M. I. Faley

Josephson junctions, interconnects, and crossovers on chemically etched edges of YBa2Cu3O7−x

A technique of ultraviolet photolithography of YBa2Cu3O7−x and PrBa2Cu3O7−x films combined with nonaqueous Br‐ethanol chemical etching was developed. Josephson junctions, interconnects, and crossovers on the basis of chemically etched edges of c‐axis oriented YBa2Cu3O7−x thin films were prepared and investigated. For the Josephson junctions with a PrBa2Cu3O7−x barrier, the IcRn product values of about 10 mV at 4.2 K and up to about 0.6 mV at 77 K were achieved. Shapiro steps were observed in the temperature range up to about 89 K. Critical current spreads of about ±10% were observed. At 77 K, the electrodes of crossovers carried more than 106 A/cm2. For a 160‐nm‐thick SrTiO3 intermediate layer in crossovers, a resistivity of more than 109 Ω cm was achieved at T≤100 K.

Properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films deposited on substrates and bicrystals with vicinal offcut and realization of high I/sub c/R/sub n/ junctions

The surface morphology, microstructure and transport properties of epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ and PrBa/sub 2/Cu/sub 3/O/sub 7/ thin films and heterostructures deposited on slightly vicinal substrates of SrTiO/sub 3/ by high pressure oxygen sputtering were studied. The vicinal angles of the substrates and bicrystals were less then 13/spl deg/. Depending on the tilt angle of the substrate a transition from spiral or island to step-flow growth leading to an improvement of the surface roughness was observed. Atomic force and transmission electron microscopy were used for these investigations. Furthermore, electrical and structural properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on vicinal offcut SrTiO/sub 3/ bicrystals with different grain boundary types were studied. This included junctions with a 2/spl times/12/spl deg/ tilt or twist of the YBa/sub 2/Cu/sub 3/O/sub 7/ c-axis across the grain boundary. In comparison to conventional [001]-tilt grain boundaries bicrystal Josephson junctions [100]-tilt grain boundaries showed high I/sub c/R/sub n/-products of up to 1.2 meV at 77 K and up to 8 meV at 4.2 K. IV-curve instabilities, probably of magnetic origin due to flux flow in the electrodes, often could be observed for junctions biased with high current densities.

Effect of chemical and ion‐beam etching on the atomic structure of interfaces in YBa2Cu3O7/PrBa2Cu3O7 Josephson junctions

The atomic structure of the interfaces of Josephson junctions formed by epitaxial YBa2Cu3O7/PrBa2Cu3O7/YBa2Cu3O7 triple‐layer films was investigated by high‐resolution transmission electron microscopy. The samples were fabricated by sputter deposition on surfaces which were etched ex situ either chemically, using a nonaqueous Br‐ethanol solution, or by an Ar ion beam. In the interfaces produced after ion etching a thin intermediate layer with a thickness of a few nanometers was observed. The main part of this layer consists of cubic PrBa2Cu3O7 or YBa2Cu3O7 which is cation disordered. The interfaces formed during deposition on Br‐ethanol‐etched surfaces did not contain such an intermediate layer but exhibited high structural perfection similar to that of interfaces produced in situ. These observations permit a qualitative explanation of the difference in the electrical properties of junctions produced by these two techniques.

High-

We have investigated the microstructural and electron transport properties of 45° step-edge Josephson junctions grown on MgO substrates and used them for the preparation of superconducting quantum interference device (SQUID) magnetometers intended for magnetoencephalography (MEG) measurement systems. The high-Tc SQUID magnetometers also incorporate 16 mm multilayer superconducting flux transformers on the MgO substrates and demonstrate a magnetic field resolution of ~ 4 fT/√Hz at 77 K. Results are illustrated for the detection of auditory evoked magnetic responses of the human cortex and compared between high-Tc SQUIDs and a commercial low-Tc MEG system. Our results demonstrate that MEG systems can be upgraded using high-Tc SQUIDs to make them independent of helium and more user-friendly, saving operating costs and leading to the widespread utilization of MEG systems in clinical practice and at universities.

T_{\rm c}

For detection of tendon ruptures in prestressed members of bridges, a four-channel SQUID system was developed. The tendons are magnetized by scanning a yoke electromagnet over the concrete surface along the hidden member. Four HTS dc-SQUID magnetometers with ramp-type junctions, optimized for high-field performance, are mounted in an orientation-independent liquid nitrogen cryostat. The SQUIDs are integrated as a linear array within the yoke and operated in magnetic fields up to 15 mT, recording the stray field during magnetization as well as the remanent field after switching off the excitation. Unwanted signals from stirrups of the mild steel reinforcement are suppressed with two types of techniques: either the comparison of remanent field signals after changing the magnetization direction of the stirrups, or a best fit of typical stirrup signals to the stray field signal and their subtraction. Subsequent correlation analysis with the dipolar signal of a typical void yields rupture signal amplitudes. A finite element program was written to simulate stray field and remanent field traces of typical steel configurations. Excellent agreement with measured data was found. Results of measurements on a prestressed highway bridge are presented. Signal amplitudes above the threshold values were verified as originating from ruptures of the steel tendon by opening the bridge deck.

