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IPHT Jena and Supracon AG develop and fabricate full tensor magnetic gradiometer (FTMG) systems, based on low-temperature-superconducting (LTS) planar-type SQUID (Superconducting Quantum Interference Device) gradiometers of first order. The systems are operated by Spectrem Air Ltd., jointly owned by Anglo American and De Beers. This paper describes details of the system setup, data processing and achieved performance parameters. The sensors work at 4.2 K in liquid helium. With a base length of only 3.5 cm a real gradient measurement is possible. With six gradiometer the full magnetic gradient tensor can be measured with redundancy, allowing for high system reliability and the application of enhanced noise reduction techniques. The data of the FTMG system will allow for a 3D inversion for the underlying magnetic sources. Examples of 2D maps of tensor components with high spatial resolution are shown.

We review the development of low parasitic capacitance sub-micrometer cross-type Josephson tunnel junctions for their use in highly sensitive and field-stable SQUID magnetometers. The potential of such junctions is shown on I – V characteristics as well as on Fraunhofer diffraction patterns. The evaluation of Fiske steps lead to a specific junction capacitance of about 62 fF/μm 2 for a critical current density of about 1.7 kA/cm 2 . The avoidance of any idle-region – the undesired overlap between superconducting electrodes around the junction – due to a self-aligned junction definition process lead to highly sensitive SQUIDs; multiloop SQUID magnetometers exhibiting exceptionally low magnetic field noise levels as low as 0.3 fT/Hz 1/2 , as well as large usable voltage swings of more than 150 μVpp. Furthermore, junction dimensions in the sub-micrometer range allowed for very high tolerable background fields during cool-down of up to 6.5 mT. In operation mode, the SQUID magnetometers recovered completely from magnetization pulses of up to 76 mT. With respect to their easy and reliable usage as well as their high sensitivity, the presented SQUID sensors are adequate for many applications, like in geophysics, biomagnetism or low-field magnetic resonance imaging.

We report on a new method suitable for the fabrication of highly sensitive SQUID magnetometers exhibiting very small effective areas. In contrast to commonly used approaches, which mostly make use of constrictions in a superconducting material to form the Josephson junctions, we have been able to fabricate such sensors with SIS cross-type Josephson tunnel junctions in an Nb/AlO x /Nb trilayer process. The small junction size and vanishing idle region of our cross-type junctions and a therefore very low capacitance lead to a high sensitivity of the resulting SQUIDs. First results on fabricated miniature SQUID magnetometer within this technology are shown. We found, that even with SQUID loop dimensions of about 10 × 10 μm 2 very low flux noise levels could be achieved, which clearly demonstrate the high potential of our approach. Furthermore we estimate the required SQUID parameter which may allow the detection of single electron spin-flips with such devices in the future.

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).

First-order integrated planar SQUID gradiometers with pick-up loops of 40 µm in diameter and a baseline of 200 µm were designed specially for the readout of signals from magnetic micro-calorimeters. The SQUID has to operate in magnetic fields up to 5 mT and at temperatures down to 7 mK. The design is a first step to develop a multi-pixel x-ray detector, which can be used in high-resolution x-ray fluorescence analysis. Chip coils with a heater switch produce a magnetic field up to a few millitesla in persistent current mode. A SQUID-array amplifier was used to readout the signal of the SQUID gradiometer. The intrinsic noise of the gradiometer at 4.2 K is lower than 1 μΦ0 Hz−1/2. The SQUID can withstand external magnetic fields up to 7 mT without losing its performance. The results of a test with the SQUID gradiometer at ultra low temperatures are reported.

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.

We present a new type of microwave circuits for Josephson voltage standards. Coplanar strips (CPS) are used instead of microstriplines or coplanar waveguides (CPW) to integrate the Josephson junctions in a microwave transmission line. Compared to microstripline and CPW circuits the CPS design doubles the scale of integration and simplifies the fabrication technology. Well-operating 10 V circuits in CPS design with 17,000 SIS junctions and 1 V SINIS circuits were tested in a new cryoprobe. In this cryoprobe a low loss dielectric waveguide consisting of a teflon strip is used for the microwave transmission in the 70 GHz frequency range. The dielectric waveguide together with a fibreglass-reinforced plastic supporting tube reduces the contribution of the cryoprobe to the liquid helium boil-off rate drastically. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

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.

