Showing papers on "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.

Theory of collective flux creep

Abstract: The nature of flux-creep phenomena in the case of collective pinning by weak disorder is discussed. The Anderson concept of flux bundle is explored and developed. The dependence of the bundle activation barrier U on current j is studied and is shown to be of power-law type: U(j) is proportional to j exp -alpha. The values of exponent alpha for the different regimes of collective creep are found.

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.

A new process with the promise of high Jc in oxide superconductors

Abstract: We report a new process which promises high critical current density in oxide superconductors. The process consists of three stages. Firstly a YBa2Cu3Ox sample is rapidly heated and quenched from the Y2O3 plus liquid region. Subsequently the quenched sample is reheated to the Y2BaCuO5 plus liquid region, and then slowly cooled with a temperature gradient in flowing oxygen. The process enables us to grow a superconducting phase unidirectionally and to suppress the second phase intrusion, leading to the production of well textured YBa2Cu3Ox which yields a high Jc value in the presence of magnetic fields. It is also found that Bean's critical state is realized in such high Jc samples.

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.

Kim model for magnetization of type‐II superconductors

Abstract: We have calculated the initial magnetization curves and complete hysteresis loops for hard type‐II superconductors. The critical‐current density Jc is assumed to be a function of the internal magnetic field Hi according to Kim’s model, Jc(Hi)=k/(H0+‖Hi‖), where k and H0 are constants. As is the case for other critical‐state models, additional assumptions are that bulk supercurrent densities are equal to Jc, and that the lower critical field is zero. Our analytic solution is for an infinite orthorhombic specimen with finite rectangular cross section, 2a×2b (a≤b), in which a uniform field H is applied parallel to the infinite axis. Assuming equal flux penetration from the sides, we reduced the two‐dimensional problem to a one‐dimensional calculation. The calculated curves are functions of b/a, a dimensionless parameter p=(2ka)1/2/H0, and the maximum applied field Hm. The field for full penetration is Hp=H0[(1+p2)1/2−1]. A related parameter is H*m=H0[(1+2p2)1/2−1]. Hysteresis loops were calculated for the di...

Onset of superconductivity in the two-dimensional limit.

Abstract: The onset of superconductivity in homogeneous ultrathin films is found to occur when their normal-state sheet resistance falls below a value close to h/4${e}^{2}$, the quantum resistance for pairs. The data further suggest that in the T\ensuremath{\rightarrow}0 limit such films are either superconducting or insulating. The existence of a threshold in systems which are not granular implies that its explanation involves more general arguments than those which follow from the modeling of films by Josephson-coupled arrays.

Scanning-tunneling-microscope observation of the Abrikosov flux lattice and the density of states near and inside a fluxoid

Abstract: The Abrikosov flux lattice is imaged in NbSe2 by tunneling into the superconducting gap edge with a low-temperature scanning-tunneling microscope. The tunneling conductance into a single vortex core is strongly peaked at the Fermi energy, suggesting the existence of core states or core excitations. As one moves away from the core, this feature evolves into a density of states which is consistent with a BCS superconducting gap.

Thermally assisted flux flow at small driving forces

Abstract: The theory for thermally assisted flux flow (TAFF) in the limit of small driving forces is used to derive exact expressions for the time-dependent behaviour of the magnetisation and permeability. The problem is especially relevant for the high-temperature superconductors where large variations of the transition temperature in small DC fields are observed as a function of the frequency of the probing AC field. The parameters of the theory are extensively discussed in relation to the present understanding of flux pinning.

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.

