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We study the temperature dependence of the upper critical magnetic field Hc2 in a layered iron-based superconducting (IBS) material of the 11-family, namely the Fe(Se,Te) thin film grown on CaF2 substrate. On the basis of intrinsic anisotropy as well as system dimensionality, it turns useful to make a comparison with an ultrathin film conventional low temperature superconductors (LTS) such as NbN, mostly used for device applications. We compare the anisotropy factors as a function of temperature and magnetic field. Both materials present a peculiar behavior: The LTS behaves as a strong anisotropic system, whereas the IBS shows very weak anisotropic features. The strong NbN character can be directly ascribed to the dimensionality of the ultrathin film employed, thus revealing a geometry effect. The weak Fe(Se,Te) trend should be related to its layered crystallographic structure, thus probing an intrinsic origin of its anisotropy. These characteristics become relevant for the potential application of this material in coated conductor technology.

In a superconducting photon detector, the effective transition to the normal state can be induced by the instability of the flux flow regime. Indeed, in the presence of self-magnetic field, the local suppression of superconductivity induced by photon absorption determines vortex nucleation and flux flow regime, which can make the superconducting state unstable. Understanding such instability can boost the performances of those superconducting devices based on this resistive switching. Here, we present the study of the geometry influence on such instability in NbN and NbTiN ultrathin films. Despite the same patterned microbridge geometry, the two superconductors show different behaviors at very low applied magnetic fields. A comparison with other superconductors outlines the possibility to tune the resistive switching by geometry effects in interesting materials for devices applications. Finally, we also report the influence of the cooling environment on the electric critical power in superconducting thin films.

Cooling efficiency and thermal stability is strictly demanding for practical applications of superconductors operating at current values close to the critical current, such as superconducting detectors. Indeed, a thermally unstable device can show premature quench, i.e. it can suddenly switch from the superconducting state to the normal one at a current value lower than the expected one, which can result in false counts. Cooling by direct contact with a liquid He bath is considered the best way to obtain thermal stability in a superconducting device. Other, cheaper cooling techniques can be suitable to achieve satisfactory working conditions. In this work, we evaluate the impact of three different cooling environments, namely liquid He in a standard cryostat and both dynamic and static He gas in a cryogen-free cryostat, on current voltage characteristics (CVCs) acquired in ultra-thin superconducting microbridges suitable for detectors. In particular, we use the Flux-Flow Instability phenomenon as a tool to analyze voltage stability in CVCs in the three different environments and we find that cryogen-free techniques have performance comparable to liquid He cooling.

Vortex dynamics is strongly connected with the mechanisms responsible for the photon detection of superconducting devices. Indeed, the local suppression of superconductivity by photon absorption may trigger vortex nucleation and motion effects, which can make the superconducting state unstable. In addition, scaling down the thickness of the superconducting films and/or the width of the bridge geometry can strongly influence the transport properties of superconducting films, e.g. affecting its critical current as well as its switching current into the normal state. Understanding such instability can boost the performances of those superconducting devices based on nanowire geometries. We present an experimental study on the resistive switching in NbN and NbTiN ultra-thin films with a thickness of few nanometers. Despite both films were patterned with the same microbridge geometry, the two superconducting materials show different behaviors at very low applied magnetic fields. A comparison with other low temperature superconducting materials outlines the influence of geometry effects on the superconducting transport properties of these materials particularly useful for devices applications. © 2017 IEEE.

The best rf accelerating cavities are made of bulk niobium. This technology is now close to it full development, with any further improvement limited by the superconducting properties of Nb itself. In the best performing cavities the rf equatorial magnetic field H is close to the Nb thermodynamic critical field (HC ≈ 200 mT). In 2006 Gurevich proposed the use of nanoscale layers of superconducting materials having HC higher than Nb. This multilayer structure is expected to shield bulk niobium and therefore to increase the cavity breakdown field. In order to explore this pioneering idea, we have studied the penetration of the magnetic field by mean of nanometric sized multi layer structures deposited on flat single crystal substrates. We have deposited high quality "model" samples by dc magnetron reactive sputtering on R-plane cut sapphire substrates. A 250 nm layer of niobium represents the bulk material as in rf cavities. Such Nb layers were coated with a single or multiple stacks of NbN layers (25 nm or 12 nm) separated by 15 nm MgO barriers, and characterized by X-ray reflectivity and dc transport measurements, dc magnetization curves have been measured by conventional super conducting quantum interference device (SQUID) to determine the first penetration field BC1. For comparison, BC1 was also mea sured with a local probe method based on 3rd harmonic analysis. The Nb samples coated with NbN multi-layers clearly exhibit a higher first penetration field.

