Table of contents

The paper reviews the main aspects of nonequilibrium hot-electron phenomena in superconductors and various theoretical models developed to describe the hot-electron effect. We discuss implementation of the hot-electron avalanche mechanism in superconducting radiation sensors and present the most successful practical devices, such as terahertz mixers and direct intensity detectors, for far-infrared radiation. Our presentation also includes the novel approach to hot-electron quantum detection implemented in superconducting x-ray to optical photon counters.

Thermal noise strongly influences the operation of RSFQ (rapid single flux quantum) logic circuits made of high-temperature superconductors (HTS). In the past, the circuit design was based on fabrication yield optimization. A new theoretical study using a method of general determination of the digital bit-error rate (BER) gives hope to develop such devices with a large immunity against noise. With regard to this study the design parameters of a circuit optimized with respect to fabrication yield are far from its minimum bit-error rate. Only for temperatures close to 4 K the parameters determined for fabrication yield match the parameters obtained with BER optimization. For this reason, a new reliable technology for fabrication of HTS circuits is required. We have developed a new fabrication process to serve as a basis for a proof of this new design approach. We have calculated the bit-error rate of a newly designed RSFQ chip with realistic values derived from a test chip which has been fabricated with this new multilayer technology. The new technology contains three superconductor thin films. An inductance smaller than 0.5 pH per square has been reached by using a ground plane.

We have developed and investigated a thin-film Hall sensor coupled with a high-T_C superconducting antenna structure. In this paper we present a theoretical model from which we can derivate the criteria for the layout and the technological realization of the sensor system. The model describes the improvement in the sensitivity of the magnetometer system by a superposition of the external field and an inductive incoupling of a field generated from the superconducting antenna. Based on the theoretical results it is possible to realize a hybrid magnetometer with a sensitivity of about 2.3 V T⁻¹ and with a field resolution of about 1 nT Hz^−1/2 at a temperature of 77 K.

We are developing a digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) logic circuits. Since the RSFQ D/A converter operates according to the Josephson frequency–voltage relationship, arbitrary waveforms can be synthesized with an accuracy equivalent to that of the Josephson dc voltage standard. We have examined the characteristics of magnetically coupled voltage multipliers (VMs). The VM is a key component in the RSFQ D/A converter for use in metrological applications.

Y-Ba-Cu-O samples with additions of Y₂O₃ and CeO₂ were quenched during seeded isothermal melt processing and examined by optical microscopy and scanning electron microscopy. Large YBa₂Cu₃O_7−y (Y123) particles in the starting powder were found to form a distinct type of melt during heating, which was unaffected by the Y₂O₃ or CeO₂ additives. This type of melt later formed regions with a low concentration of Y₂BaCuO₅ (Y211) particles in the Y123 matrix. The maximum growth rate of Y123 that could be sustained in the sample was found to be lower in the melt formed from large Y123 particles, and this may lead to growth accidents and subgrains in some samples.

We calculate the tunnelling conductance spectra of a normal-metal/insulator/triplet superconductor using the Blonder–Tinkham–Klapwijk (BTK) formulation. Possible states for the superconductor are considered with horizontal lines of nodes, breaking the time-reversal symmetry. These results would be useful to discriminate between pairing states in the superconductor Sr₂RuO₄ and also in UPt₃.

Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB₂. The materials included a hot-pressed sintered MgB₂ pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB₂ filament and a pulsed-laser-deposited thin film grown on a single crystalline [111] oriented SrTiO₃ substrate. Thick plate-shaped grains with an aspect ratio of ∼3 and large faces parallel to (0001) planes dominated the microstructure in all four types of sample. The intermediate-sized plate-shaped grains (0.1 μm × 0.3 μm on average) in the electromagnetically most homogeneous parts of the hot-pressed pellets were strongly facetted, but not textured. Large (3–5 μm) plate-shaped grains were seen in the pellet reacted directly from stoichiometric Mg and B. A tendency for parallel alignment of the [0001] axes of the considerably larger grains (∼0.25 μm × 1 μm) in the filament was observed near its W core, but degradation of this texture away from the core was apparent. The very small grains (∼10 nm) of the thin film possessed a well-defined fibre texture with [0001] parallel to the film normal and no preferred orientation in the plane of the film. Electrical resistivity of the finest grain samples was some 10³ times higher than the largest grain sample and their critical current density about one order of magnitude higher. We conclude that, in contrast to the cuprate-based high-T_c superconductors, grain boundaries do not limit the critical current density of polycrystalline MgB₂ and indeed act as flux-pinning centres, which enhance the critical current density.

The newly developed transformation-heat-based up-quenching (TRUQ) process causes an improvement of the stoichiometry of the A15 Nb₃Al phase and an enhancement of the high-field J_c performance in Nb₃Al superconductors. J_c properties have been optimized in the fabrication steps for TRUQ Nb₃Al superconductors. The TRUQ process in the vicinity of 1000 °C gives the best high-field and low-field J_c's as well as T_c and B_c2. J_c's are not so sensitive to the holding time of the TRUQ process, which makes rapid cooling unnecessary. The TRUQ process can be well applied to conventional Cu-clad stabilized conductors. The J_c of 550 A mm⁻² for the superconducting area is obtained at 23 T and 2 K; this is the highest measured value for A15-type superconductors. Thus, TRUQ Nb₃Al conductors are very promising for high-field applications, e.g. GHz-class NMR magnets. Temperature profiles of the samples have been measured during the TRUQ process and it has been shown that TRUQ can be induced, even if the heating rate is less than 20 °C s⁻¹. The TRUQ process may be suitable for practical coil fabrications.

We have studied the structure of the superconducting gap in MgB₂ thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a T_c of 38.6 ± 0.3 K and resistivity of about 20 μΩ cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB₂, with two distinct values of the gap, Δ₁ = 2.3 ± 0.3 meV and Δ₂ = 6.2 ± 0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder–Tinkham–Klapwijk model reveals that both gaps close simultaneously at the T_c of the film. Possible mechanisms that can explain the intrinsic coexistence of two values of the gap are discussed.

The floating zone method has been optimized to process the (light rare earth) Ba₂Cu₃O_y (LRE123) compound. The effects of processing rate, thermal gradient and maximal temperature have been investigated. Nd₁Ba₂Cu₃O_y, Sm₁Ba₂Cu₃O_y, Gd₁Ba₂Cu₃O_y and (Nd_0.33Eu_0.33Gd_0.33)₁Ba₂Cu₃O_y have been processed under air and exhibited an optimal critical temperature greater than 93 K with a sharp transition. Critical current densities under self-field of 55000 A cm⁻² have been reached and a fishtail effect appears under 2 T with different mixing of light rare earth. Large single domains have been obtained with the (a, b) plane oriented at 90° of the bar axis. Moreover, the composition homogeneity has been tested along the Nd123 bar. Thus, the floating zone method is an interesting process to investigate under air new compositions of LRE123.

We optimized the aspect ratio of a sapphire puck for a Hakki–Coleman-type high-temperature superconductive (HTS) thin film shielded sapphire dielectric resonator (SDR) to get the maximum Q-value, Q₀, of the SDR at the frequencies 10, 15 and 20 GHz. For the simulation, we used the exact expressions for the geometrical factors of various parts of the SDR obtained by the Q-value definition. The SDR analysis is made by using HTS thin films deposited on 1 inch diameter and 10 mm × 10 mm substrates. The analysis suggests that the optimum aspect ratio of the sapphire puck depends on the surface resistance of the HTS thin film to get the maximum Q-value of the SDR. This analysis can be very useful for the design of a HTS shielded sapphire SDR of desired size within the available space for its application.

