Showing papers on "High-temperature superconductivity published in 1989"

Phenomenology of the normal state of Cu-O high-temperature superconductors.

Abstract: The universal anomalies in the normal state of Cu-O high-temperature superconductors follow from a single hypothesis: There exist charge- and spin-density excitations with the absorptive part of the polarizability at low frequencies \ensuremath{\omega} proportional to \ensuremath{\omega}/T, where T is the temperature, and constant otherwise. The behavior in such a situation may be characterized as that of a marginal Fermi liquid. The consequences of this hypothesis are worked out for a variety of physical properties including superconductivity.

High current density in bulk YBa2Cu3Ox superconductor

Abstract: A liquid phase processing method for the fabrication of bulk YBa2Cu3Ox superconductors with large current carrying capacity has been developed. Slow cooling through the peritectic transformation (1030–980 °C) has been shown to control the microstructure of these superconductors. A cooling rate of 1 °C/h in this temperature range has yielded a microstructure with long plate type, thick grains oriented over a wide area. Current density up to 18 500 A/cm2 has been obtained by continuous direct current measurements and in excess of 62 000 A/cm2 with pulse current of 10 ms duration and 75 000 A/cm2 using 1 ms pulse. The strong magnetic field dependence observed in sintered bulk 1‐2‐3 superconductors is also minimized to a large extent where a current density in excess of 37 000 A/cm2 is obtained in a field of 6000 G.

Superconductivity produced by electron doping in CuO 2 -layered compounds

Abstract: We have discovered that the Ce4+doping and subsequent annealing in reducing atmosphere give rise to 24-K superconductivity in the Nd2CuO4-type structure with sheets of Cu-O squares. In contrast to the previously reported high-Tccuprates, the charge carriers in the new superconductors are doped electrons, not holes; this was confirmed by the measurements of Hall and Seebeck coefficients as well as by chemical analysis of the effective copper valence. An anomalous dependence ofTcon the concentration of doped electrons is shown for these electron-doped superconducting cuprates.

Levitation in Physics

Abstract: Several physical effects allow free floatation of solid and even liquid matter. Materials may be levitated by a jet of gas, by intense sound waves, or by beams of laser light. In addition, conductors levitate in strong radio-frequency fields, charged particles in alternating electric fields, and magnets above superconductors or vice versa. Although levitation by means of ferromagnets is unstable, supper-conductors may be suspended both above and below a magnet as a result of flux pinning. Levitation is used for containerless processing and investigation of materials, for frictionless bearings and high-speed ground transportation, for spectroscopy of single atoms and microparticles, and for demonstrating superconductivity in the new oxide superconductors.

AC susceptibility of high temperature superconductors in a critical state model

Abstract: A critical state model for a granular superconductor is employed to calculate the temperature and AC and DC magnetic field dependence of the complex susceptibility, χ = χ ′ + iχ ″, of a sintered bulk YBa 2 Cu 3 O 7- δ superconductor. Inter granular Josephson vortices are assumed to sweep in and out of the weak-link network while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis losses. The predictions of the model for χ′ and χ″ are consistent with experimental data and model parameters which characterize a high temperature granular superconductor can be determined. These parameters are the inter- and intragranular pinning force densities, the fraction of the superconducting grains, the grain size distribution and a London penetration depth which neglects grain anisotropy.

Critical currents and thermally activated flux motion in high-temperature superconductors

Abstract: We have measured the resistance below Tc of single crystals of the high‐temperature superconductors Ba2YCu3O7 and Bi2.2Sr2Ca0.8Cu2O8+δ in magnetic fields up to 12 T. The resistive transition of both compounds is dominated by intrinsic dissipation which is thermally activated, resulting in an exponential temperature dependence of the resistivity well below Tc. The dissipation is significantly larger and of different character in the Bi‐Cu compound than in Ba2YCu3O7. The relation between the activated behavior and the depinning critical current is discussed.

Superconducting gap in bi-sr-ca-cu-o by high-resolution angle-resolved photoelectron spectroscopy.