A new method for preparing Nb/Al–AlO x /Nb Josephson junctions with very low critical current densities is reported: an ultraviolet light assisted aluminum oxidation process in pure oxygen atmosphere. A better reproducibility of the critical current density of the junctions is reached in comparison to junctions prepared by the double oxidation technique. Series arrays of Josephson tunnel junctions with up to 960 junctions are produced and tested for Josephson voltage standard applications. The thickness of the aluminum oxide barrier is determined by Rutherford backscattering spectrometry and is compared with the results of specific capacitance measurements.

In 1997, the Institute for Physical High-Technology (IPHT) in Jena, Germany, started a program with the aim of developing an airborne full-tensor magnetic gradiometer superconducting quantum interference device (SQUID) using liquid-helium-based thin-film technology. Since that time, the sensor, system electronics, data acquisition system, and cryogenics have been developed and extensively tested. This article reports on successful tests of the intrinsic planar LTS (low-temperature superconductor) SQUID gradiometers which were conducted by towing the system from a helicopter as well as for a stinger-mounted configuration in a Cessna 208 aircraft.

EUCAS2013 Volume: 507 Host publication title: 11th European Conference on Applied Superconductivity (EUCAS2013) We have demonstrated the operation of a 12-channel Beyer-style SQUID-based time domain multiplexer. It was manufactured using a fabrication process that is cross-compatible between VTT and IPHT-Jena. The multiplexer consists of twelve 12-SQUID series arrays, each shunted by a Zappe-style interferometer array acting as a flux-controlled superconducting/normal conducting switch. By keeping all switches but one in the superconducting state, it is possible to select one active readout channel at a time. A flux feedback coil common to all SQUID arrays allows realization of a flux-locked loop. We present characteristics of the multiplexer and measurement data from experiments with a 25-pixel X-ray calorimeter array operated at T

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.

We present an innovative grating design based on conical diffraction which acts as an almost perfect and low-loss beamsplitter for extreme ultraviolet radiation. The scheme is based on a binary profile operated in grazing incidence along the grating bars under total external reflection. It is shown that periods of a few 10(2) nm may permit an exclusive (±1)(st) order diffraction with efficiencies up to ~ 35% in each of them, whereas higher evanescent orders vanish. In contrast, destructive interference eliminates the 0(th) order. For a sample made of SiO(2) on silicon, measured data and simulated results from rigorous coupled wave analysis procedures are given.

We implement the impedance measurement technique in order to characterize superconducting structures designed for applications in quantum computing. We report an experimental study of the circuit consisting of three small Josephson junctions incorporated in a low-inductance superconducting loop. Measurements of the circuit response to an applied weak ac bias as a function of the external magnetic flux φe yield complete information on the properties of the circuit. We found that the system displays two metastable states. From experimental data, we have determined the magnetic field dependent Josephson energy U(φe) of these states, the potential barrier ΔU between these states, as well as the difference of the critical currents of the junction in the loop.

The magnetic coupling behaviour of dc-magnetron sputtered Co/Cu multilayers with wedge shaped Cu interlayers was investigated by magnetoresistance (MR). The well known oscillatory behaviour of both the amplitude of the MR effect and the saturation field H s was found. Within the first region of antiferromagnetic coupling around 1 nm Cu there is a different thickness dependence of H s and MR presumably caused due to the imperfect structure of our multilayer system. Domain nucleation and reversal behaviour were studied by Kerr microscopy and correlated with the observed coupling states determined by MR. There were no clear hints for a biquadratic coupling term.

