How to optimize high-Tc superconductive cuprates

Different ways to optimizeT _c 's and the magnetic properties of high-T _c superconductive cuprates are described and discussed. Oxygen intercalation-desintercalation phenomena and cationic substitutions (M³⁺ for M²⁺ or vice versa) lead to a variation of the hole carrier density and can have a drastic influence onT _c 's, as shown for bismuth, thallium, and lead-based cuprates, as well as for La₂CuO₄ and 123-type superconductors. In the former, the role of hole reservoirs for the rock salt type layers is outlined. The modification of the hybridization of orbitals can also explain the variations ofT _c 's observed in some systems like Y_1–x Ca _x Ba₂Cu_3–x :Fe _x O₇ in which the oxidation state of copper remains constant. A critical current density enhancement can be realized by irradiating high-T _c materials by high-energy heavy ions because of the peculiar columnar structure of the heavy ion-induced tracks. The effects of such columnar defects, 70 Å in diameter, on flux pinning, magnetic relaxation, and location of the irreversibility line of bismuth-based 2212 crystals irradiated by 6-GeV Pb ions are reported. We observe a strong shift of the irreversibility line toward high fields and temperatures, indicating that pinning effects must be taken into account in the vortex lattice motion. Such induced changes are accompanied by a strong enhancement of the critical current density and a significant enlargement of the irreversibility region in theH,T plane.     

RF impedance of high-Tc cuprates

Surface impedance of a high-T_c superconductor (HTS) is described in terms of equivalent lumped-circuit elements, i.e. resistance and inductance, based on the concept of the complex conductivity of a hard type II superconductor containing weak links (WLs), modelled as a parallel combination of a pure superconducting branch and a branch containing WLs.     

Quantum oscillations in the high-Tc cuprates

We review recent progress in the study of quantum oscillations as a tool for uniquely probing low-energy electronic excitations in high-T(c) cuprate superconductors. Quantum oscillations in the underdoped cuprates reveal that a close correspondence with Landau Fermi-liquid behaviour persists in the accessed regions of the phase diagram, where small pockets are observed. Quantum oscillation results are viewed in the context of momentum-resolved probes such as photoemission, and evidence examined from complementary experiments for potential explanations for the transformation from a large Fermi surface into small sections. Indications from quantum oscillation measurements of a low-energy Fermi surface instability at low dopings under the superconducting dome at the metal-insulator transition are reviewed, and potential implications for enhanced superconducting temperatures are discussed.     

Fourfold symmetry in the ab-plane of the upper critical field for the high-Tc cuprates

We have found clear anisotropy in the ab plane with fourfold symmetry of the resistive superconducting transition under magnetic fields for single-crystal Pb₂Sr₂Y_0.62Ca_0,38Cu₃O₈, which is regarded as anisotropy of the upper critical field H_c2. This is not only qualitatively but also quantitatively similar to that formerly observed in La_1.86Sr_0.14CuO₄. The observed fourfold symmetry is explained as being mainly due to the anisotropy of the superconducting energy gap owing to pairing. H_c2, one of the bulk properties, supports pairing in the high-T_c superconductivity.     

Electron tunneling into superconductive lead-indium alloys subjected to hydrostatic pressure

Results are presented for electron tunneling studies of superconducting lead-indium alloys (Pb_0.88In_0.12 and Pb_0.64In_0.36) as a function of pressure. The pressure dependence of the energy gap ₀, the reduced energy gap 2₀/kT_c, and the electron-phonon coupling constant show that under pressure the alloys move toward weaker coupling. The numerical agreement with estimates from the strong coupling theory is excellent. The pressure dependence of the transition temperature was measured, and was found to be in good agreement with various theoretical estimates forT _c. Grüneisen constants were obtained for the transverse, longitudinal, and localized peak. The well-known McMillan relation for the average square phonon frequency,M² = const, was investigated. It was found that this relation does not hold as function of pressure in these alloys. An estimate was obtained for the pressure dependence of the band-structure density of states of pure lead. It is found to be about 6.52 times the value predicted for the free-electron case.Supported by grants from the National Research Council of Canada.     