We have designed a band-pass sigma-delta modulator made in niobium nitride technology. The basic components of such a 10 GHz carrier frequency modulator are a band filtering resonator and an SFQ comparator synchronized with a clock frequency up to 200 GHz sampling rate. We describe the clock design including an internal JTL and its testing circuit based on frequency division. The technology involves NbN/Ta/sub x/N/NbN internally shunted Josephson junctions with Jc/spl sim/20kA/cm/sup 2/ and high RnIc product (/spl sim/1 mV) at 4.2 K. Junction parameters are shown to be controlled by the reactive sputtering conditions of the Ta/sub x/N barrier. SiO/sub 2//MgO bi-layers are used as dielectrics associated with NbN wiring layers and ground-plane. All layers are prepared in a way compatible with a full processing at about 350/spl deg/C in order to minimize the London penetration depth of NbN layers and thus to allow circuits operation at about 9 K. High frequency simulations of the modulator's circuits are presented, including the comparator, the DEMUX, and the resonator response.

We report here two processes for fabricating quantum devices based on local anodization induced by Atomic Force Microscope (AFM). The first process involves ultra-thin films of doped silicon-on-insulator (SOI) passivated with hydrogen. AFM-drawn oxide lines create a chemical contrast that is used as a mask for silicon wet etch. Etching is performed down to the buried silicon oxide layer, thus leading to silicon nanowires supported on insulator. We show that this process appears well suited to obtain SOI nanostructures and investigate electrical transport though silicon wires with sub-1000 nm 2 cross-section. In the second process, diffusion of oxygen species is performed through the whole layer of metallic ultra-thin films, which provides a technique for direct writing of insulating regions. This technique was applied to ultra-thin niobium films for fabricating mesoscopic structures. A superconducting quantum interferometer useful for nanomagnetism is demonstrated.

Superconducting hot electron bolometer mixers are now a competitive alternative to Schottky diode mixers in the terahertz frequency range because of their ultra wideband (from millimeter waves to visible light), high conversion gain, and low intrinsic noise level. High Tc superconductor materials can be used to make hot electron bolometers and present some advantage in term of operating temperature and cooling. In this paper, we present first a model for the study of superconducting hot electron bolometers responsivity in direct detection mode, in order to establish a firm basis for the design of future THz mixers. Secondly, an original process to realize YBaCuO hot electron bolometer mixers will be described. Submicron YBaCuO superconducting structures are expitaxially sputter deposited on MgO substrates and patterned by using electron beam lithography in combination with optical lithography. Metal masks achieved by electron beam lithography are insuring a good bridge definition and protection during ion etching. Finally, detection experiments are being performed with a laser at 850 nm wavelength, in homodyne mode in order to prove the feasibility and potential performances of these devices.

The temperature behaviour of the first critical field ($B_{C1}$) of superconducting thin film samples can be determined with high accuracy using an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of the sample, a non zero third harmonic voltage $V_{3}$ is induced in it when Abrikosov vortices enter the sample. Conditions to be satisfied for the quantitative evaluation of $B_{C1}$ using this technique are detailed. As validation test, different type II superconductors (Nb, NbN, MgB$_{2}$ and Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-d}$ under the form of thin films) have been measured. The comparison between experimental results, data presented in literature and theoretical predictions is presented and discussed., Comment: to be published in Journal of Applied Physics

We present a systematic study of microwave nonlinearity, undertaken on different types of superconductors (Nb, NbN, MgB2 Y-1 beta a(2)Cu(3)O(7-delta)), in thin film form. Experiments are performed in a dielectrically loaded cavity operating at 7 GHz. The dependence of the surface impedance Z(s) and of the third order intermodulation (IMD) products on the power feeding the cavity is analysed, with the aim of shedding a light on the primary mechanisms of nonlinearity. Data from different superconductors are quantitatively compared.

NbN/MgO/NbN Josephson tunnel junctions and multilayers have been achieved on 3 inch. Si and SOI substrates in order to develop high-speed RSFQ logic gates with terahertz cut-off frequency junctions and high operating temperature (10 K). Small area junctions (∼1 μm 2 ) with 20 kA/cm 2 J C and low sub-gap leakage current ( V m >15 mV) at 4.2 K (and J C ∼10 kA/cm 2 at 12 K) are reproducibly obtained from multilayers post-annealed at 250°C. Deposition of a thin MgO buffer layer underneath the NbN base electrode is critical to insure lower NbN surface resistance values ( R s =7 μΩ at 10 GHz and 5 K) and lower London penetration depth λ 0 (∼270 nm). NbN penetration depth and intrinsic capacitance of NbN/MgO/NbN junctions have been investigated by measuring resonant Fiske steps in the I – V curves of junctions coupled to an integrated resonant stub. The extension of our present NbN technology for fabricating highly integrated planarized RSFQ circuits, based on self-shunted 0.5 μm 2 area, 20 kA/cm 2 junctions is now made possible on large area substrates (up to 8 inch).