We analysed x-ray powder diffraction data of spray pyrolytic precursor powders for the oxide-powder-in-tube (OPIT) technology by means of the Rietveld method. For calcinations in air we find a typical phase composition of 2212, 14-24, 11, CP and 3321 at 800 °C (13 h). At 820 °C 2212 is in equilibrium with 14-24 and CP after 48 h calcination. In 8% O₂ at 800 °C 2212 exists together with 14-24, 21, CP and CuO, in 99.999% N₂ at 760 °C the stable phase assemblage consists of 2212, 21 and CuO. The highest amount of 2212 can be found in the powder calcined at 8% O₂ (72.4 wt%) compared with 68.9 wt% for the powder calcined in 21% O₂ and 69.3 wt% for the powder calcined in flowing N₂. A first trace of 2223 can be observed in powders calcined in air at temperatures of 838 °C and 842 °C. The crystallographic features of the 2212 phase correlate with the calcination atmosphere: under reducing atmospheres the satellite reflections of the x-ray powder diffractogram disappear and the orthorhombic lattice distortion z increases due to Pb²⁺-incorporation into 2212. With increasing O-content the c lattice parameter becomes significantly shorter in samples calcined in oxidizing atmospheres. It is possible to infer a cationic ratio of Sr:(Sr + Ca) ≈ 0.12 for the 11 phase and Ca:(Ca + Sr + Bi) ≈ 0.57 for 14-24 in precursors from lattice parameter data.

High-resolution (sub-microsecond to nanosecond) time-resolved measurements of the dynamics of superconducting electronic devices, such as Josephson tunnel junctions and SQUIDs, are indispensable for fundamental physics research, such as quantum mechanics of macroscopic variables. We describe the development of a time-resolved measurement technique that enables direct measurements, in the time domain, of the temporal evolution of Josephson junctions with a nanosecond resolution. The technique was applied to study escape dynamics of Josephson junctions, as macroscopic quantum systems, in the quantum tunnelling regime. The measured probability of junctions remaining in the initial metastable state—the survival probability—as a function of time decayed exponentially, agrees very well with the theoretical prediction of the incoherent tunnelling process.

The resonance curve shape of a thin film parallel plate HTS resonator was observed as a function of delay time from 0.3 to 12 s at each frequency step across the resonance at various microwave surface magnetic field levels up to 28 mT. Skewed resonance curves and a pronounced shift of resonance frequency were encountered as the microwave input power was increased. A simple phenomenological model was set up in order to explore whether the skewed shape and frequency shift observed could originate from a global temperature rise. The simulation produces resonance curve shapes that agree well with the experimental observation whether the frequency is swept up or down. However, the model fails to produce a sufficiently large resonance frequency shift unless input powers two orders of magnitude larger than in experiment or unrealistically small values for the thermal conductivity are used. The delay time experiments in combination with the simulation suggests that the films fail due to a complex combination of mechanisms.

A polymerized organic–inorganic synthesis of a mixture containing 69% Y-123, 30% Y-211, 1% CeO₂ in weight has been studied with the aid of polyvinylic alcohol (PVA). The advantages of this method are (a) avoidance of extensive and repetitive firing steps and (b) no need for mechanical mixing of components. By heating the solution of stoichiometric quantities of the nitrate salts, a viscous gel is obtained which turns into an aerogel by subsequent heating. The resulting aerogel was calcined in a single step and the powder was used to prepare Y-123/Y-211 composite monodomain rods by the vertical Bridgman solidification technique. Characterization of both powders and grown monodomains have been performed. Fine and dispersed Y-211 phase distribution was successfully achieved. Finally, magnetic measurements were performed by SQUID magnetometry.

AC losses are of great importance when designing devices for electric power applications based on high-temperature superconducting (HTS) tapes. In order to obtain an optimized design one needs to be able to calculate the AC losses that appear in an actual application. One important quantity is the AC losses that appear at the ends of a coil where the magnetic field contains a component perpendicular to the face of the tape. In this study we calculate the AC losses due to the combined action of a perpendicular magnetic field and a transport current. The results are compared to calorimetrically measured data of the AC losses in a BSCCO/Ag HTS tape.