Abstract: Detailed studies indicate a superconducting gap in the high-temperature superconductor Bi2Sr2CaCu2O8. Photoemission measurements with high energy and angle resolution isolate the behavior of a single band as it crosses the Fermi level in both the normal and superconducting states, giving support to the Fermi liquid picture. The magnitude of the gap is 24 millielectron volts.

High‐temperature superconductivity in ultrathin films of Y1Ba2Cu3O7−x

Abstract: We have grown ultrathin films of Y1Ba2Cu3O7−x in situ on (001) SrTiO3 by pulsed laser deposition The zero resistance transition temperature (Tc0) is >90 K for films >300 A thick The critical current density (Jc at 77 K) is 08×106 A/cm2 for a 300 A film and 4–5×106 A/cm2 for a 1000 A film The Tc0 and Jc deteriorate rapidly below 300 A, reaching values of 82 K and 300 A/cm2 at 77 K, respectively, for a 100 A film Films only 50 A thick exhibit metallic behavior and possible evidence of superconductivity without showing zero resistance to 10 K These results are understood on the basis of the defects formed at the film‐substrate interface, the density of which rapidly decreases over a thickness of 100 A We have studied these defects by ion channeling measurements and cross‐section transmission electron microscopy Our results suggest that the superconducting transport in these films is likely to be two dimensional in nature, consistent with the short coherence length along the c axis of the crystals

Nature of the charge carriers in electron-doped copper oxide superconductors

Abstract: The charge carriers in all of the well-known layered copper oxide high-temperature superconductors1– are holes located in the CuO2planes, the common structural element of these systems. Recently, Tokura et al.5 have reported that doping of Ln2CuO4, where Ln represents Pr, Nd or Sm, with formally tetravalent Ce can produce a high-temperature superconductor with electron carriers. To determine the character of the carrier states in Nd2–xCexCuO4, we have performed X-ray absorption measurements at the Cu K and Ce L3 edges for several different Ce concentrations. We find clear evidence that the electrons introduced by doping fill 3d holes on the Cu atoms, thus converting some Cu2+ ions to Cu+. Hence the nature of the states nearest to the Fermi energy (that is, those responsible for conduction) is qualitatively different from that in the hole-doped superconductors, in which the holes move in a band formed predominantly from O 2p states8–10. As a result, the details of the electron-pairing mechanism could be very different in the two systems11.

Superconductivity at 27 K in fluorine-doped Nd2CuO4

Abstract: THE recent report1 of superconductivity in Nd1,85Ce015CuO4 is the first example of a high-transition-temperature (high– Tc) super-conductor with electrons as the charge carriers and a formal copper oxidation state of less than +2 All previously characterized copper oxide superconductors are hole-doped, and the copper is formally oxidized Here we show that substitution of fluorine for oxygen in T′-phase Nd2CuO4 provides an alternative route to achieving formal reduction of copper, electron conductivity and superconductivity at temperatures as high as 27 K This result is unusual, because anionic rather than cationic substitution gives rise to the superconductivity

Unusually simple crystal structure of an Nd-Ce-Sr-Cu-O superconductor

Abstract: Recently, a new copper oxide superconductor with an onset tem-perature of 28 K was found in the Nd–Ce–Sr–Cu–O system1. The superconducting phase was identified by X-ray diffraction and electron microscopy2, and the metal-atom positions are identical with those in the K2NiF4 or Nd2CuO4 structure3, except for the order of the cations Nd(Ce) and Sr. Here we report the refinement of the crystal structure using neutron powder diffraction. We find that it comprises alternating slabs of the K2NiF4 and Nd2CuO4 structures, and that all the copper atoms are crystallographically identical, with oxygen in fivefold (square-pyramidal) coordination. In having only one kind of copper site, this new compound is structurally simpler than the other CuO5-bearing superconductors such as YBa2Cu3O7, and may therefore be particularly helpful in the effort to understand the origins of high-temperature superconductivity.

