Search

We designed first-order integrated planar SQUID gradiometers especially for readout of signals from magnetic micro-calorimeters used for the detection of single x-ray quanta. These SQUIDs must operate in magnetic fields up to 5 mT and at temperatures down to 10 mK. Various new designs of eight-pixel gradiometer SQUID arrays on one chip were developed. The dimensions of the gradiometers are adapted to the magnetic thermometer of the micro-calorimeter. The intrinsic noise of the SQUID gradiometers measured at 4.2 K is lower than 2.5/spl mu//spl Phi//sub 0//Hz/sup 1/2/. They withstand external magnetic fields up to 7 mT without degradation of their performance. A new concept for thermal contact of the detector to the environment based on normal-conducting metallic wiring on the chip is introduced. Field coils with a persistent current switch were integrated on chip to produce the necessary magnetizing field. Appropriate changes in niobium technology compared to our previous work were introduced to achieve necessary critical current of the coil. Thermal coupling between the switch heater and the superconducting short was optimized. A persistent current of up to 80 mA can be injected in the field coil using a heater power of 4 mW. This current corresponds to magnetizing field of 6 mT at the paramagnetic sensor. During the heat pulse the SQUID remains superconducting.

We present the first steps toward a 288 pixel sub-millimeter wavelength bolometer camera. We have used bolometers of the transition edge type, choosing a thermometer made of a proximity bilayer of Mo and the alloy Au/Pd with a designed transition temperature of about 500 mK. We have adapted our well-developed niobium based technology for SQUID manufacturing in order to be able to fabricate the current sensor and the bolometer on a common substrate. The function of both devices was successfully tested at the operating temperature of 300 mK.

We have built and demonstrated a simple system with open geometry that measures biomagnetic signals such as magnetoencephalogram (MEG), magnetocardiogram (MCG) and magnetomyogram (MMG) simultaneously with low field nuclear magnetic resonance (NMR) free induction decay signals (FID). The system employs LT/sub C/ SQUID gradiometers and can operate with proton Larmor frequency in the 80 Hz-10 kHz range. A pre-polarizing field of up to 60 mT is generated by resistive coils. Two different types of SQUID gradiometers were used: a tangential thin-film planar first-order gradiometer and an axial second-order gradiometer. The gradiometers were placed inside a fiberglass dewar at about 1 cm distance from a subject. All measurements were performed inside a single-layer magnetic shielded room. This system is the prototype for a system that will ultimately be capable of measuring biomagnetic signals together with magnetic resonance images (MRI).

We have developed a very simple method for manufacturing submicrometer Josephson junctions (JJs) based on niobium and titanium layers. Together with the use of a microwave circuit consisting of modified coplanar strips this type of JJs offers the possibility of easy integration of several thousand submicrometer junctions in a microwave circuit. Up to this point we have manufactured and successfully tested arrays with 2000 super-/normal-/super-conductor junctions with areas from 0.3 /spl mu/m/spl times/0.3 /spl mu/m to 2.0 /spl mu/m/spl times/2.0 /spl mu/m. First measurements with microwave irradiation showed stable Shapiro steps at voltages up to 40 mV and, thereby, the proper operation of all 2,000 junctions was seen.

First-order planar superconducting quantum interface device (SQUID) gradiometers were fabricated on the basis of the Nb/AlO/sub X//Nb technology developed at IPHT Jena. The gradiometers have two pickup loops each with quadratic dimensions of 2 cm and a baseline of 4 cm, integrated on chip. The intrinsic noise corresponds to a field resolution in one loop better than 3 fT//spl radic/Hz. Because of the high gradiometer balance (>10/sup 5/) the main signals masking a cardiogram are magnetic field gradients caused by nearby sources (laboratory equipment). To suppress the disturbances a second-order gradiometer was realized electronically. In addition, three washer-SQUID's arranged as a three-axis magnetometer were used to improve the disturbance suppression. All SQUID's were operated with directly coupled low-noise high-speed electronics controlled by a computer via a RS232 port. They have an input voltage noise of about 0.33 nV//spl radic/Hz, a 1/f corner frequency of 0.1 Hz and bandwidth of 5 MHz. The SQUID signals were digitized using a commercially available 16-bit ADC and digitally filtered. The system shows stable operation in the laboratory without magnetic shielding. A human cardiogram could be recorded in real time with a signal-to-noise ratio better than 20. First results of the measurement of foetal cardiogram in a magnetically shielded room are discussed.