A process has been developed to fabricate NbN tunnel junctions and 1.5 THz SIS mixers with Al electrodes and Al/SiO2/Al microstrip tuning circuits on thin Si membranes patterned on silicon on insulator wafers (SIMOX). High Josephson current density (Jc up to 2×104 A/cm2) NbN/AlN/NbN and NbN/MgO/NbN SIS junctions have been fabricated with a reasonably good Vm quality factor and energy gap values close to 5 meV at 4.2 K on (100) oriented 3 inches SIMOX wafers covered by a thin (∼8 nm) MgO buffer layer. The sputtering conditions critically influence the dielectric quality of both AlN and MgO tunnel barriers as well as the surface losses of NbN electrodes. 0.6-μm Si/SiO2 membranes are obtained after processing of a whole wafer and etching the individual chips in EDP. Such a technology is applied to the development of a waveguide/membrane SIS mixer for use around 1.5 THz.

A generic NbN Superconducting Tunnel Junctions (STJ) technology has been developed using conventional substrates (Si and SOI-SIMOX) for making THz spectrometers including SIS receivers and RSFQ logic gates. NbN/MgO/NbN junctions with area of 1 /spl mu/m/sup 2/, Jc of 10 kA/cm/sup 2/ and low sub-gap leakage current (Vm>25 mV) are currently obtained from room temperature sputtered multilayers followed by a post-annealing at 250/spl deg/C. Using a thin MgO buffer layer deposited underneath the NbN electrodes, ensures lower NbN surface resistance values (Rs=7 /spl mu//spl Omega/) at 10 GHz and 4 K. Epitaxial NbN [100] films on MgO [100] with high gap frequency (1.4 THz) have also been achieved under the same deposition conditions at room temperature. The NbN SIS has shown good I-V photon induced steps when LO pumped at 300 GHz. We have developed an 8 levels Al/NbN multilayer process for making 1.5 THz SIS mixers (including Al antennas) on Si membranes patterned in SOI-SIMOX. Using the planarization techniques developed at the Si-MOS CEA-LETI Facility, we have also demonstrated on the possibility of extending our NbN technology to high level RSFQ circuit integration with 0.5 /spl mu/m/sup 2/ junction area, made on large area substrates (up to 8 inches).

Narrow ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ films, excited by pulses of supercritical current, had their nanosecond electric response monitored in zero applied magnetic field. Delayed voltage steps plus constant differential resistance, characteristic of phase-slip centers (PSC), are observed at all temperatures. The duration of the initial zero voltage state is well fit by Ginzburg-Landau based theories, with a gap relaxation time controlled by phonon escape. At higher levels of excitation, PSC's give birth to slowly spreading normal hot spots.

A thin film of NbN (thickness t = 300 A), has been deposited on an MgO and a Si wafer. Both samples have been studied by transmission from 10 or 20 to 120 cm−1, and have exhibited one maximum of transmission at a given frequency like the classical superconductors as Pb, Sn or Hg in the superconductive state. From the Far IR experimental data, the characteristic temperature θc, and the gap frequency (νgap (θ) = 2 Δ (θ), Δ(θ) being the energy gap) are immediately obtained (for instance for the NbN / MgO sample, θc = 15.5 K; νg (5 K) = 39.7 cm−1), and it is seen that \(\frac{{{\text{2}}\vartriangle (4{\text{K)}}}}{{{\text{k}}_{\text{B}} \theta _c }} \approx 3.5\) as expected from the BCS theory for a weak coupling. To fit the data we had to adjust only two additionnal parameters: collision and plasma frequency, νc (θ) and νp (including all carriers). At θ = 5 K, thebest fit for the NbN / MgO sample is obtained with νc = 371 cm−1 and νp = 12,600cm−1.

The characterization of high temperature superconductor grain boundary junctions under a magnetic field in the mT range is reported. The devices were obtained by patterning narrow stripes in a YBa2Cu3O7−x (YBCO) film deposited on SrTiO3 and MgO symmetric bi-crystal substrates. To allow the investigation of devices having different current density, two grain boundary disorientations were considered: 24° and 45° for SrTiO3 and 24° for MgO. Current-voltage characteristics as a function of the applied field for several temperatures have been collected. The experimental data are discussed on the basis of the electrical parameters obtained for the different substrates. The periodic modulation of the Josephson critical current with the magnetic field indicates that the behavior of YBCO grain boundary junctions can be approximated by the standard overlap junctions model. A comparison between the experimental variation of the critical current with theoretical behavior allows for the determination of the current de...

