Showing papers on "High-temperature superconductivity published in 2005"

Introduction to Unconventional Superconductivity

Abstract: This lecture gives a basic introduction into some aspects of the unconventional superconductivity. First we analyze the conditions to realize unconventional superconductivity in strongly correlated electron systems. Then an introduction of the generalized BCS theory is given and several key properties of unconventional pairing states are discussed. The phenomenological treatment based on the Ginzburg‐Landau formulations provides a view on unconventional superconductivity based on the concept of spontaneous symmetry breaking. Finally some aspects of two examples of unconventional superconductors will be discussed: high‐temperature superconductivity and spin‐triplet superconductivity in Sr2RuO4.

Frontiers in Superconducting Materials

Abstract: Superconducting Materials - A Topical Overview. Superconductivity at Ultra-Low Temperatures and its Interplay with Nuclear Magnetism. Superconductivity in Heavy Fermion Compounds. One Dimensional Organic Superconductors. Superconductivity in Fullerides. Chemical Design of Copper-Oxide Superconductors: Homologous Series and Oxygen Engineering. Oxygen in High-Tc Cuprate Superconductors. Rutheno-Cuprates: The Superconducting Ferromagnets. A Phase Diagram Approach to Magnetic Superconductors. Superconductivity in Quaternary Borocarbides. Heterogeneous Magnetic Superconducting Systems. High Pressure Effects on Superconductivity. Disordered Superconductors. Irradiation of HTS for Enhancement of Critical Current. Stacking Manner of Charge Reservoir Blocks in Superconducting Copper Oxides. Superconductors with Nanoscale Flux Pinning Centres. Physics and Chemistry of the Cobalt Oxide Hydrate Superconductor. Progress in Naxcoo2 Single Crystal Growth. Development of NB-Based Conductors. Bi-Based Superconducting Cuprates: Materials Aspects, Crystal Growth and Properties. (Re)Ba2cu3o7 Coated Conductors for AC And DC Applications. Thallium-Based Superconducting Cuprates. Materials Developments And Applications Of Bulk Re-Ba-Cu-O Superconductors. Synthesis of Bulk Superconductors and their Properties for Permanent Magnet Guideway. High TC Cuprates - Thin Film Processing, Atomic Layer Controlled Deposition. Superconductivity in Mgb2. Superconductivity of Magnesium Diboride: Theoretical Aspects. Critical Current Density In Superconducting Mgb2. Preparation and Properties of Advanced Mgb2 Wires and Tapes. Mgb2 Thin Films.

Enhanced superconducting properties of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5)

Abstract: In the present studies, the interplane couplings in Cu0.5Tl0.5Ba2Ca2Cu3O10-δ superconductor have been enhanced by Mg doping. With the improved interplane coupling a- and c-axis lengths are decreased while critical current density [Jc(H=0)] is enhanced by two orders of magnitude and the zero resistivity critical temperature [Tc(R=0)] is increased by 10 K. Predominant single phase of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5) was prepared by the solid state reaction of Ba(NO3)2, Ca(NO3)2, MgO, and Cu(CN). The material was checked by x-ray diffraction for crystallinity and was found to be the predominant single phase of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5) with a- and c-axes lengths 3.840 and 14.20 A, respectively. The c-axis length decreases with the increased concentration of Mg in the compound. Fourier transform infrared absorption measurements have shown softening of apical oxygen modes with increased Mg doping and improved interplane coupling.

Fabrication of high performance light rare earth based single-grain superconductors in air

Abstract: Large, single-grains of (LRE)-Ba–Cu–O (LRE=light rare earth: Nd, Sm, and Gd) bulk high-temperature superconductors with significantly improved properties have been fabricated in an air atmosphere using a practical processing technique based on seeded infiltration growth. This process involves the use of a generic seed crystal, developed recently at the IRC in Superconductivity, to promote epitaxial grain nucleation. The formation of a solid-solution phase within the nucleated single grain is then suppressed effectively by providing excess Ba to the growth front under an air processing atmosphere. Critical current densities in excess of 105A∕cm2 at 77K have been observed in the resulting large grain samples, with an associated and significant improvement in trapped magnetic field. This air-based seeded-infiltration-growth process offers a significant degree of freedom both in terms of the processing parameters and the reproducibility in growth of oriented single grains.