For a more wide-spread use of Josephson voltage standards, cryogen-free operation, by means of an appropriate closed-cycle refrigerator, is highly desirable. In this work we present a low-noise pulse tube cooler (PTC) that is capable to cool the voltage standard circuits to temperatures below 4 K. As a low loss dielectric waveguide with a very small thermal conductivity a Teflon strip was used for the microwave transmission. The matched transition from the WR12 rectangular waveguide to the Teflon strip and vice versa was made using special exponential tapers. Programmable 1 Volt 14 bit SINIS and 10 Volt SIS arrays assembled in the PTC operated well, i.e., demonstrated a similar performance to that measured in liquid helium. The Josephson junction arrays were integrated in coplanar strips instead of the commonly used microstriplines for the microwave transmission lines. The 1 Volt SINIS array contained 8192 Josephson junctions (JJs). It generated an inherently stable Josephson voltage step of 1.19 V at a driving microwave frequency of 70 GHz. The step width was 150 /spl mu/A and the critical current of the junctions was 550 /spl mu/A. Similar results were obtained under liquid helium cooling conditions. In another experiment a 10 Volt voltage standard chip with 19700 SIS JJs was installed. It was possible to generate stable Shapiro steps at the 10 V level.

Fabrication of miniaturized tunnel junctions based on high-melting metals by the shadow evaporation technique is rather complicated. The thermal load of the suspended bridge mask during metal evaporation is assumed to be the most serious problem. As an alternative we have developed a preparation technique using e-beam direct-writing lithography in conjunction with material deposition by sputtering. To test the preparation process, we have fabricated single electron transistors (SETs) based on the metals Al and Nb, including mixed Al/Nb samples. For SETs made completely of Nb, we preferred Al0/sub x/ to the natural oxide NbO/sub x/ for barrier generation. The yield of functioning samples amounted to about 80%. By means of simple considerations we have estimated the tunnel capacitances to be of the order of a few 10/sup -16/ F, the tunnel resistance spread was less than one order of magnitude.

Superconductor-normal metal-superconductor (SNS) junctions are well-suited for large-scale integrated superconducting circuits. Potential applications are programmable voltage standards, digital-to-analog converters and large-scale RSFQ circuits. The application of SNS junctions provides some advantages, e.g. a simplified fabrication technology and reduced parasitic inductances. But it requires a technological process with dimensions down to the submicrometer region to achieve sufficiently high characteristic voltages I/sub C/R/sub N/. In our process the normal metal interlayer is titanium. Therefore the sidewall insulation of the junctions can be made by anodization, as it is known from the Nb-Al technology. Nb-Ti-Nb Josephson junctions and junction arrays were fabricated down to the submicrometer range with I/sub C/R/sub N/ values up to 87 microvolt depending on the titanium thickness. Their measured critical currents have a small spread of about 10 percent. Shapiro steps are observed under the influence of microwave irradiation. For the application of such junctions in a programmable Josephson voltage standard a new type of microwave circuit using coplanar strips is proposed and successfully tested in 10 V SIS circuits.

Several improvements of Josephson voltage standard systems are described. These concern optimized microwave chips for the voltage generation, a new cryoprobe with an attenuation-reduced waveguide and a simplified microwave source. All these improvements contribute to a marketable cryogenic voltage standard.

As an alternative to the shadow evaporation method for the preparation of ultrasmall tunnel junctions we have established the so-called self-aligned in-line technique. It was applied to the fabrication of common Al/AlO/sub x//Al-type and, for the first time, Nb/AlO/sub x//Nb-based single-electron transistors. The characterization of the samples at temperatures in the range of a few hundred millikelvins reveals charging effects (Coulomb blockade and gate modulation) of the quasiparticle current.

Planar dc SQUID gradiometers with integrated antenna were designed and investigated. With low-T/sub c/ superconductors they were realized as washer type and as transformer type SQUID gradiometers. With high-T/sub c/ superconductors we tested galvanometer SQUIDs. Field gradient sensitivities of 32 fT/(cm/spl radic/(Hz)) could be achieved with low-T/sub c/ SQUIDs and of 520 fT/(cm/spl radic/(Hz)) with high-T/sub c/ SQUIDs, respectively. All types of gradiometers could work stable in disturbed environments. The influence of the SQUID gradiometer layout and the construction of the sample holder on the degree of disturbance suppression is shown.

A resonant type of cryoelectronic microwave circuit for the generation of frequency-scaled Josephson voltages is presented. The circuits consist of superconducting microstripline resonators with series arrays of Josephson tunnel junctions located on the microwave current antinodes. This microwave design was chosen to obtain highly accurate Josephson voltages at low microwave power levels and low drive frequencies. The resonator circuits have been fabricated in both niobium-lead and all-niobium technology. At present, we have observed long-term stable Josephson voltages up to 0.75 V for niobium-lead circuits with 1320 junctions and 0.66 V for all-niobium circuits with 1456 junctions, respectively.