Observation of X-band SQUID signals in a High-Tc superconducting film device

A microwave superconducting magnetometer is described in which a microstrip resonator is coupled to a two-hole high-T _c thin-film SQUID device. Both the microstrip circuit and the thin-film SQUID were fabricated by photolithography techniques. The YBCO thin film was deposited on single-crystal substrate of yttria-stabilized zirconia [YSZ(100)] by an ion beam sputtering technique producing a superconducting transition measured at a critical temperature ofT _c =92 K to within T 3 K. Non linear oscillatory behavior was observed in the microstrip resonator when inductively coupled to the SQUID. This nonlinear behavior yielded a microwave device in which the reflected microwave power varied with applied DC magnetic flux.     

Development of High-T c SQUID System for Nondestructive Evaluation

Nondestructive evaluation (NDE) system with high spatial resolution power based on high-T _c superconducting quantum interference device (SQUID) has been developed. The SQUID is fabricated by YBCO thin film and mounted in vacuum at the end of a cold finger fixed at the inner wall bottom of the dewar, which is filled with liquid nitrogen (77 K) to cool the SQUID sensor. The sensor faces a thin flat sapphire window on the outer wall of the dewar and is thermally isolated from it by a vacuum space at a distance of less than 1 mm. The magnetic field sensitivity of SQUID is about 40 pT/ Hz in magnetically unshielded environment. This NDE system was used to detect the flaws buried into materials. The remnant magnetic field inducted by the local damages in a stainless steel sample was measured. Incorporated a gradiometric double-D excitation coil, the eddy current NDE was carried out. Small cracks in the aluminum below the surface can be detected effectively in unshielded environment.      

Development of High-T c SQUID and Application to Ultra-Sensitive Contaminant Detection System

We have developed magnetic metallic contaminant detectors using multiple high-T _c SQUIDs (superconducting quantum interference devices) for food and industrial products. For manufacturers producing items such as processed food and industrial products, problems with metallic contaminants are critical issues. When contamination occurs, the manufacturer of the product suffers a great loss from recalling the tainted product. The lower detection limit for practical X-ray usage is on the order of 1 mm. A detection system using a SQUID is a powerful tool for sensitive inspections. For practical use, the detection width of the system should be increased by employing multiple sensors. We designed and set up a multi-channel high-T _c SQUID inspection system with a width of at least 130 mm for food and 70 mm for industrial products. As a result, a small ?0.3-mm stainless steel particle for food and a ?27-μm iron particle for industrial products were successfully detected.     

High-T c superconducting Nb-Ga wires prepared by heating and quenching in liquid helium

High-T _c Nb-Ga wires were prepared by a diffusion process. The Nb wires dip-coated with Ga were Joule-heated to high temperatures in a liquid He bath and then quenched by switching the current off. The highest onset transition temperature was 17.4 K. It was increased to 20.1 K by annealing for 24 h at 700°C. The midpoint temperature of the resistive transition was 19.8 K.     

Lorentz Microscopy Observation of Unconventional Behaviors of Vortices in High-T c Superconductors

Vortices in high-T _c superconductors have been predicted to behave in unconventional manners that reflect the layered structure of the materials. Our newly developed 1-MV field-emission electron microscope has opened the way to enable individual vortices to be observed inside high-T _c superconducting thin films. This makes it possible to investigate the inner structures of vortices, such as those trapped along tilted columnar defects and those of chain vortices that are formed when a magnetic field is obliquely applied with respect to the normal of the layer plane of the materials.     

High-T c superconductor in an a.c. field

The impedance of the layered high-T _c oxide is evaluated. The response of the S-N proximity system is studied. The mechanism of intrinsic losses due to the low-lying collective mode is proposed.     

High-T c superconductor in an a.c. field

The behavior of high-T _c oxides in an alternating field is studied. The short coherence length and the layered structure require an unconventional microscopic analysis. The complex conductivityδ(Ω, k) is evaluated. The frequency and temperature dependences of the impedance are obtained (in the region of small frequencies). The effect of impurities is discussed. The microwave properties of the proximity system S_h-N (S_h and N are high-T _c and normal films, respectively) are discussed.     