DC SQUIDs for Magnetoencephalography

A multilayer thin film epitaxial passivation of single crystal MgO substrates was developed. YBa2Cu3O7−x films on the buffered MgO substrates demonstrate pure c-axis orientation, absence of in-plane disoriented grains, transition temperature Tc>91K, and critical current density Jc∼5MA∕cm2 at 77.4K and were deposited in thicknesses of up to several micrometers without cracks. High-temperature superconductor multilayer flux transformers of 2μm thickness on the buffered MgO substrates demonstrated improved insulation between the superconducting layers and an increased dynamic range compared to flux transformers on SrTiO3 substrates.

SQUID array for magnetic inspection of prestressed concrete bridges

We have optimized the preparation of submicrometer wide symmetric 20° bicrystal Josephson junctions on the basis of YBa2Cu3O7−x (YBCO) films and integrated them in SQUID magnetometers and gradiometers with multilayer flux transformers. The achieved reduction of the curvature of the grain boundary leads to an improvement of the homogeneity of the current flow in the junction due to the d-wave symmetry of the order parameter in YBCO. The junctions have typical normal resistance Rn above 10 Ω and a IcRn product of about 0.4 mV at 77.4 K. The high resistance of the junctions has required an extensive use of low pass filters, rf-shields and microwave absorbers to protect the junctions from increased interference of high frequency electromagnetic fields. Integration of such junctions in SQUID magnetometers with a square 16-mm multilayer flux transformer allowed us for the first time to reach a field resolution of about 3.5 fT/√Hz at frequencies above 10 Hz and about 7 fT/√Hz at 1 Hz at an operation temperature of 77.4 K. SQUID gradiometers with a 10 mm base length of the planar multilayer flux transformer have achieved resolutions of about 15 fT/cm√Hz at frequencies above 10 Hz and about 30 fT/cm√Hz at 1 Hz and temperature 77.4 K.

Multilayer buffer for high-temperature superconductor devices on MgO

Nonaqueous Br-ethanol chemical etching was successfully used for the preparation of the Josephson junctions, vias and crossovers in magnetometers including flux transformers. PrBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films were used for the barrier layer in the Josephson junctions of the SQUIDs and as an insulation in the junctions and in the flux transformers. Dc-SQUID magnetometers with small inductances and even without flux antennas were used for NDE applications which mainly require a high dynamic range. Highly sensitive devices were prepared with flip-chip flux antennas. An ac-bias SQUID electronics significantly improves the sensitivity of the magnetometers at low frequencies.

A New Generation of the HTS Multilayer DC-SQUID Magnetometers and Gradiometers

We present a new design of a portable nitrogen cryostat for operation of moving SQUIDs. A mixture of liquid and gaseous nitrogen fills a reservoir in direct contact with a copper part for the SQUID integration. The temperature at the SQUID position is 77.8 K or 78.8 K depending on orientation, and varies within /spl plusmn/10 mK during lateral movement. The cryostat can operate as a portable system for 7 hours without refilling. Washer rf SQUIDs and dc gradiometers were integrated with the cryostat. We proved the operation of the system as a moving magnetometer in an unshielded laboratory environment. Noise spectra in shielding and outside were independent of orientation. The system was equipped with a differential eddy current excitation. We show the first non-destructive material evaluation results for fatigue crack detection on stationary samples with moving SQUID sensors.

Dc-SQUID magnetometers and gradiometers on the basis of quasiplanar ramp-type Josephson junctions

To detect extremely small magnetic fields generated by the human brain, currently all commercial magnetoencephalography (MEG) systems are equipped with low-temperature (low-Tc) superconducting quantum interference device (SQUID) sensors that use liquid helium for cooling. The limited and increasingly expensive supply of helium, which has seen dramatic price increases recently, has become a real problem for such systems and the situation shows no signs of abating. MEG research in the long run is now endangered. In this study, we report a MEG source localization utilizing a single, highly sensitive SQUID cooled with liquid nitrogen only. Our findings confirm that localization of neuromagnetic activity is indeed possible using high-Tc SQUIDs. We believe that our findings secure the future of this exquisitely sensitive technique and have major implications for brain research and the developments of cost-effective multi-channel, high-Tc SQUID-based MEG systems.