In this work we present the fabrication and characterization of artificially generated bi-epitaxial YBCO grain boundaries on SrTiO/sub 3/ and buffered R-plane sapphire. The grain boundary is obtained by partly interposing a MgO seed layer between a bare or buffered substrate and a CeO/sub 2/ layer. We find that the structural perfection of the YBCO films decrease as the complexity of the stacking sequence increases. By comparing the structural properties of the YBCO film on the different stacking sequences with the transport properties of the fabricated devices, we found a strong correlation between the normalized junction resistance /spl rho//sub N/ and the degree of structural perfection in the superconducting film. The electrical behavior of the obtained junctions can be explained considering the grain boundary as a Josephson structure where the barrier transparency is related with the degree of structural and textural perfection of each superconducting electrode, justifying the correlation between the normalized junction resistance and the disorder in the YBCO films.

Vortex penetration severely limits high field performances in bulk niobium RF cavities for accelerators. To reach higher fields, Gurevich [Appl. Phys. Lett. 88, 012511 (2006)] proposed to deposit nanometric layers (d

40th International Conference on Metallurgical Coatings and Thin Films (ICMCTF), San Diego, CA, APR 29-MAY 03, 2013; International audience; For thermal applications, one needs to manufacture titanium nitride based films with as high as possible critical superconductivity temperature while minimizing the mechanical constraints. We investigated the structural and mechanical properties of titanium oxynitride thin films TiNxOy with 0.385

The growth characteristics of La 2− x Sr x CuO 4 thin films of various thicknesses (10–150 nm), epitaxially grown by laser ablation on (100) SrTiO 3 substrates, were analysed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). A periodic surface roughness with an important peak-valley amplitude (about 20% of the film thickness) was clearly observed in the films thicker than 25 nm. The period of the undulations did not depend on the deposited thickness. In contrast, AFM experiments performed on thinner films showed a perfect smoothness of the surface and a complete coverage of the substrate. These results were examined in connection with the fabrication of YBCO/LSCO(YBa 2 Cu 3 O 7−δ /La 2− x Sr x CuO 4 ) high T c superconducting multilayer films.

Several electric field effect devices have been built with YBaCuO/PrBaCuO layers and multilayers; The layers are deposited by an inverted cylindrical magnetron sputtering process (ICM) on MgO substrates. This process has been proved to give very good performances even for thinner and ultra thin films. The films used for the field effect transistor devices consist of one or two bilayers of YBaCuO and PrBaCuO with a nominal thickness of each material of about 4 nm. Small channel geometries of 50/spl times/60 microns are etched by argon ion milling and the channel is covered by a silicon dioxide dielectric layer (50-100 nm). The gate contacts as well as the drain and source contacts are made by sputtered gold pads. These devices are tested in static and dynamic measurements: the silicon dioxide layer inhibits symmetric I-V characteristics of the dielectric layer with high breakdown field strength up to 4 MV/cm. The gate capacity of a few pF allows the test of the device in a kHz frequency range. Tests with sine wave gate voltages show low distortion of the output signal with a voltage gain of about 0.01. Smaller dielectric layers could raise the gain close to one. The results of these field effect experiments are compared with optically induced effects. The optical measurements are performed with a helium-neon laser (633 nm) with a power density of 600 W/cm/sup 2/. The laser pulses with frequencies up to 1 MHz give responses somewhat similar to the observed field effect responses. Combining field effect and optical irradiation, it Is found that the laser pulses seem to create charge carriers, screening the field effect. Based on this principle a new photodetector is proposed. >

In this paper we report the in-plane fabrication and the characterization of artificially engineered biepitaxial grain boundaries (GB) obtained by partly interposing a MgO seed layer between a bare or even-buffered substrate and a CeO2 thin layer. The main peculiarity of this technique can be summarized by the capability to locate and engineer a single 45°-tilted GB at any stage of the device preparation. The junctions, realized by patterning the grain boundary occurring in the overhanging YBCO film, show Josephson current modulation in a large temperature range with anI c R n value of about 200 μV at 4.2K, theR n value being constant over the whole superconducting region. Under microwave irradiation, theI–V characteristics display several Shapiro steps while, according to the Resistively Shunted Junction (RSJ) behaviour, the step heights have the typical current biased junction dependence.