High-T c superconducting Bi(Al)-Ca-Sr-Cu-O glass-ceramic fibres drawn from glass preforms

Bi(Al)-Ca-Sr Cu-O glass-ceramic fibres over 100 cm in length were successfully drawn from a glass preform above the crystallization temperature,T _x. The diameters of the uniformly drawn fibres with circular cross-section could be controlled in the range from 25–200 m and the drawing speed was as high as 200 cm min^–1. In this work Al₂O₃ was used to modify the properties of the glass. It increased the glass transition and crystallization temperatures but did not significantly increase the glass working range. Shrinkage and increase of density during heat treatment of the glass fibres were observed. The annealed (825°C/12 h in air) Bi₄Al_0.1Ca₃Sr₃Cu₄O_y glass-ceramic fibre showed aT _c(onset) of 82 K and aT _c(zero) of 71 K.     

Phonon Mechanism of High-T c Superconductivity

For a long time the majority view has been that phonons are irrelevant to the mechanism of high-temperature superconductivity. However, recent experimental results, including the neutron inelastic scattering measurements reviewed here, seriously challenge this view. We point out that the electron–phonon coupling in the cuprates can be substantially different from that in simple metals because of the covalency and strong electron correlation. In particular certain phonon branches induce substantial intersite charge transfer that can result in a negative electronic dielectric susceptibility. Such a strong electron–phonon coupling of certain modes could form the basis for the phonon mechanism of superconductivity in the cuprates.     

Observation of superconducting transition temperature oscillations in metal-nonmetal film systems

For technetium-carbon, molybdenum-carbon, and vanadium-carbon thin-film layer structures an oscillatory dependence of the superconducting transition temperature on carbon layer thickness is found. The oscillations take place for thin metal layers (d<300 Å) and for very small thicknesses of the nonmetal film (d _C<30 Å). In addition to the oscillations ofT _c , oscillations of the normal-state electrical resistance remaining up to room temperature are found in the layered samples. A correlation is observed between critical temperature and resistance changes: Maxima in the curveT _c (d _C) correspond to minima in (d _C). The amplitude of the critical temperature oscillations is 10–15% of theT _c value and depends on the metal layer thickness, increasing with decreasingd approximately as 1/d. Also found in the thin-film layer structures are oscillations of the critical magnetic fieldsH _c andH _c correlating withT _c oscillations. A mutual influence of nonmetal layers located on opposite sides of the metal film was observed.     

Josephson Plasma in Various High-T c Cuprates

Josephson plasma in various high-T _c cuprates with and without magnetic field is studied by using the sphere resonance method. For Bi ₂ Sr ₂ CaCu ₂ O ₈₊, the plasma in a zero magnetic field exists at 5 cm ^–1 for a slightly overdoped sample (T _c = 85 K) and shifts to 11 cm ^–1 as the doping increases (T _c = 71 K). For SmLa _1–x Sr _x CuO _3.95 (T* phase), two peaks appear at 11 and 30 cm ^–1 in a zero magnetic field, and both peaks shift to lower frequencies as the magnetic field increases. These peaks are identified as the Josephson plasma of the intrinsic Josephson junction at the fluorite-type Sm ₂ O ₂ block layer and the rocksalt-type (La,Sr) ₂ O _2– block layer, respectively. This indicates that the T* phase can be regarded as the ···S/I/S/I/S/I/S/I/S··· (···superconductor/insulator1/superconductor/insulator2/superconductor···) -type Josephson junction array.     

Multiband Responses in High-T c Cuprate Superconductors

We report on the interplay of localized and extended degrees of freedom in the metallic state of high-temperature superconductors in a multiband setting. Various ways in which the bare magnetic response may become incommensurate are measured against both phenomenological and theoretical requirements. In particular, the pseudogap temperature is typically much higher than the incommensurability temperature. When microscopic strong-coupling effects with real-time dynamics between copper and oxygen sites are included, they tend to restore commensurability. Quantum transport equations for low-dimensional multiband electronic systems are used to explain the linear doping dependence of the dc conductivity and the doping and temperature dependence of the Hall number in the underdoped LSCO compounds. Coulomb effects of dopands are inferred from the doping evolution of the Hartree–Fock model parameters.     