We present a microwave circuit for Josephson voltage standards. Here, the Josephson junctions are integrated in a microwave transmission line designed as coplanar strips (CPS). The new layout offers the possibility of achieving a higher scale of integration and to considerably simplify the fabrication technology. The characteristic impedance of the CPS is about 50 Ω, and this should be of interest for programmable Josephson voltage standard circuits with SNS or SINIS junctions. To demonstrate the function of the microwave circuit design, conventional 10 V Josephson voltage standard circuits with 17 000 Nb/AlOx/Nb junctions were prepared and tested. Stable Shapiro steps at the 10 V level were generated. Furthermore, arrays of 1400 SINIS junctions in this microwave layout exhibited first-order Shapiro steps.

For four decades semiconductor electronics has followed Moore's law: with each generation of integration the circuit features became smaller, more complex and faster. This development is now reaching a wall so that smaller is no longer any faster. The clock rate has saturated at about 3-5 GHz and the parallel processor approach will soon reach its limit. The prime reason for the limitation the semiconductor electronics experiences is not the switching speed of the individual transistor, but its power dissipation and thus heat. Digital superconductive electronics is a circuit- and device-technology that is inherently faster at much less power dissipation than semiconductor electronics. It makes use of superconductors and Josephson junctions as circuit elements, which can provide extremely fast digital devices in a frequency range - dependent on the material - of hundreds of GHz: for example a flip-flop has been demonstrated that operated at 750 GHz. This digital technique is scalable and follows similar design rules as semiconductor devices. Its very low power dissipation of only 0.1 mu W per gate at 100 GHz opens the possibility of three-dimensional integration. Circuits like microprocessors and analogue-to-digital converters for commercial and military applications have been demonstrated. In contrast to semiconductor circuits, the operation of superconducting circuits is based on naturally standardized digital pulses the area of which is exactly the flux quantum Phi(0). The flux quantum is also the natural quantization unit for digital-to-analogue and analogue-to-digital converters. The latter application is so precise, that it is being used as voltage standard and that the physical unit 'Volt' is defined by means of this standard. Apart from its outstanding features for digital electronics, superconductive electronics provides also the most sensitive sensor for magnetic fields: the Superconducting Quantum Interference Device (SQUID). Amongst many other applications SQUIDs are used as sensors for magnetic heart and brain signals in medical applications, as sensor for geological surveying and food-processing and for non-destructive testing. As amplifiers of electrical signals. SQUIDs can nearly reach the theoretical limit given by Quantum Mechanics. A further important field of application is the detection of very weak signals by 'transition-edge' bolo-meters, superconducting nanowire single-photon detectors, and superconductive tunnel junctions. Their application as radiation detectors in a wide frequency range, from microwaves to X-rays is now standard. The very low losses of superconductors have led to commercial microwave filter designs that are now widely used in the USA in base stations for cellular phones and in military communication applications. The number of demonstrated applications is continuously increasing and there is no area in professional electronics, in which superconductive electronics cannot be applied and surpasses the performance of classical devices. Superconductive electronics has to be cooled to very low temperatures. Whereas this was a bottleneck in the past, cooling techniques have made a huge step forward in recent years: very compact systems with high reliability and a wide range of cooling power are available commercially, from microcoolers of match-box size with milli-Watt cooling power to high-reliability coolers of many Watts of cooling power for satellite applications. Superconductive electronics will not replace semiconductor electronics and similar room-temperature techniques in standard applications, but for those applications which require very high speed, low-power consumption, extreme sensitivity or extremely high precision, superconductive electronics is superior to all other available techniques. To strengthen the European competitiveness in superconductor electronics research projects have to be set-up in the following field: - Ultra-sensitive sensing and imaging. - Quantum measurement instrumentation. - Advanced analogue-to-digital converters. - Superconductive electronics technology.

The magnetic behaviour of one Co layer (thickness 0.4 to 2 nm) sandwiched between a 22 nm Pt underlayer and a 3 nm Pt top layer was investigated by torquemeter. At a mean thickness of 1.2 nm a transition was observed from a perpendicular easy axis to an easy plane. Near this transition fourfold components of the torque curves were recognized. This behaviour can be described either by a second-order anisotropy term or in terms of an inhomogeneous magnetization distribution. The latter was tried using a discrete atomic-scale model.