Dynamic spin fluctuations and the bag mechanism of high-T c superconductivity

Abstract: The spin-bag approach to the high-temperature superconductivity is presented in detail The general argument that the local supression of the electronic pseudogap leads to an attractive interaction of the quasiparticles is substantiated by the detailed calculation of the pairing potential mediated by the collective modes of the spin-density-wave background In particular, the spin-wave spectrum, the sublattice magnetization, and the spectral distribution of the collective modes are studied within the random-phase approximation In the low-doping limit, different shapes of the Fermi surface give rise to a superconducting gap which formally has $d$-wave- or $p$-wave-like symmetry, however the gap has no nodes on the Fermi surface Therefore, the superconducting properties of our model are analogous to those of a conventional $s$-wave (ie, nodeless) BCS superconductor We also discuss possible bag effects in the large-$U$ Hubbard model and in charge-density-wave systems Finally, the relation of our work with other approaches and with experiment are discussed briefly

Theory of melted flux liquids

Abstract: Novel intermediate flux states should be accessible in high-${T}_{c}$ superconductors, where it appears that the conventional Abrikosov flux lattice is melted over a significant portion of the (H,T) plane. We discuss the Lindemann criterion, and argue that fluctuations in a flux crystal are highly anisotropic, so that an asymptotically two-dimensional melting transition is possible as the shear modulus drops toward zero for many sample geometries and field orientations. We then describe the ``entangled flux liquid'' which arises at high-flux densities or thick samples. The statistical mechanics of this liquid is closely related to the physics of two-dimensional superfluids. The decay of vortex line correlations along the field direction is controlled by the superfluid excitation spectrum. A renormalization-group analysis shows how line wandering changes the nature of the B(H) constitutive relation near ${H}_{c1}$. We suggest that a heavily entangled flux liquid could exhibit a shear modulus on experimental time scales, in analogy with viscoelastic behavior in dense polymer melts.

On the Angle Dependence of the Magnetoresistance in Quasi-Two-Dimensional Organic Superconductors

Abstract: The dependence of the title is shown to originate in a singular angle dependence of the distribution width of the area of the semiclassical closed orbits under the magnetic field in the reciprocal space. In quasi-two-dimensional metals with a cylindrical Fermi surface, the distribution width of the area of the orbits on the Fermi surface vanishes at certain inclination angles of the magnetic field, entailing the appearance of remarkable effects. The observed peaks of the coarse-grained magnetoresistance are found to correspond to these special angles.

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.

Distribution of activation energies for thermally activated flux motion in high-T_{c} superconductors: An inversion scheme

Abstract: Within a thermally activated flux-motion model we derive an exact inversion scheme which makes it possible to calculate the distribution m(${\mathrm{E}}^{\mathrm{*}}$) of activation energies ${\mathrm{E}}^{\mathrm{*}}$ for flux motion from experimental magnetic relaxation data M(t,T). The distributions determined from relaxation data for polycrystalline and single-crystalline ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ strongly resemble a log-normal distribution function. The results show that within this model existing data imply that structural disorder is present in both ceramic and single-crystalline samples.

Nuclear magnetism logging tool using high-temperature superconducting squid detectors

Abstract: A logging device is disclosed for measuring the nuclear magnetism response of earth formations. The device employs a detector that is capable of detecting sinusoidal and slowly varying changes in magnetic field caused by precession of mobile nuclei about the earth's magnetic field. Such a detector may be one or more laser-pumped helium magnetometers, or one or more high-temperature Superconducting Quantum Interference Devices (SQUID). The invention uses a micro-miniature Joule-Thomson refrigerator to maintain the high-temperature SQUIDs below their superconducting transition point. The SQUIDs are flux coupled in an axial gradiometer configuration to reduce motion-induced magnetic noise.

Onset of superconductivity in ultrathin granular metal films.