Anomalous absence of pressure effect on transition temperature in the electron-doped superconductor Nd1.85Ce0.15CuO4−δ

Abstract: THE recent discovery of the electron-doped superconductors1 added fresh fuel to controversies about the mechanism of high-temperature superconductivity in copper oxide compounds. The superconducting transition temperature (Tc) of almost all the hole-doped copper oxide compounds increases with increasing pressure2. The pressure coefficient has a large positive value compared with a typical conventional (BCS) superconductor. A systematic survey of the TcS of Y–Ba–Cu–O and La–Sr–Cu–O compounds has shown that Tc is strongly correlated with hole concentration3–5, but the Hall coefficient is only weakly dependent on pressure6,7. These results indicate that the large change in Tc with pressure is caused, not by a change in hole concentration, but by other factors related to changes in the interatomic distances. Here we report that the newly discovered electron-doped superconductor Nd,1.85Ce0.15CuO4–δ exhibits almost no pressure effect on Tc up to 2.5 GPa, in remarkable contrast to the hole-doped superconductor Nd1.3Ce0.3Sr0.5CuO4–δ. The Cu–O pyramids characteristic of the hole-doped compounds lose their apical oxygens to become square planes in the electron-doped materials. If the pressure effect on Tc does arise from changes in bond lengths, the difference in behaviour of the two compounds points to the involvement of the bond between copper and apical oxygen, which is missing in the electron-doped material.

Renormalized Fluctuation Theory of Resistive Transition in High-Temperature Superconductors under Magnetic Field

Abstract: We attempt to explain the broad resistive transition observed in high-temperature superconductors under an applied magnetic field by a renormalized theory of the order parameter fluctuations using a model of superconductors with layered structures The results are compared with single-crystal resistivity data They are in good agreement, at least in the higher temperature side of the resistive transition, where the theory is expected to be valid

Mechanisms of Field-Dependent Microwave Absorption in High-Tc Ceramic Superconductors

Abstract: New experimental features of the field-modulated microwave absorption in high-Tc ceramic superconductors are presented. The proposed model for the loss mechanism can explain the complicated field and temperature dependences of the absorption signals.

Absence of current direction dependence of the resistive state of high temperature superconductors in magnetic fields

Abstract: Resistive transition of Bi 2 Sr 2 CaCu 2 O 8+ y film in magnetic fields is studied in the light of the currently proposed flux creep model. Measurements with precise angular alignment have clarified that the resistive transition does not depend on the relative direction between magnetic field and transport current. This calls for a significant revision of the Lorentz-force-driven flux creep picture. A fluctuation induced phase-slip phenomenon as an alternative model is also discussed.

Preparation and superconducting properties of ultrathin YBa2Cu3O7−x films

Abstract: Ultrathin films of YBa2Cu3O7−x with thicknesses down to 2 nm were grown on(100)SrTiO3 and (100)MgO by inverted cylindrical magnetron sputtering. Metallic behavior and zero resistance temperatures above 4.2 K were obtained in 3‐nm‐thick films on SrTiO3. Thinner films revealed temperature‐activated conductivity and only partial transitions to superconductivity due to inhomogeneities in the film morphology. On MgO, the critical film thickness leading to deteriorations of the transport properties was 6 nm. An analysis of the fluctuation‐enhanced conductivity near Tc in terms of the Aslamazov–Larkin theory [Phys. Lett. A 26, 238 (1968)] revealed three‐dimensional behavior even in the thinnest fully superconducting films.

Electronic properties of high-Tc superconductors.

Abstract: We study a generalized Hubbard Hamiltonian which includes the Cu and O orbitals in the CuO/sub 2/ planes of high-/ital T//sub /ital c// superconductors. We use slave-boson technique to account for the intra- and interatomic correlations. In the saddle-point approximation we obtain the metal-insulator phase diagram and conclude that, for one hole per unit cell in the CuO/sub 2/ plane, La/sub 2/CuO/sub 4/ is a charge-transfer insulator. Our results show that doping produces a rapid metallization of the system and destroys antiferromagnetic order. Transport properties and mechanism for superconductivity are briefly discussed.