We have developed a novel and simple method for the formation of the Y₂Ba₄Cu₇O_15−x (Y-247) superconductor at atmospheric oxygen pressure, using cuprous cyanide as the starting copper compound. Cuprous cyanide was found to act as a reaction accelerator and getter of oxygen during the formation of Y-247. Y-247 is prepared by solid-state reaction of Y₂O₃, BaCO₃ and Cu₂(CN)₂ at the thermal stability temperature of Y-247 (860 °C) for 12 h. However, we found that the formation of the Y-247 phase could be accomplished in just a few minutes due to the high reactivity of cuprous cyanide. We carry out a longer time reaction to reduce the amount of simultaneous growth of a small amount of impurity phases. A predominantly single-phase Y-247 compound (confirmed by x-ray diffraction) with T_c(R = 0) = 93 K and T_c(onset) = 95 K could be achieved very easily by this method. It is worth emphasizing that, even in the quenched Y-247 from 860 °C, T_c(R = 0) = 93 K is confirmed by resistivity and magnetic measurements. The observations have shown that oxygen-deficient YBa₂Cu₃O_6+x blocks in the Y-247 unit cell had enough oxygen required for 95 K superconductivity even at 860 °C. However, the amount of superconductivity was found to increase with post-annealing in an oxygen atmosphere at 350 °C.

The use of high-temperature superconducting films for the fabrication of antennas allows us to realize radiating structures that are small in size and have a very high efficiency. In this paper, we investigate the properties of a novel compact antenna obtained by crossing a square patch with diagonal and transverse slots. Single patch antennas, with both linear and circular polarization operating in the X-band, with a size reduction of about 40% as compared to conventional square patches, have been designed and tested at prototype level. They have been realized by using double-sided YBa₂Cu₃O_7−x(YBCO) and Tl₂Ba₂Ca₁Cu₂O₈(Tl-2212) superconducting films grown on MgO substrates and have been measured using a portable cryocooler. At T = 77 K, the experimental scattering parameter, S11, agrees well with simulations, showing a power handling of 23 dBm for YBCO devices. The miniaturized patch antenna investigated has allowed us to realize a 4 × 4 array of linear polarization antennas at 11.5 GHz, using a YBCO film grown on a MgO substrate with a 2'' diameter, by considering a reduced interspace (more than 20%) between the elements. The far-field pattern and the 16-array gain are in good agreement with the simulated results.

We have measured the temperature dependence of top and bottom voltage signals of a Y_0.8Er_0.2Ba₂Cu₃O_7−δ crystal in magnetic field up to 7.5 T parallel to the c-axis using the pseudo-flux-transformer method. The results show that in a dissipative regime the voltage signal on the side of the applied current contacts is more than that on the opposite side; meanwhile the bottom voltage signal shows a field-dependent peak effect near the critical temperature T_c. We calculated the true resistivities ρ_ab and ρ_c of the crystal within an anisotropic model. The difference between the calculated ρ_ab and ρ_c and the experimental values ρ_ab^st and ρ_c^st is helpful in understanding flux dynamic behaviour. Based on the Yeshurun–Malozemoff model, we have investigated temperature and field dependences of effective activation energy along the ab-plane and c-axis obtained from the ρ_ab(T) and ρ_c(T), respectively. The temperature dependence of effective activation energy along the c-axis displays a good thermally activated flux flow (TAFF) behaviour. In the TAFF regime the field dependence of effective activation energy follows a power behaviour, U(H) = 4.24 × 10⁴ H^−1.1. The TAFF regime and the coupling between the layers have been discussed.