Airborne geophysics is a commonly used tool for the geophysical prospecting of large areas with high spatial resolution. Especially the mapping of anomalies of the Earth’s magnetic field is well suited for airborne operation. Up to now the measurement of the total field and it’s gradients with caesium vapour magnetometers is the most widely used method in airborne magnetic surveys. But there are advantages of using the tensor gradient instead of the total field gradients, which will be shortly discussed.

We have developed highly sensitive SQUID-based magnetometers and gradiometers, fabricated in a standard Nb/AlOX/Nb technology. The SQUID itself is designed as a current sensor having an input coil and a feedback coil. The number of turns of the input coil can be adjusted to ensure optimal coupling to the pickup loops with an inductance in the range from 5 nH to 300 nH. Several types of planar pickup loop configurations have been realized. The magnetometer has a pickup loop with a size of 1 cm × 1 cm. With a typical white noise level better than 4 µ0 Hz-1/2 and an effective area of 2.6 mm2 a field resolution of 3.2 fT Hz-1/2 results for the magnetometer. Two loops connected in series with an area of 2 cm × 2 cm each and a baseline of 4 cm were used in the gradiometer. We measured a 7.5 mm2 effective area for each loop and a field gradient resolution of 36 fT m-1 Hz-1/2 corresponding to a field resolution in the loop of 1.6 fT Hz-1/2.

Several applications of Josephson junctions (JJs) in low-temperature superconducting circuits such as programmable voltage standards and SFQ logical circuits or SQUID sensors require JJs with sub-µm dimensions. A method for preparing superconductor-isolator-superconductor (SIS) JJs as well as superconductor-normal metal-superconductor (SNS) JJs was developed which allows the preparation of both types of junctions by nearly the same technological process. Titanium is used as the normal metal for the SNS JJ. Ti can be anodically oxidized and a technology similar to the well-tried Nb-Al technology for SIS JJs can be applied to the SNS system, too. By electron-beam exposure of a negative deep UV photoresist (ma-N 2400) a stable resist mask is generated which has to withstand the following three self-aligned process steps: etching of the Nb counterelectrode by RIE, isolation by anodization and lifting of a SiO planarization layer. By this process SNS (Nb-Ti-Nb) and SIS (Nb-Al/AlOx-Nb) Josephson junctions with smallest junction areas of 0.49 µm2 were prepared and characterized by dc measurements and under the influence of microwave irradiation. The Nb-Ti-Nb junctions have IcRn products of 87 µV. They can be used for programmable Josephson voltage standard circuits at frequencies of 70 GHz.

We present the experimental results and a bolometer model of the voltage-biased superconducting bolometer on the low stress silicon nitride (Si3N4) membrane, developed in collaboration between the Max-Planck-Institut fur Radioastronomie (MPIfR), Bonn and the Institute for Photonic Technology (IPHT), Jena, Germany. The superconducting thermistor, de- posited on the low stress silicon nitride membrane, is a bilayer of gold-palladium and molybdenum and is designed for a transition temperature of 450 mK. Bolometers for the 1.2 mm atmospheric window were designed, built and tested. The thermal conductance of the bolometer is tuned by structuring the silicon nitride membrane into spider-like geometries. The incident radiation is absorbed by crossed dipoles made from gold-palladium alloy with a surface resistance of 10 Ω/square. Using the COSMOS finite element analysis package, the thermal conductance is obtained for the bolometers of different geometries. FEA simulations showed that the deposition of a gold ring around the absorbing area could increase the sensitivity of the bolometer. Therefore, a gold ring is deposited around the center absorbing patch of the silicon nitride membrane. For the bolometer with a gold ring, the measured NEP is 1.7 × 10 −16 W/ √ Hz and the time constant is in the range between 1.4 and 2 ms.

Single nonhysteretic superconductor-metal-superconductor Josephson junctions consisting of Nb-Ti-Nb were prepared using the well-known anodization process for the isolation of the junctions. The 4×4 μm2 junctions show overdamped Josephson current-voltage characteristics at 4.2 K. The characteristic voltage IcRn is about 8.0 μV at a critical current density jc of about 29 kA/cm2. Under the influence of 10.5 GHz microwave irradiation the constant-voltage steps of n=0 and ±1 are observed with current amplitudes of about 3 mA.