Magnetoresistive amplifiers with superconductive elements

The construction and properties of certain superconductive amplifiers in which the resistance of a gate element is controlled by the electromagnetic field of a signal coil are described in this paper. Vacuum-deposited films of tin, which have narrow resistive transition widths, have been used as the gate elements, and the field has been applied perpendicularly to the film surface. The parameters of small-signal analysis are determined, and the influence of temperature, bias magnetic field, and zero-signal gate current upon power gains are shown. The limitations and merits of the device in its present form are discussed.     

Electron-lattice interaction in cuprates

Experimental studies of local structure by pulsed neutron PDF analysis on manganites and cuprates suggest that these oxides are locally distorted due to a strong electron-lattice interaction, and charge carriers organize themselves most likely into a tweed-like state. Measurements by inelastic neutron scattering as well as calculations on the Hubbard model show that the electron-lattice interaction is strongest at the zone boundaries, enhanced by phonon-induced spin-correlations. Such a strong electron-lattice interaction must be the origin of these local distortions, and may form the basis for the microscopic mechanism of superconductivity.     

Josephson-type superconductive tunnel junctions and applications

The physics and applications of the Josephson effect are reviewed in detail. A brief introduction to superconductivity and tunneling is provided. The configuration most fully discussed is the oxide junction. However, properties of bridges, point-contacts, and solder-blobs are also described. Among the applications reviewed are magnetometers, infrared detectors, microwave generators and mixers, voltmeters, computer logic and memory devices, relaxation oscillators, thermometers, and spectrometers.     

High-temperature superconductive filters and multiplexers

This paper presents measured and computer-simulated results for a number of high-temperature superconductor (HTS) thin filters. The CAD algorithm used to design these filters is described. Experimental results are also presented for an integrated 4-channel C-band multiplexer employing dual-mode hybrid dielectric/HTS thin-film filters. The results presented in this paper demonstrate the feasibility of building C-band superconductive multiplexers that have the potential of large reduction in mass and volume over conventional technologies.     

High-speed superconductive circuitry: An overview

Superconductive digital circuits have been demonstrated with speeds >2 GHz. Simulations show that some should be capable of much higher speed, including an analog-to-digital converter clocked at 20 GHz and a shift register with 60-GHz clocking. This article reviews the experimental status and the developments expected on the basis of simulations. The emphasis is on low-temperature superconductors.     

A method to optimize evacuation instructions

In this paper, an optimization method is presented for instructions to evacuate by car the population of a region threatened by a hazard. By giving the optimized instructions to the evacuees, traffic conditions and, therefore, the evacuation efficiency are optimized. The instructions, containing a departure time, a safe destination, and a route, are created using an optimization method consisting of two phases: the generation of the search space and the algorithm AES⁺ evacuation, a version of ant colony optimization. The main contributions of the proposed optimization method are the unique approach to generate the search space in which network degeneration is taken into consideration, the possibility to optimize instructions under the assumption of both full and partial compliance of the evacuees with the instructions, and the flexibility in the sense that the user of the method can define his or her own objective function and choose a suitable traffic simulation model. The paper contains a comprehensive case study. The case study shows that the effectiveness of the optimized instructions is more than doubled when compared with the effectiveness of instructions set up by straightforward rules (like evacuating to the nearest destination using the shortest route). Further, the case study shows that the number of arrivals under optimized, but possibly sub-optimal instructions is equal to at least 90% of the theoretical upper bound on this number of arrivals.     

Anisotropy of the superconductive to normal transition in tantalum-nitrogen single crystals

Systematic experiments on the transition to the normal state in superconducting tantalum single crystals doped with interstitially dissolved nitrogen impurities are presented. On the low- side of the magnetic phase diagram, where at some low temperature 0 <T ^* <T _ca transition from type I to type II superconductivity occurs, a new effect has been observed, as reported briefly elsewhere: because of the correlation ofH _{c 2} with crystal directions, at fixed temperatures nearT ^*, whether the sample is a type I or a type II superconductor depends on which crystal direction is oriented parallel to the external field. In the type II superconducting range all samples display anH _{c 2} anisotropy effect, which is commonly observed in cubic superconductors, even in the extreme dirty limit (~12). However, the second anisotropy coefficient (l = 6 component) is found to be negligibly small in all tantalum samples.     