Abstract: The evolution of superconductivity in ultrathin films of Sn, Pb, Ga, Al, and In has been examined as a function of thickness and temperature. The films were grown in increments by condensation from the vapor onto substrates held at temperatures below 18 K. For each metal, global superconductivity or zero electrical resistance was found when the normal-state sheet resistance ${R}_{N}$ fell below a value close to h/4${e}^{2}$, or 6.45 k\ensuremath{\Omega}/\ensuremath{\square}, an observation uncorrelated with either structural or material parameters such as thickness or transition temperature. Prior evidence of superconductivity with nonzero resistance, local superconductivity, was found at earlier stages of film growth. All evidences of superconducting behavior were observed at temperatures close to the bulk transition temperature beginning in the range of thicknesses for which normal-state resistivities were greater than 200 \ensuremath{\mu}\ensuremath{\Omega}-cm and were rapidly changing with thickness. This implies that the films consisted of fully superconducting grains connected by tunneling junctions. The strong disorder represented by a broad distribution of junction parameters can be renormalized into weak disorder. Thus theoretical calculations based on regular arrays of superconducting sites coupled by (Josephson) junctions appear to be relevant. The extreme thinness of the films implies very small junction capacitances leading to large quantum fluctuations of the phase differences of their superconducting order parameters. Two classes of theories explaining a nearly universal resistance threshold for superconductivity have emerged. Both classes involve the quenching of these quantum fluctuations. In the limit of very small junction capacitances the threshold occurs at resistance values near h/4${e}^{2}$, and is essentially independent of the capacitance and the energy gap, in good agreement with the experimental data. Not contained in any of the models is an explanation of the observed regular variation of the low-temperature resistances of the films with the normal-state sheet resistance for values just above the resistance threshold.

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.

Correspondence between two-dimensional bosons and a bulk superconductor in a magnetic field

Abstract: We study the partition function of 2D lattice bosons at T = 0 and derive a dual representation which is isomorphic to a bulk superconductor with fluctuating-gauge field, in an applied magnetic field. This allows us to relate boson ground states to thermodynamic phases of the superconductor. A density-wave bose insulator corresponds to an Abrikosov flux-lattice phase, whereas boson superfluidity implies a nonsuperconducting flux-line liquid phase. By analogy with a boson supersolid, we suggest the possibility of an exotic Abrikosov flux-lattice phase with no superconducting long-ranged order.

Gap Energy and Long Range Order in the Boson - Fermion Model of Superconductivity

Abstract: A partir des observations de petites longueurs de coherence pour tous les supraconducteurs a haute temperature, on conclut que l'etat de paire est raisonablement bien localise dans l'espace des coordonnees et peut par consequent etre represente de maniere phenomenologique pour un champ de bosons local (φ). Le mecanisme de la supraconductivite sous-jacent est suppose s'exercer a travers la reaction de «canal-s»: 2e→φ→2e. Ceci conduit a un modele boson-fermion melange. Etude de l'ordre a grande distance, du gap d'energie, et de l'effet Meissner dans une telle theorie

Origin of the incommensurate modulation of the 80-K superconductor Bi 2 Sr 2 CaCu 2 O 8.21 derived from isostructural commensurate Bi 10 Sr 15 Fe 10 O 46

Abstract: The compound ${\mathrm{Bi}}_{10}$${\mathrm{Sr}}_{15}$${\mathrm{Fe}}_{10}$${\mathrm{O}}_{46}$ is shown to be isostructural with the 80-K superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{O}}_{8.21}$. The incommensurate modulation of the Cu compound is commensurate in the crystal of the Fe compound that was selected. The modulation is caused by the insertion of extra oxygen atoms in the Bi layers and results in corrugated slabs. The structure of the Bi-O layers can be described as roughly 70% of rocksalt type and 30% of oxygen-deficient perovskite type. The results explain the excess oxygen in the superconductor, the mechanism causing the corrugation, and the inelastic bending property displayed by single crystals. They also reconcile apparently discrepant results about the oxygen distribution in the bismuth layers.

Critical-state model for harmonic generation in high-temperature superconductors.

Abstract: High-temperature superconductors exhibit harmonic generation when immersed in an ac magnetic field. To explain this phenomenon, we propose a macroscopic critical-state model as an alternative to the loop model used by Jeffries et al. While the original Bean model of the critical state only predicts odd harmonics, our extended model also predicts even harmonics by taking into account the dependence of the critical current upon magnetic field. The results of our measurements of harmonic signals as a function of ac magnetic field, dc magnetic field, temperature, and harmonic number are consistent with the proposed model. In particular, we find that, as the magnetic field is increased, the critical current crosses over from the Bean regime, where ${J}_{c}$ is independent of field, to the Anderson-Kim regime, where ${J}_{c}$ is approximately inversely proportional to the field.