We present the experimental results of voltage-biased superconducting bolometers (VSB) on silicon nitride (Si 3 N 4 ) membranes with niobium wiring developed in collaboration between the Institut fur Physikalische Hochtechnologie (IPHT), Jena, Germany and the Max-Planck-Institut fur Radioastronomie (MPIfR), Bonn, Germany. The bolometer current is measured with the superconducting quantum interference device (SQUID), and as expected, the current responsivity is proportional to the inverse of the bias voltage. The experiments were performed with bilayer gold-palladium molybdenum thermistor at 300 mK 3 He cooled cryostat and the desired transition temperature of T c = 450 mK is achieved. The strong negative electro-thermal feedback of the VSB maintains the constant bolometer temperature and reduces the response time from 4 ms to 100 μs. We have tested thermistors of various size and shape on a continuous membrane and achieved a noise equivalent power (NEP) of 3.5 × 10 -16 W/√Hz. The measured NEP is relatively high due to the comparatively high background and high thermal conductance of the unstructured silicon nitride (Si 3 N 4 ) membrane. We have fabricated 8-leg spider structured membranes in three different geometries and the relation between the geometry and the thermal conductance ( G ) is studied. Using the COSMOS finite element analysis tool, we have modeled the TES bolometers to determine the thermal conductance for different geometries and calculated the various parameters. Due to the demands of large number pixel bolometer camera we plan to implement multiplex readout with integrated SQUIDs in our design.

Ever since the first proposal of the voltage-biased transition-edge bolometer the astrophysics community desired bolometer arrays with as many pixels as possible. With respect to the technical problem due to the need of lots of readout SQUID sensors only with multiplexing it is possible to go beyond a few hundred pixel. A technology which allows the manufacture of detector and readout on one chip would simplify this task substantially. Here we demonstrate the fabrication of a transition edge sensor based on a thermistor out of a molybdenum / gold-palladium bilayer. The alloy of gold-palladium (Au-Pd), which allows the tuning of molybdenum's critical temperature by one order of magnitude, is taken from our foundry process for SQUID manufacturing. Au-Pd can further be used for shunt resistances, absorber patterns and bond pads, and, therefore, it is a good choice for a combined technology. The thermistor is placed on a moderately patterned silicon nitride membrane in the shape of an 8-legged spider. The radiation band of interest is coupled via a conical feed horn to a simple grid of dipole-like antenna patterns. This removes the need for the poorly reproducible high-resistance absorption films for the matching of the free space impedance. The simple detector technology is compatible with the SQUID manufacturing. Hence, some of the SQUID layers can be merged with the corresponding detector layer, i.e. the thermistor wiring and the SQUID washer are made in a single niobium layer. The concept of feed horn coupling eases the design requirements, consequently the SQUID can be placed close to the detector, thereby allowing a simpler wiring to be used and in theory a better performance to be obtained.

Experimental observation of the strong influence of an array of ferromagnetic nanodots on the critical current of a short overlap Josephson junction is reported. Pronounced commensurability effects are detected due to the presence of the additional peaks in the magnetic field induced diffraction pattern. The changes in the Fraunhofer pattern of the Josephson junctions are account for by the formation of Abrikosov vortices trapped in the electrodes which induce a phase inhomogeneity in the junction area., 5 pages, 4 figures

Magnetization behavior in antiferromagnetically (AFM) coupled multilayer systems was calculated by using an atomic layer model. Comparisons with the experimental results obtained on sputtered Co/Cu multilayers reveal remarkable differences in the magnetization reversal and in the field dependence of the magnetoresistance. Kerr loops measured from both sides of the stack display strong vertical differences. At the lower side near the Fe seed layer the magnetization reversal is in good agreement with that of our calculations whereas near the surface in large portions of the stack the AFM coupling is destroyed or varied. These effects are presumably caused by magnetic short circuits at defects in the multilayer structure. Cross‐section transmission electron microscopy reveals growth defects which seem to be responsible for the deviations from the calculated ideal behavior.