At high flux-flow velocities in type-II superconductors the nonequilibrium distribution of the quasiparticles leads to an electronic instability and an abrupt switching into a state with higher electric resistivity, as predicted by Larkin and Ovchinnikov (LO). We report the first observation of this effect in a high-temperature superconductor, namely in epitaxial $c$-axis oriented films of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. Using the LO theory, we have extracted from our results the inelastic quasiparticle scattering rate ${\ensuremath{\tau}}_{\mathrm{in}}$, which strongly decreases with decreasing temperature below ${T}_{c}$.

The critical current density (Jc) for zero magnetic field has been measured as a function of temperature in c-oriented thin layers of YBa2CU3O7-x prepared by different synthesis methods. One direct and two indirect Jc(T) methods were employed for each film: transport measurements on patterned film constrictions, DC remanent magnetization studies using the Bean model and non-linear AC screening analysis. Results from the indirect methods were calibrated against transport data. Possible origins of observed dependences are suggested and calibration procedures are discussed. The investigation demonstrated the feasibility of using AC screening analysis in rapid determinations of Jc(T) close to Tc.

A superconducting 3-terminal device based on NbN ultra-thin nano-strips has been realized. The device is capable of pulse discrimination and shaping and provides power gain with standard 50 ohm input and output impedance. It operates at 4.2 K and is well suited for the realization of output interfaces for superconducting digital circuits and as readout of cryogenic detectors. Successful realization of the integrated readout of a nano-strip single photon detectors is reported.

A superconducting pulse discriminator based on a cascade switch to the normal state of parallel ultrathin NbN nanostrips has been fabricated and carefully investigated. Correct operation was achieved using 1 ns input pulses with amplitudes down to 15 µA. The discriminator had a peak current gain of 12 and an FWHM timing jitter of 80 ps, limited by our measurement instrument resolution. These characteristics, together with simple on-chip integration, small area and low dissipation, make this device suitable for applications such as readout of fast cryogenic detectors and the output stage of superconducting digital circuits.

YBa 2 Cu 3 O 7−x thin films have been grown on (100) MgO substrates by the Pulsed Laser Deposition (PLD) technique. Films properties of relevance for microwave applications were studied for different substrate preparation and deposition parameters. It is reported on the influence of composition deviations, substrate surface preparation and deposition parameters on Tc (R=0), Jc (77 K), the crystalline texture and the surface morphology of the films. An improvement of the PLD technique aimed at the optimizing of HTS thin film growth for electronics and microwave systems is proposed.

Transmission spectroscopy in the very far IR (10 to 50 cm−1) is shown to be efficient to get dc conductivity, plasma frequency, and collision frequency at different temperatures in the normal state (e.g. 210 and 125 K), and the superconducting state (e.g. 79 to 65 and 8 K), for an YBa2Cu3O7—δ (YBaCuO) thin film. For a given sample a plasma frequency vp = 6500 ± 1000 cm−1 independent of temperature, a collision frequency depending linearly on temperature, vc = 0.69T (in cm−1) and a negligible transmission τ at v = 10 cm−1 for T = 8 K (τ < 2 × 10−4) are found, showing that the amount of YBa2Cu3O6 insulating impurity is very low. A two-fluid model is used to explain the spectra at T = 65 and 80 K, with a concentration Ns of superconductive electrons given by a T4 law, Ns = No[1 — (T/Tc)4], which seems to be acceptable as a first approximation where the collision frequency for normal electrons is assumed to be saturated around 40 cm−1 at T < 65 K, to fit the data. Such a collision frequency is corresponding to a mean free path l = 212 A if we take a Fermi velocity vF = 200 km s−1 and l = vF/2πcvc. La spectroscopie des supraconducteurs par transmission dans L' IR lointain (10 a 50 cm−1) permet d'obtenir la frequence de plasma et la frequence de collision avec une precision meilleure que ne donnet les mesures par reflection, si L'on peut disposer d'un film supraconducteur tres mince (p. ex. 30 nm). Pour un echantillon donne, on a trouve une frequence de plasma vp = 6500 ± 1000 cm−1 independante de la temperature, une frequence de collision dependant lineairement de T : vc = 0.69T (en cm−1) et une transmission a 8 K negligeable pour v = 10 cm−1, montrant que le film est relativement exempt d'impuretes isolantes. Un modele a deux fluides permet d'interpreter les spectres a 65 et 80 K en adoptant une loi en T4 pour la concentration en quasiparticules, en admettant que leur frequence de collision se sature a 40 cm−1 pour T < 65 K.