Electronically competing phases and their magnetic field dependence in electron-doped nonsuperconducting and superconducting Pr 0.88 LaCe 0.12 CuO 4 ± δ

Abstract: We present comprehensive neutron scattering studies of nonsuperconducting and superconducting electron-doped Pr0.88LaCe0.12CuO4 +/-delta (PLCCO). At zero field, the transition from antiferromagnetic (AF) as-grown PLCCO to superconductivity without static antiferromagnetism can be achieved by annealing the sample in pure Ar at different temperatures, which also induces an epitaxial (Pr, La, Ce)(2)O-3 phase as an impurity. When the superconductivity first appears in PLCCO, a quasi-two-dimensional (2D) spin-density-wave (SDW) order is also induced, and both coexist with the residual three-dimensional (3D) AF state. A magnetic field applied along the [(1) over bar, 1, 0] direction parallel to the CuO2 plane induces a "spin-flop" transition, where the noncollinear AF spin structure of PLCCO is transformed into a collinear one. The spin-flop transition is continuous in semiconducting PLCCO, but gradually becomes sharp with increasing doping and the appearance of superconductivity. A c-axis aligned magnetic field that suppresses the superconductivity also enhances the quasi-2D SDW order at (0.5, 0.5, 0) for underdoped PLCCO. However, there is no effect on the 3D AF order in either superconducting or nonsuperconducting samples. Since the same field along the [(1) over bar, 1, 0] direction in the CuO2 plane has no (or little) effect on the superconductivity, (0.5, 0.5, 0) and (Pr, La, Ce)(2)O-3 impurity positions, we conclude that the c-axis field-induced effect is intrinsic to PLCCO and arises from the suppression of superconductivity.

Magnetization losses in multifilament coated superconductors

Abstract: We report the results of a study of the magnetization losses in experimental multifilament, as well as control (uniform) coated superconductors exposed to time-varying magnetic fields of various frequencies. Both the hysteresis loss, proportional to the sweep rate of the applied magnetic field, and the coupling loss, proportional to the square of the sweep rate, have been observed. A scaling is found that allows us to quantify each of these contributions and extrapolate the results of the experiment beyond the envelope of accessible field amplitude and frequency. The combined loss in the multifilament conductor is reduced by about 90% in comparison with the uniform conductor at full field penetration at a sweep rate as high as 3 T/s.

Critical current versus strain measurement up to 21T and 1000A of long length superconducting wires and tapes

Abstract: A device has been developed to measure the critical current of long length superconductors under uniaxial strain up to 1000A and up to 21T The latter is based on a modified Walters spring (WASP) where zero applied strain can be precisely controlled The length of the investigated conductor is on the order of 1m with a typical gauge length between the voltage taps of about 05m This facilitates the measurement of critical currents at an electric field criterion as low as 001μV∕cm, which is particularly important for superconductors to be used in magnets which are operated in the persistent mode The operation of the WASP was simulated by finite element calculations indicating that the radial stress (contact pressure) of the superconductor is less than 5% of the axial stress The performance of the probe is demonstrated for several Nb3Sn conductors with round and rectangular cross section, respectively, as well as for a Bi-2223 tape

Influence of flux creep on dynamic behavior of magnetic levitation systems with a high-T/sub c/ superconductor

Abstract: Significant flux creep may be generated in some high-T/sub c/ superconductors with weak pinning, which could yield an influence on the dynamic behavior of a high-T/sub c/ superconductor-magnet levitation system. To investigate this influence, this article presents a numerical analysis of dynamic features of the levitation generated by an interaction between a high-T/sub c/ superconductor (HTSC) and a permanent magnet (PM) after the flux creep in the superconductor is taken into account in a macro-model of superconductivity. The influence is comprehensively displayed by comparing the predictions of dynamic responses of such systems in which the flux creep in the superconductor is and is not taken into account. The obtained results show that whether or not the flux creep results in a noticeable influence to the levitation of superconductor-magnet systems is mainly dependent upon properties of superconductivity and applied excitation, e.g., critical current density of superconductors, and amplitude and frequency of external excitations. When the critical current density is less than 4.5/spl times/10/sup 8/ A/m/sup 2/, and the system is subjected to a periodic excitation, the influence of flux creep should be taken into account in the theoretical analysis.