Superconducting state in an oxygen hole metal.

Abstract: We study within BCS theory the properties of an effective Hamiltonian to describe conduction by holes through an oxygen anion network. The Hamiltonian contains an on-site repulsive interaction ${U}_{p}$ and a modulated hopping interaction $\ensuremath{\Delta}t$ that yields a larger hopping amplitude between sites when other holes are present on those sites. The superconducting state is found to be $s$ wave with an energy-dependent gap. Superconductivity is restricted to low hole densities and the critical temperature increases with the hopping amplitude. The particular form of the interaction allows for superconductivity even in the presence of large Coulomb repulsion, up to $\frac{{U}_{p}}{\ensuremath{\Delta}t}\ensuremath{\approx}30$. We discuss the behavior of the tunneling density of states, specific heat, gap ratio, and coherence length as a function of hole density and parameters in the Hamiltonian, and the relationship between our results and existing, as well as possible future, experimental results on high-${T}_{c}$ oxides. Our model provides a natural explanation for the spread in gap values observed in different experiments, for the observed broadening of the resistive transition in a field, and for the observed superconducting glass behavior.

SQUID magnetometers for low-frequency applications

Abstract: We present a novel formulation for SQUID operation, which enables us to evaluate and compare the sensitivity and applicability of different devices. SQUID magnetometers for low-frequency applications are analyzed, taking into account the coupling circuits and electronics. We discuss nonhysteretic and hysteretic single-junction rf SQUIDs, but the main emphasis is on the dynamics, sensitivity, and coupling considerations of dc-SQUID magnetometers. A short review of current ideas on thin-film, dc-SQUID design presents the problems in coupling and the basic limits of sensitivity. The fabrication technology of tunnel-junction devices is discussed with emphasis on how it limits critical current densities, specific capacitances of junctions, minimum linewidths, conductor separations, etc. Properties of high-temperature superconductors are evaluated on the basis of recently published results on increased flux creep, low density of current carriers, and problems in fabricating reliable junctions. The optimization of electronics for different types of SQUIDs is presented. Finally, the most important low-frequency applications of SQUIDs in biomagnetism, metrology, geomagnetism, and some physics experiments demonstrate the various possibilities that state-of-the-art SQUIDs can provide.

Direct measurement of the temperature-dependent magnetic penetration depth in Y-Ba-Cu-O crystals.

Abstract: The temperature dependence of the magnetic penetration depth lambda is determined directly from low-field dc magnetization measurements on single crystal Y-Ba-Cu-O (T/sub c/ = 89.7 K, ..delta..T/sub c/ = 0.2 K). The results are consistent with the behavior expected from a BCS (s wave) superconductor.

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

Growth and anisotropic superconducting properties of Nd2-xCexCu04-y single crystals

Abstract: ELECTRON-DOPED copper oxide superconductors have recently been discovered in the system Ln2–xCexCuO4–y, where Ln is Pr, Nd or Sin1,2. Here we report the growth of single crystals of Nd2–xCexCuO4–y, and measurements of their anisotropic superconducting properties. A sharp superconducting transition is observed at 22.5 K in Nd1.84Ce0.16CuO4–y. The resistivity shows a metallic temperature dependence both parallel and perpendicular to the basal plane. Applying a magnetic field perpendicular to the basal plane causes a parallel shift of the resistive transition curve to lower temperatures, as in conventional type II superconductors but unlike the hole-doped copper oxide superconductors. The Ginzburg–Landau coherence lengths estimated from the resistively defined upper critical magnetic field are 70.2 A in the basal plane and 3.4 A along the c axis, showing an anisotropy factor of 21. This value is much larger than that in La2–xSrxCuO4–y despite their apparent structural similarities. Crystallographic differences from the hole-doped systems and the concomitant changes in electronic properties may offer clues to the role of the Cu–O planes in the microscopic pairing mechanism of high-temperature superconductors.