The best rf bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H≈200 mT close to the thermodynamic critical field H_{c}. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of H_{c}>H_{c}^{Nb} for magnetic shielding of bulk niobium to increase the breakdown magnetic field of superconducting rf cavities. Depositing good quality layers inside a whole cavity is rather difficult, so as a first step, characterization of single layer coating and multilayers was conducted on high quality sputtered samples by applying the technique used for the preparation of superconducting electronics circuits. The samples were characterized by x-ray reflectivity, dc resistivity (PPMS), and dc magnetization (SQUID) measurements. Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating exhibit different behaviors and clearly show an enhancement of the magnetic penetration field. Because SQUID measurements are influenced by edge and shape effects, we propose to develop a specific local magnetic measurement of H_{C1} based on ac third harmonic analysis in order to reveal the true screening effect of multilayers.

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.

YBaCuO (YBCO) based multilayers have been deposited independently by three techniques: laser ablation, inverted cylindrical target sputtering and on-axis planar d.c. magnetron sputtering. The last technique is used to cover uniformly R-plane sapphire and LaAlO3 2 in wafers with YBCO or multilayers to produce optoelectronic devices such as IR detectors. Very thin (about 3 nm) yttria-stabilized zirconia and MgO dielectric films have been studied as tunnel barriers for making such high T c tunnel junctions.

Primarily motivated by the similarities between the underdoped superconducting cuprates and the granular systems in regards of electric conductivity, phase fluctuations of the order parameter, and nuclear spin-lattice relaxation, a study has been carried out in a NbN(111) textured film at controlled granularity by means of superconducting quantum interference device magnetization and $^{93}\text{N}\text{b}$ NMR measurements. The Meissner diamagnetism in zero-field-cooling and field-cooling conditions and for different orientation of the magnetic field and the isothermal magnetization curves around the superconducting transition temperature ${T}_{\text{c}}$, are studied. $^{93}\text{N}\text{b}$ spectra and relaxation measurements have been performed for two values of the external magnetic field in parallel and perpendicular geometry, in the temperature range 4\char21{}300 K. In the superconducting phase the experimental findings for the textured film are similar to the one in bulk NbN. The nuclear spin-lattice relaxation process is the same as in bulk NbN in the temperature range 50\char21{}300 K, confirming a dominant contribution to the density of states at the Fermi energy arising from the Nb $4d$ band. At variance, on cooling from about 40 K down to ${T}_{\text{c}}$ $(H)$, the $^{93}\text{N}\text{b}$ relaxation rate in the film dramatically departs from the expected behavior for the Fermi gas and mimics the opening of a spin gap. The interpretation of the spin-gap opening in terms of depletion in the density of states at the Fermi energy can justify the anomalous temperature behavior of the $^{93}\text{N}\text{b}$ relaxation rate on approaching ${T}_{\text{c}}$ $(H)$ from above. The experimental findings suggest the occurrence of superconducting fluctuations (density-of-states term) in one-dimensional regime, coupled to a reduction in the time of flight of the electrons, both effects being related to the granularity. The data also suggest that the spin-gap phase in underdoped cuprates could be connected more to granularity, rather than to exotic mechanisms of magnetic origin.

After several years, the Far IR (FIR) results from several laboratories seem to converge. The very FIR transmission data at different temperatures in the normal phase, combined with dc resistivity measurements, lead to a constant plasma frequency in that phase, and a linear variation of collision frequency vc versus temperature T. Approximately the same plasma frequency can be used to explain the FIR transmission spectra at 7 K from 20 to 200 cm−1. These remarks obtained from purely FIR data are in good accordance with the recent conclusions of Fiory et al obtained by a quite different method, i.e. electrostatic measurements.

In situ direct deposition at approximately 700 degrees C of thin YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconductive films and multilayers has been done by three techniques using stoichiometric YBa/sub 2/Cu/sub 3/O/sub 7-x/ sintered targets. Excimer laser ablation in a DC magnetron system with hollow and planar targets leads to 0.5-, 1.2-, and 2.5-in diameter uniformly superconductive layers under static conditions. High critical current densities (>10/sup 6/ A cm/sup 2/ at 77 K) associated with low resistivity and good epitaxial behavior are achieved on top of MgO, SrTiO/sub 3/, LaAlO/sub 3/, and YSZ single-crystal wafers. High-quality c-oriented films (T/sub c/>80 K) are routinely obtained by means of a DC magnetron on large sapphire (>2 in) substrates covered by a YSZ RF sputtered buffer layer. The infrared properties of such films have been checked at 1.15- mu m wavelength. In order to achieve active devices (such as optical detectors and high-frequency mixers), small (

The roadmap gives an overview on status and future developments in Superconducting Digital Electronics (SDE). Key areas in SDE under focus are applications, circuit simulation and design, circuit fabrication, interfacing and testing, cooling and system aspects, and new devices and materials. Care was taken to establish the vital link between research and development on the one hand and the industrial view on the other hand. The present roadmap is based on extensive input from the roadmap working group on SDE established by SCENET – the European Network for Superconductivity, intensified by the activities of the FLUXONICS Network – the European Foundry for Superconducting Electronics. It is the result of many years of discussion in the group and of consultations with experts in the field, on the way to bring together industrial expectations and visionary extrapolation and current status of technology.