A new type of cryoelectronic microwave circuits in Nb/Al/sub 2/O/sub 3//Nb technology 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 rf current antinodes. The circuits are designed to obtain highly accurate Josephson voltages at low rf power levels and low drive frequencies. At present, we have observed Josephson voltages up to 0.66 V in a resonator circuit with 1456 junctions.

Thin films of CoCr are considered to be the most promising media for high-density perpendicular magnetic recording. It is shown that when depositing single CoCr layers onto a Si substrate by dc magnetron sputtering excellent magnetic and morphological properties can be achieved. The growth of nodular defects hindering a small head to medium spacing is characterized by cross-sectional TEM and discussed on the basis of a shadowing model treated by computer simulations.

The SAIL (self-aligned in-line) technique has been applied to the preparation of ultrasmall metallic tunnel junctions. By using e-beam lithography and sputter methods the area of both Al/AIO x /Al and Nb/AIO x /Nb contacts has so far been reduced to less than 0.005pm 2 . At low temperatures high-ohmic double junctions with a small metallic island in between show the Coulomb blockade effect. The current through such a device can be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single-electron transistor). Both these single-charge phenomena have been observed at temperatures of a few hundred mK.

We present a configuration of LTS dc SQUID magnetometers that is suited for an absolute measurement of the vector components of the Earth’s magnetic field with a white noise level of about 6 fT Hz 1=2 . Due to its periodic voltage‐flux characteristic, a SQUID’s output voltage generally corresponds to a set of equidistant fluxes or magnetic field strengths. To resolve this ambiguity, we introduce a configuration of coplanar SQUIDs integrated on a single chip, which exhibit effective areas differing by several orders of magnitude. The set of possible magnetic field strengths matching the output voltages of these SQUIDs is thereby significantly reduced and especially unique for magnetic field strengths less than a certain threshold value of about 10 T in our current implementation. The SQUIDs are realized with 0.8 m cross-type Josephson junctions that withstand high background fields of up to 3.9 mT during cool down and operation. A first one-dimensional experimental implementation successfully measured the modulation of the magnetic field component perpendicular to the sensor surface with amplitudes exceeding 50 T. The overall dynamic range of the SQUID magnetometer system achieves 190 dB.

Superconducting NbNx thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using the metal organic precursor (tert-butylimido)-tris (diethylamino)-niobium (TBTDEN) and hydrogen plasma. The transition temperature TC and the resistivity of the NbN thin films were measured by four-point probe measurement. Their composition was analyzed by x-ray diffraction and Rutherford backscattering spectroscopy. The deposition process was optimized to obtain a low resistivity as well as a high superconducting transition temperature. A TC close to 10 K and a resistivity of 2.5 μΩ m as well as a critical current density of 8.9 × 105 A cm−2 were achieved. Originally, a high oxygen concentration was detected in the compound. By variation of the plasma parameters, the concentration could be reduced from 57 atom (at.)% to 11 at.%. Because of the excellent thickness control and conformality, such ALD films may be suited very well for applications in superconductor electronics and sensing devices.

A SQUID current sensor with integrated input coil has been designed and fabricated using our new technology with Nb–AlOx–Nb cross-type Josephson junctions. The white noise level of the SQUID was measured to be of 1 μΦ0 Hz−1/2, corresponding to a current resolution of about 110 fA Hz−1/2. A superconducting transformer with a turns ratio of 19:1 was fabricated with our standard Nb technology. The transformer chip size is 12.5 mm × 12.5 mm. Three transformer chips were assembled in flip-chip configuration yielding an effective ratio of turns of 48:1. The current sensor SQUID with this transformer at the input has a current sensitivity of 4.1 nA/Φ0. The second stage SQUID amplifier was used to characterize the noise performance of the current sensor SQUID. The measured noise level of the current sensor corresponds to the noise limited current resolution of 3 fA Hz−1/2 in the white noise region and at 100 Hz and 10 Hz it is lower than 5 fA Hz−1/2 and 10 fA Hz−1/2, respectively.