The fundamental and third harmonic ac susceptibility measurements as a function of temperature (20–120 K), frequency (5 Hz–1 kHz) and ac magnetic field amplitude (1–1600 A m⁻¹) on a bulk Bi_1.84Pb_0.34Sr_1.91Ca_2.03Cu_3.06O₁₀ high-T_c superconductor prepared by the ammonium nitrate fusion technique are reported. The height and temperature of the peak in the out-of-phase component of the fundamental susceptibility increase with increasing field amplitude and the measuring frequency employed. In the presence of a superimposed H_dc = 1520 A m⁻¹, an additional small peak appears and the larger peak broadens even at lower ac fields of 20 A m⁻¹. The third harmonic susceptibility exhibits an interesting temperature dependence with increasing ac field amplitude. Experimental results are qualitatively discussed in the framework of the critical state model. The results are in good agreement with the theory at fields lower than the least field needed for full flux penetration, H_p, below which ac losses are essentially due to the hysteretic intergranular coupling. The critical state model fails to explain the data at high temperatures with ac fields larger than H_p, where the ac losses are interfered by the irreversible flux penetration into the grains. This is presumably due to the weakening of the intergrain coupling with magnetic field penetration.

Pulsed laser depositions of double-buffer and triple-buffer YBa₂Cu₃O_7−y (YBCO)/Y₂O₃(YSZ)/CeO₂ heterostructures have been performed in situ onto commercially available biaxially textured NiFe 50%/50% tape. The deposition in the forming gas (4% H₂/Ar) from a CeO₂ target and the deposition in vacuum from a CeO₂:Pd composite target have been explored as two possible routes for cube-on-cube growth of the first buffer layer. The influence of the critical processing parameters on the texture is investigated and some of the issues involved in the reduction of NiO (111) and the formation of cube-on-cube NiO (200) growth are discussed. X-ray diffraction has been used for texture evaluation of the substrate and subsequent deposited layers. The substrate–buffer interface region has been studied by focused ion beam cross section electron microscopy. Both the buffers and YBCO layers show biaxial alignment with ω and ϕ scans having optimum YBCO full width at half maximum (FWHM) values of 4.3° and 8.8°, respectively. The morphology has been characterized using atomic force microscopy and scanning electron microscopy. The value of T_c (onset) has been measured at 90 K (ΔT_c = 10 K). The critical current density, J_c, has been measured by transport measurements and magnetic measurements performed in a dc SQUID magnetometer.

We have studied the magnetization of a melt-processed Sm–Ba–Cu–O bulk superconductor by magnetic pulses at 77 K. The amount of trapped magnetic flux (Φ_T) as a function of the pulse amplitude exhibited a peak, which is attributable to the heating of the sample caused by the fast motion of flux lines in the presence of resistive forces. The peak value of Φ_T was about half the amount of magnetic flux that was trapped when the sample was field cooled to 77 K, where the field-cooling (FC) value corresponds to the full capability of trapping magnetic flux by the sample. We found that sandwiching the sample with ferromagnetic materials and iteratively applying magnetic pulses while progressively decreasing the amplitude increases the amount of trapped magnetic flux. By combining these two techniques, Φ_T increased to about 90% of that of FC magnetization.

