in nondestructive evaluation (NDE) performed with the superconducting quantum interference device (SQUID) magnetometer since the first work was reported in the mid-1980s The SQUID is the most sensitive detector of magnetic flux known The energy sensitivity of the SQUID may make it the most sensitive detector of any kind The research on SQUIDs for NDE is based on the promise of that sensitivity and on the various other desirable properties developed for SQUID instrumentation in biomagnetism and other fields The sensitivity of SQUID instruments down to very low frequencies allows them to function as eddy-current sensors with unparalleled depth resolution, and to image the static magnetization of paramagnetic materials and the flow of near-dc corrosion currents The wide dynamic range of the SQUID makes it possible to image defects in steel structures and to measure the magnetomechanical behaviour of ferromagnetic materials with high sensitivity In the last decade SQUID instrumentation designed specifically for NDE has appeared and improved the spatial resolution of most work to roughly 1 mm, with promise of another order of magnitude improvement within the next five years Algorithms for flaw detection and image deconvolution have begun to flourish With many talented, industrious people in the field, the future of SQUID NDE looks bright, provided the crucial first niche can be found

http://iopscience.iop.org/article/10.1088/0022-3727/30/3/002/pdf

SQUIDs for nondestructive evaluation

The microstructure of YBa2Cu3O7 films epitaxially grown on step-edge (100) SrTiO3 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 YBa2Cu3O7 film.

Microstructure of epitaxial YBa2Cu3O7 films on step-edge SrTiO3 substrates

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. >

Sensitive RF-SQUIDs and magnetometers operating at 77 K

A superconducting quantum interferometer device (SQUID) electronics with tank frequencies near 150 MHz was constructed to enhance the rf SQUID sensitivity. By these means we characterized our YBa2Cu3O7 thin‐film rf SQUIDs with step‐edge junctions immersed in liquid N2. We obtained transfer function values of over 40 μV/Φ0, at a 50 Ω input impedance, for both hysteretic and dispersive mode of rf SQUID operation. The rms white and 1/f flux noise levels were significantly lower than at the tank frequency of 20 MHz. The best magnetic field sensitivity at 77 K was 0.9 pT/Hz1/2, and the energy resolution 1.4×10−28 J/Hz, down to 0.3 Hz.

Low‐noise YBa2Cu3O7 rf SQUID magnetometer

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.

/pdf/yba2cu3o7-thin-film-squid-gradiometer-for-biomagnetic-s2xu3sy4fv.pdf

YBa2Cu3O7 thin film SQUID gradiometer for biomagnetic measurements

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.

/pdf/microwave-rf-squid-integrated-into-a-planar-yba2cu3o7-1hf9ae885q.pdf

C. Heiden

Papers

Microstructure of epitaxial YBa2Cu3O7 films on step-edge SrTiO3 substrates

Sensitive RF-SQUIDs and magnetometers operating at 77 K

Low‐noise YBa2Cu3O7 rf SQUID magnetometer

YBa2Cu3O7 thin film SQUID gradiometer for biomagnetic measurements

Microwave rf SQUID integrated into a planar YBa2Cu3O7 resonator