Abstracb This paper presents the fabrication of Ta Superconducting Tunnel Junction detector working at 0.2 K to be used for photon counting instruments in astronomical applications. We would like to operate this type of detectors up to 2.5 pm with a moderate energy resolution in order to offer innovative instrumental perspectives to the astronomical community. The Ta junction fabrication and characterization as well as photon counting experiments in the near-infrared are presented. Fabrication process improvements are discussed at the end of this paper.

Thin layers of YBa2Cu3O7−x (YBCO) deposited on LaAlO3 substrates have been prepared in situ by coevaporation of Y, Ba, and Cu. Incorporation of oxygen was accomplished by an atomic oxygen beam source with high cracking efficiency. The total oxygen flow at the substrate could be kept low enough to permit the use of a quadrupole mass spectrometer for evaporation rate monitoring. Films were strongly c‐axis oriented with rocking curve full width at half‐maximum values of 0.6°. Transport measurements on patterned films yielded critical current densities of 6×106 A/cm2 at 77 K. The deposition method was demonstrated to be feasible for preparation of Y2O3/YBCO heterostructures.

We present a fabrication method of superconducting quantum interference devices (SQUIDs) based on direct write lithography with an Atomic Force Microscope (AFM). This technique involves maskless local anodization of Nb or NbN ultrathin films using the voltage biased tip of the AFM. The SQUIDs are of weak-link type, for which two geometries have been tested: Dayem and variable thickness nanobridges. The magnetic field dependence of the maximum supercurrent Ic(flux) in resulting SQUIDs is thoroughly measured for different weak link geometries and for both tested materials. It is found that the modulation shape and depth of Ic(flux) curves are greatly dependent on the weak link size. We analyze the results taking into account the kinetic inductance of nanobridges and using the Likharev-Yakobson model. Finally we show that the present resolution reached by this technique (20nm) enables us to fabricate Nb weak-links which behavior approaches those of ideal Josephson junctions., Comment: 22 pages, 8 figures

A frequency dependent inductance estimation has been carried out for Rapid-Single-Flux Quantum (RSFQ) applications in NbN multilayer technology up to frequencies above the NbN gap frequency. For this purpose an extension of the fullwave modified transverse resonance method (MTRM) has been used, in which each superconducting layer is characterized by its complex conductivity according to the BCS derived model of Mattis and Bardeen. Our estimation shows significant influence of the gap frequency on the NbN multilayer structure inductance at high frequencies.