Cu_1−xTl_xBa₂Ca₃Cu₄O_12−y (Cu_1−xTl_x-1234) thin films have been found to be very attractive candidates in the cuprate family due to their low superconductor anisotropy, long coherence length and, consequently, high J_c. The method of preparation has been reported previously, however the kinetics of their formation has not yet been studied. In this paper, we report on the growth kinetics of superconducting Cu_1−xTl_xBa₂Ca₃Cu₄O_12−y thin films. In the preparation, we use the amorphous phase epitaxy method, which is a thallium treatment of the amorphous phase at elevated temperatures. The amorphous phase was deposited on a SrTiO₃ substrate by rf-sputtering from a stoichiometric target with a composition of CuBa₂Ca₃Cu₄O_y. The thallium treatment of the amorphous phase was carried out in a gold capsule for 45 min. The mechanism of the growth kinetics has shown that the formation of Cu_1−xTl_xBa₂Ca₃Cu₄O_12−y thin films was accomplished from Cu_1−xTl_xBa₂Ca₁Cu₂O_8−y (Cu_1−xTl_x-1212) and Cu_1−xTl_xBa₂Ca₂Cu₃O_10−y (Cu_1−xTl_x-1223) by the successive introduction of CuO₂ planes in these phases. We also studied the effect of the time and temperature of the thallium treatment on the growth of Cu_1−xTl_x-1234 films. The best synthesis temperature for Cu_1−xTl_x-1234 films was found to be 910 °C, but this phase could also be isolated as a single phase at lower temperatures (∼890 °C). However, the low-temperature synthesis results in a higher thallium content in the final compound. From the x-ray diffraction measurements the c-axis length was found to increase with the increase of the thallium content. The pole figure measurements of the (103) reflection of the films have shown a-axis oriented crystals with Δϕ = 0.8°. The observed critical temperature (T_c) for Cu_1−xTl_x-1212, Cu_1−xTl_x-1223 and Cu_1−xTl_x-1234 are 78 K, 103 K and 110 K respectively. Current density measurements have shown a maximum J_c ∼ 2 × 10⁶ A cm⁻².

A less controversial method is used to analyse the imaginary part of the ac susceptibility measurement of MgB₂. The pinning potential barrier U_o(T, H) is obtained near the melting curve. It is found that U_o(T, H) depends linearly on both the temperature and the applied field.

Using the tape-in-rectangular tube (TIRT) process, we have made multi-core Bi-2223/Ag tapes with various numbers of filaments (10–162), and with different filament architectures and orientations. We have measured the angular dependence of the transport current of the tape samples with 'parallel' and 'perpendicular' filaments. The transversal I_c distribution obtained by spatially resolved transport measurements ('magnetic knife') illustrates that the filament quality of the TIRT tapes is better at the tape edges than in the centre. The tapes were stressed by two types of tensioning set-up (a short straight sample and a U-shaped spring) and by bending at 77 K. The I_c degradation shows different behaviour for parallel and perpendicular filaments, which is attributed to the difference in filament density and crack propagation.

This paper deals with a new kind of superconducting limiter. Usually, superconducting current limiters are based on the transition from superconducting state to the normal state by overtaking the critical current. This new superconducting current limiter has no current transport in the superconductor. Indeed, in our structure, the limitation is based on the saturation of a high temperature bulk superconductor by the magnetic flux. We show that our structure enables one to limit the current.

In order to realize superconductive microwave devices with large tuning characteristics, we have studied a mechanically tunable superconductive resonator beneath a dielectric and/or magnetic floating plate (FP) as a first step. The resonator consists of a half-wavelength coplanar waveguide (CPW). The principle of the mechanically tunable method is that variations of the effective permittivity and/or the effective permeability for the microwave waveguide line can be obtained by changing the distance between the resonator and the FP. In computer simulations of electromagnetic fields for the mechanically tunable half-wavelength CPW resonator, we have obtained a large shift of the resonant frequency with a band of 1.75 GHz to 7 GHz (tunability is approximately 25%) using the (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃ (LSAT) single crystal (dielectric constant (ε_r) = 22.8, loss tangent (tanδ) = 1.7 × 10⁻⁴) as the FP. Based on the results, we have performed an experiment using a half-wavelength CPW resonator made from YBa₂Cu₃O_7−δ thin film and the LSAT FP. The result shows a drastic resonant frequency shift of approximately 1.36 GHz (tunability is approximately 20%) when the LSAT FP is moved, using an electromagnetic actuator. In this experiment, it is also noted that the insertion loss is quite low, less than 0.8 dB, compared to previous reports for various tunable filters. Moreover, we have also measured the tuning characteristics with a TiO₂ single crystal (ε_r = 85.4, tanδ = 2.5 × 10⁻⁴) as the FP. As a result, we have obtained quite large tuning characteristics of approximately 2 GHz (tunability is approximately 28%).