Nonlinear magnetic flux response in high temperature superconductors

Abstract: We have spectrum analysed the voltage induced in a pick-up coil wound around a cylindrical bulk sample of YBa 2 Cu 3 O 7−δ when driven by a weak low frequency AC magnetic field, superimposed on a weak DC magnetic field, at 77 K. The harmonics generated by the highly nonlinear response are explained quantitatively in great detail by employing a critical state model with a field independent pinning force density, where intergranular vortices sweep in and out of the Josephson weak link regions during each AC cycle. The symmetry relation for the local flux density, which relates the first and second half of an AC cycle, is broken in the presence of a DC magnetic field, causing even harmonics to appear. This suggests high-sensitivity applications in magnetometry.

Mechanisms of high temperature superconductivity

Abstract: The focal points of the symposium presented in this book were the following: For the high temperature copper oxide superconductors, the general consensus was that electron correlation effects are of essential importance. Based on this understanding a number of discussions took place: how to include the correlation effects and on the relative importance of various interactions; whether the ground state of a normal state is Fermi-liquid-like or not; where the doped holes mainly exist and on the nature of these holes; the value of the ratio of the energy gap to the critical temperature; whether the mechanics s are magnetic, or charge fluctuation, or excitonic, or very unusual ones quite different from the Fermi-liquid picture; whether a normal phase is normal or abnormal, etc. For the Bi-O superconductors, such as the BaKBiO system, the discussion concentrated on whether or not the mechanism of superconductivity is the same as that of the high temperature copper oxide superconductors.

Theoretical issues for critical currents of bulk polycrystalline YBa2Cu3O7−δ superconductors

Abstract: The influence of grain orientation and weak‐link coupling between grains on the critical current of polycrystalline Y‐Ba‐Cu‐O superconductors has been investigated by numerical methods. The theoretical results indicate that there is size effect on jC in bulk polycrystals, and that it is not necessary to consider the influence of grain orientation when jC is less than 105 A/cm2.

Pinning and thermal fluctuations of a flux line in high-temperature superconductors.

Abstract: Using a simple model of single vortex depinning, we show that the temperature dependence of both resistivity and ac susceptibility in the superconducting state in moderate magnetic fields can be understood in terms of the activation of vortices from homogeneous and densely distributed pinning centers. Furthermore, the field dependence of the activation energy is explained by including the interaction of vortices above H\ensuremath{\sim}1 T, below which the distribution of pinning strengths becomes important.

Superconducting and mechanical properties of YBCO-Ag composite superconductors

Abstract: The measurements of electrical properties and X-ray structural analysis were made for sintered composite materials prepared by mixing the powders of the ceramic superconductor YBa2Cu3O7−x (YBCO) and the powders of the pure metal element M with with M = Ni, Cu and Ag at 50 vol% proportions. The superconducting and mechanical properties were studied further on the YBCO-Ag system in the wide volume fractions 0 to 92%Ag. The onset of the superconductivity is found to occur even for samples containing more than 80 vol % Ag. The value of the critical current density is increased initially by adding silver. The mechanical strength against fracture is also improved. The X-ray structural analysis coupled with scanning electron micrographs revealed that silver and YBCO remain intact after sintering and that fine YBCO particles form continuous networks around homogeneously distributed silver particles. This explains well both superconducting and mechanical properties in the present YBCO-Ag composite system.

Laser-induced formation and surface processing of high-temperature superconductors

Abstract: The use of lasers in the formation and surface processing of high-temperature superconductors (HTS) is reviewed. Presently, thin film fabrication by reactive laser sputtering, and surface patterning by laser-induced reduction/metallization and ablation are the most promising applications. The great majority of the investigations have been performed for Y-Ba-Cu-O.

Measurements of the magnetic penetration depth in YBa2Cu3O7−δ thin films by the microstrip resonator technique

Abstract: We have utilized the superconducting microstrip resonator technique to measure the magnetic penetration depth in high Tc oxide thin films in the 1–25 GHz regime. This technique is particularly well suited for thin films, where the absolute value of the penetration depth can be accurately determined. Results for high Tc superconducting thin films show that the value of the penetration depth is sensitive to the preparation conditions of the film, and the temperature dependence is that expected of conventional superconductors.