YBaCuO-PrBaCuO-YBaCuO a-axis oriented trilayer structures was realized by in-situ ICM (Inverted Cylindrical Magnetron) sputtering method, including a template step. Cross type junctions of micrometer size were patterned with a trilayer SNOP process. Their Josephson and tunneling characteristics have been investigated for various thicknesses of the PrBaCuO barrier (10- 100 nm). The critical current was found proportional to exp(-(alpha) T1/2) (SNS type in the dirty limit) where (alpha) was a constant. Strong and reproducible non-linearities were observed on the I-V curves especially above 10 mV and analyzed in terms of tunneling process. Tunnel barrier and interface models are proposed to explain the low critical current-normal resistance products (0.4 mV at 4.2(Kappa) ) as well as the I-V curves.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Several electric field effect devices have been built with YBaCuO/PrBaCuO layers and multilayers. The layers are deposited by an inverted cylindrical magnetron sputtering process (ICM) on MgO substrates. This process has been proven to give very good performances, even for thinner and ultra thin films. The films used for the field effect transistor devices consist of one or two bilayers of YBaCuO and PrBaCuO with a nominal thickness of each material of about 4 nm. Small channel geometries of 50 microns x 60 microns are etched by argon ion milling and the channel is covered by a silicon dioxide dielectric layer (50-100 nm). The gate contacts, as well as the drain and source contacts, are made by sputtered gold pads. These devices are tested in static and dynamic measurements: the silicon dioxide layer inhibits symmetric I-V characteristics of the dielectric layer with high breakdown field strength up to 4 MV/cm. The gate capacity of a few pF allows the test of the device in a kHz frequency range. Tests with sine wave gate voltages show low distortion of the output signal with a voltage gain of about 0.01. Smaller dielectric layers could raise the gain close to one. The results of these field effect experiences are compared with optically induced effects. The optical measurements are performed with a helium-neon laser (633 nm) with a power density of 600 W/cm2. The laser pulses with frequencies up to 1 MHz gives responses somewhat similar to the observed field effect responses. Combining field effect and optical irradiation, it is found that the laser pulses seem to create charge carriers screening the field effect. Based on this principle a new photodetector will be proposed.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report bolometric photoresponse of YBa2Cu3O7 thin film and YBa2Cu3O7ILa2CuO4 multilayers (YBCOILCO)at temperature of liquid nitrogen irradiated by a semiconductor laser diode at 0.83 microns. We develop a heat transfer model which takes into account the lateral diffusion of heat in the substrate. Using this model we explain the time dependence of the photoresponse. From the analysis of the photoresponse voltage versus time we calculate the opticalabsorption of the YBa2Cu3O7 ILa2CuO4 multilayers and of the YBaCuO monolayer.2. INTRODUCTIONExperiments of bolometric photoresponse of superconducting YBa2Cu3O7 thin films irradiated by visible or infraredlight have been reported15. Until now, no such photoresponse have been obtained on S/N/S/N. . . multilayers where S is asuperconducting film and N a normal metal or an insulator. In such multilayers, we can expect that the penetration depth of the light will be larger than in the superconducting monolayers. We report on comparative photoresponse of

Superconducting films consisting of alternating layers of La 2− x Sr x CuO 4 (22 nm thick) and YBa 2 Cu 3 O 7−δ (26 nm thick) were deposited on (100) SrTiO 3 substrates by excimer laser ablation. The films, five bilayers thick, exhibited good superconducting properties. X-ray diffraction and transmission electron microscopy investigations showed that they were heteroepitaxially grown with the c -axis of both materials normal to the substrate surface. Particular attention was paid to the microstructure of the layers as well as to the different interfaces. Cross-sectional TEM investigations showed that all the interfaces were quite sharp but not equally rough. The YBCO over LSCO interface was wavy while the LSCO over YBCO interface was nearly atomically flat. The interpretation of these different shapes and their influence on the next deposited layer will be reported, as well as the presence of different types of structural defects in the layers.

Superconductive multilayered thin films have been deposited by excimer laser ablation and d.c. magnetron sputtering. The laser deposited films were grown on (100) SrTiO 3 substrates and consisted of alternating layers of La 2− x Sr x CuO 4 (LSCO) and YBa 2 Cu 3 O 7−δ (YBCO) while the sputtered films, deposited on (100) MgO substrates, consist of a 3 nm MgO layer sandwiched in 100 nm YBCO layers. Cross-sectional transmission electron microscopy (TEM) specimens have been prepared so as to investigate the different interfaces and the local structure of the films. The results show that the laser deposited films are epitaxially grown with the c- axis normal to the surface. The interfaces are rather sharp but some roughness of the YBCO over LSCO interface, possibly related to the presence of shear defects in the LSCO layers, is evidenced. The observations of the sputtered films reveal the presence of some a- axis oriented YBCO grains and the partial crystallization of the amorphous deposited MgO layer. Some a- axis oriented grains are also observed in the laser deposited films but only in the upper layers. The different types of structural defects as well as the different interfaces, characterized by high resolution electron microscopy (HREM), will be reported.

The discovery of high Tc superconductors has revealed promising applications in optoelectronics. However a basic requirement for the development of successful novel optronic devices is the ability to fabricate high quality Josephson, tunnel or three terminal devices in a reproducible, controllable and reliable way.Large area deposition of YBCO multilayers, whole wafer processing techniques are essential and in great progress. Nevertheless, the degradation of the YBa2Cu3O7-x surface layer close to the junction contacts is observed and induces severe limitations in the devices performances. Experimental evidence of such problems and possible solutions will be presented. 1. INTEREST OF HTC JUNCTIONS FOR PHOTON DETECTION Optoelectronic applications using high-Tc superconducting oxides have recentlyattracted much attention [ 1-5J . For instance the feasibility of sensitive high-bandwidth infrared detectors with a wide optical band should provide important applications in the field of space, communications and military industries.In the past highly sensitive detectors have been made in Grain-Boundary Josephsonjunctions (GBJ) formed in BaPbl-xBixO3 (BPBO) 16] and more recently in Bal-xKxBiO3