Hotspot propagation in superconductive strip detectors

Hotspots formed in superconductive strips due to interaction with particles are considered. The superconductive strip detector is considered in terms of a model of one-dimensional superconductor. D.C. electric current flowing through a film leads to Joule heating of the normal phase and causes either expansion or collapse of the hotspot. It is shown that the energy of a particle can be obtained by measuring the length of the expanding hotspot.     

Transverse resistivity in multifilament superconductive composites

The electric field on the outermost shell of superconducting filaments depends directly on the current distribution inside a composite. Calculations are carried out taking into account the axial diffusion due to the resistivity of the matrix. Moreover, the critical state model is modified to account for the dependence of the local current density on the electric field. It is shown that a simple self-field measurement is a very good way to evaluate the average transverse resistivity in a multi-filament composite. Our samples constructed of very fine filaments show a resistivity in the range of a few 10^?9 ? m, that is, much higher resistivity than that of the copper matrix itself. This resistivity decreases as the filament diameter increases. This fact shows that there exists a high resistive barrier at the interface between the superconductor and the copper.     

Anomalously strong small-q electron-phonon coupling in cuprates due to dielectricity

We consider the role of the enormous ionic dielectric constant ε₀≈ 30–50 in the cuprates. The ionically-screened Thomas-Fermi screening parameter ~q_TF = [4θe²N(E_F)/εin0 ^1/2 is extremely small, namely 0.2-0.3 A^-1. The electron-phonon coupling constant I(q,w) is found to be anomalously large forq ≈ ~q_TF and smallco, accounting for the CDW observed in overdoped BSCCO with wavevector 0.24 A^-1. Using the Eliashberg theory we derive from this I(q,w) a maximumT _c of about 200 K. The small electron-phonon scattering angle △θ = ~q _{F ≈} 0.3 rad acounts for the observedd-wave pairing, as originating from a phonon-mediated mechananism.     

Absolute Raman efficiencies as a tool to investigate HTc cuprates

Absolute Raman efficiencies of phononic, electronic and spin excitations have been determined for doped and undoped HTc-cuprates. Anomalous temperature dependence of the Raman efficiency of the phononic 500cm^–1 mode is explained as a change of the apical oxygen position. The normal state efficiency of the. electronic background in YBa₂Cu₃O_6.9 has been obtained for various scattering geometries for Raman shifts up to 8000cm^–1. The 2-magnon contribution to the magnetic Raman signal of the undoped and lightly doped cuprates has been measured quantitatively and compared with recent calculations based on the 3-band-Hubbard model. Possible contributions of magnetic Raman scattering to the electronic B_1g background of the superconducting samples are discussed.     

Signal processing: Opportunities for superconductive circuits

Prime motivators in the evolution of increasingly sophisticated communication and detection systems are the needs for handling ever wider signal bandwidths and higher data processing speeds. These same needs drive the development of electronic device technology. Until recently the superconductive community has been tightly focused on digital devices for high speed computers. The purpose of this paper is to describe opportunities and challenges which exist for both analog and digital devices in a less familiar area, that of wideband signal processing. The function and purpose of analog signal-processing components, including matched filters, correlators and Fourier transformers, will be described and examples of superconductive implementations given. A canonic signal-processing system is then configured using these components in combination with analog/digital converters and digital output circuits to highlight the important issues of dynamic range, accuracy and equivalent computation rate. Superconductive circuits hold promise for processing signals of 10-GHz bandwidth. Signal processing systems, however, can be properly designed and implemented only through a synergistic combination of the talents of device physicists, circuit designers, algorithm architects and system engineers. An immediate challenge to the applied superconductivity community is to begin sharing ideas with these other researchers.