High-T c superconductivity in thed-p electron system

The relaxation time with spin flipτ _s and the parametersξ, δ, χ of superconducting phase have been calculated on the basis of the kinematical mechanism of superconductivity in strongly correlated oxide models. An inter-relation between the superconducting gap Δ₀ and the specific heat jump Δ _c allowing the experimental verification was obtained and the Ginsburg-Landau equation derived.     

Features of the microwave breakdown of a high-T c superconducting film at defects

The influence of nonsuperconducting defects on the power density for microwave breakdown of a high-T _c superconducting film is investigated theoretically. It is found that the scenario of the destruction of superconductivity depends substantially on the shape of the defect and on the ratio of the absorption coefficients of the defect and film. It is shown that the breakdown power density at a defect may be higher than or equal to the threshold power density for microwave breakdown of a homogeneous film. Pis’ma Zh. Tekh. Fiz. 24, 74–80 (September 12, 1998)     

Physical Picture of High-T c Superconductivity in Cuprates

A simple physical picture of high-T _c superconductivity of CuO₂ planes is proposed. It possesses all characteristic features of HTS, such as a high superconducting transition temperature, the $d_{x^{2}-y^{2}}$ symmetry of order parameter, and the coexistence of a single-electron Fermi surface and a pseudogap in the normal state. Values of pseudogap are calculated for different doping levels.     

Protection device for superconducting magnets with a superconducting switch

A device, operating in the persistent current mode, was developed for quick energy removal from superconducting magnets. A magnet protection circuit which has a superconducting switch and a superconducting power switch connected in series is proposed. The performance of the system tested in connection with a laboratory solenoid (8T, 150 A, 50 kJ) was demonstrated.     

Research and Some Applications of High-T c SQUIDs

In the work, we studied the noise characteristics of electronic gradiometers in unshielded environments. In the off-axis electronic gradiometers for biomagnetic measurements, we optimized low frequency noise and performed two-dimensional magnetic mapping of magnetic signal from human heart. The measured magnetocardiography (MCG) signal was averaged according to the simultaneously recorded electrocardiography signal to further enhance the signal-to-noise ratio. The off-axis configuration in our gradiometer system offers the flexibility for multichannel SQUID-based MCG applications. In the study of the SQUID microscope for circuit detection in unshielded environment, we fabricated SQUIDS, considered the design of cryostat, and used the lock-in technique to examine the circuit board. We also examined the magnetic field pattern from the magnetized magnetic thin film.     

The Maximum Levitation Force of High-T c Superconductors

In this paper we present the dependence of the maximum levitation force (F _z ^max) of a high-T _c superconductor (HTS) on the structural factors of high-T _c superconducting systems based on the Bean critical state model and Ampère’s law. A transition point of the surface magnetic field (B _s ) of a permanent magnet (PM) is found at which the relation between F _z ^max and B _s changes: while the surface magnetic field is less than the transition value the dependence is subject to a nonlinear function, otherwise it is a linear one. The two different relations are estimated to correspond to partial penetration of the shielding currents inside the superconductor below the transition point and complete penetration above it respectively. The influence of geometric properties of superconductors on the dependence is also investigated. In addition, the relation between F _z ^max and the critical current density (J _c ) of the HTS is discussed. The maximum levitation force saturates at high J _c . An optimum function of the J _c and the B _s is presented in order to achieve large F _z ^max.      

Dissipative states in a current-carrying superconducting film

The system of the generalized time-dependent Ginzburg-Landau equations (GTDGLE) describing dirty superconductors nearT _c is solved numerically for a thin quasi-two-dimensional, current-carrying layer. In a certain current range one has superconducting states with dissipation which should lead to a step structure in the current voltage characteristic as observed in experiments.¹ Depending on the parameters one either has phase slip line (PSL) solutions, which are the two-dimensional analogs of the one-dimensional phase slips (PSCs) or vortex street (VS) solutions, where vortices which are nucleated spontaneously in pairs move perpendicular to the direction of the current. We discuss the range of stable existence of these states and transitions between different states.Dedicated to L. Tewordt on the occasion of his 65th birthday.