We report on a two-stage system consisting of a dc SQUID magnetometer inductively coupled to an amplifying second stage realized by a SQIF. The SQIF is a series or parallel connection of dc SQUIDs with various inductances, which facilitates a unique global minimum in the voltage?flux characteristic and ensures a stable operating point, a high voltage swing and a large transfer function of the system. The noise of the amplified signal is dominated by the input stage and its voltage swing exceeds 1?mV, which reduces the demands on the readout electronics. A part of the current of the SQIF can optionally be fed back to the feedback coil of the first-stage SQUID, so that an on-chip linearization of the voltage?flux characteristic is realized. The short feedback path enables a substantially higher bandwidth compared to a flux-locked loop using a room temperature amplifier. From experimental results it can be expected that this set-up is capable of measuring magnetic fields with a high slew rate and dynamic range.

We demonstrate the transfer of single photon triggered electrical pulses from a superconducting nanowire single photon detector (SNSPD) to a single flux quantum (SFQ) pulse. We describe design and test of a digital SFQ based SNSPD readout circuit and demonstrate its correct operation. Both circuits (SNSPD and SFQ) operate under the same cryogenic conditions and are directly connected by wire bonds. A future integration of the present multi-chip configuration seems feasible because both fabrication process and materials are very similar. In contrast to commonly used semiconductor amplifiers, SFQ circuits combine very low power dissipation (a few microwatts) with very high operation speed, thus enabling count-rates of several gigahertz. The SFQ interface circuit simplifies the SNSPD readout and enables large numbers of detectors for future compact multi-pixel systems with single photon counting resolution. The demonstrated circuit has great potential for scaling the present interface solution to 1,000 detectors by using a single SFQ chip.

The microwave properties of microstrip lines used for Josephson voltage standard circuits at frequencies from 70 to 75 GHz were investigated at a temperature of 4.2 K. Our measurements quantify the different loss sources. The excitation of a parasitic substrate mode is postulated, demonstrated by microwave modelling and confirmed by experiments. These modes dominate the losses for wafers with a thickness above 500 µm. The substrate modes can induce an inhomogeneous microwave distribution in the Josephson junction array, thereby causing poor operation of the voltage standard circuits. Silicon dioxide fabricated in various PECVD processes was used as the dielectric layer in the microstrip line circuits. Permittivity and losses of silicon dioxide and the silicon substrate were determined, as well as the surface resistance of niobium.

We report on the development of a new family of magnetic field sensors with exceptionally low magnetic field noise, as low as 0.3 fT Hz − 1/2. Beside this, they exhibit high usable voltage swings of more than 150 µVpp and tolerable background fields during cool-down of up to 6.5 mT. In operation mode they recover completely from magnetization pulses of up to 76 mT, which makes them well suited for applications such as low-field magnetic resonance imaging. With respect to their easy and reliable use as well as their field resolution in the sub-fT Hz − 1/2 range, the presented SQUID sensors are adequate for many applications, such as in geophysics or in biomagnetism.

The sensitivity of superconducting electronics operated in the sub-Kelvin temperature range is usually limited by the hot-electron effect. Here, an increased thermal resistance due to a weakened electron–phonon coupling leads to a higher temperature of the electrons in the thin-film shunt resistors of the Josephson junctions. Cooling fins can be attached to weaken this effect. We characterized different configurations of resistors in PdAu with or without attached cooling fins by dissipating power and determining the effective electron temperature. This was done by directly measuring the Johnson noise with a SQUID amplifier. The results are compared to theory and numerical calculations on the electronic heat transport. The latter turns out to be a useful tool for the optimization of the thermal design of superconducting electronics.

We report on a technology for the fabrication of sub-micrometer sized cross-type Josephson tunnel junctions in niobium technology. We present the fabrication scheme and properties of cross-type junctions with linear dimensions from 10 down to 0.6??m. Sidewall passivation of the junctions is achieved by anodization as well as by planarizing the junctions with SiO in a self-aligned deposition step. The measured ratio of the sub-gap resistance to the normal resistance is about 35. Because of their low sub-gap current and low parasitic capacitance such junctions are well suited for applications like high resolution SQUIDs.