Superconductivity and Superfluidity

Abstract: This 1998 book describes the physics of superconductivity and superfluidity, macroscopic quantum phenomena found in many conductors at low temperatures and in liquid helium 4 and helium 3 In the first part of the book the author presents the mean field theory of generalized pair condensation This is followed by a description of the properties of ordinary superconductors using BCS theory The book then proceeds with expositions of strong coupling theory and the Ginzberg-Landau theory The remarkable properties of superfluid helium 3 are then described, as an example of a superfluid with internal degrees of freedom The topics covered are dealt with in a coherent manner, with all necessary theoretical background given Recent topics in the field, such as the copper-oxide high temperature superconductors and exotic superconductivity of heavy fermion systems are discussed in the final chapter This book will be of interest to graduate students and researchers in condensed matter physics, especially those working in superconductivity and superfluidity

Universal scaling relation in high-temperature superconductors

Abstract: Since the discovery of superconductivity at elevated temperatures in the copper oxide materials there has been a considerable effort to find universal trends and correlations amongst physical quantities, as a clue to the origin of the superconductivity. One of the earliest patterns that emerged was the linear scaling of the superfluid density (ρs) with the superconducting transition temperature (Tc), which marks the onset of phase coherence. This is referred to as the Uemura relation, and it works reasonably well for the underdoped materials. It does not, however, describe optimally doped (where Tc is a maximum) or overdoped materials. Similarly, an attempt to scale the superfluid density with the d.c. conductivity (σdc) was only partially successful. Here we report a simple scaling relation (ρs∝σdcTc, with σdc measured at approximately Tc) that holds for all tested high-Tc materials. It holds regardless of doping level, nature of dopant (electrons versus holes), crystal structure and type of disorder, and direction (parallel or perpendicular to the copper–oxygen planes).

MgB/sub 2/ tapes with non-magnetic sheath: effect of the sintering temperature on the superconducting properties

Abstract: A nonmagnetic sheath, namely a Ni/Cr alloy (Ni80/Cr20), was used to fabricate MgB/sub 2/ tapes through the powder-in-tube (PIT) ex-situ technique. A series of samples sintered at different temperatures between 200/spl deg/C and 950/spl deg/C were prepared in order to correlate all the superconducting properties between them and with the sintering temperature. The possibility of using a nonmagnetic but at the same time hard sheath chemically compatible with the MgB/sub 2/ phase made possible a simpler interpretation of transport and magnetic measurements in magnetic field. With the chosen cold working parameters, we found that the amount of MgB/sub 2/ lattice strain was not sufficient to produce any enhancement of the upper critical field. The effect of the heat treatment temperature was on one hand to increase the grain connectivity but also to reduce the pinning properties of the superconductor.

Erratum to Three Papers on the beta-Pyrochlore Oxide Superconductors

Abstract: The magnitude of magnetic fields given in the three papers that report on the superconductivity of the beta-pyrochlore oxides is corrected.

Phonons, electronic charge response and electron-phonon interaction in the high-temperature superconductors

Abstract: We investigate the complete phonon dispersion, the phonon induced electronic charge response and the corresponding self-consistent change of the crystal potential an electron feels as a direct measure of the electron–phonon interaction in the high-temperature superconductors within a microscopic model in the framework of linear response theory. Moreover, dielectric and infrared properties are calculated. The experimentally observed strong renormalization of the in-plane oxygen bond-stretching modes which appears upon doping in the high-temperature superconductors is discussed. It is shown that the characteristic softening, indicating a strong nonlocal electron–phonon interaction, is most likely a generic effect of the CuO plane and is driven by a nonlocal coupling of the displaced ions to the localized charge-fluctuations at the Cu and the Oxy ions. At hand of the oxygen bond-stretching modes it is illustrated how lattice-, charge- and spin-degrees of freedom may act synergetically for anisotropic pairing in the high-temperature superconductors. The different behaviour of these modes during the insulator-metal transition via the underdoped phase is calculated and from a comparison of these generic modes in the different phases conclusions about the electronic state are drawn. For the non-cuprate potassium doped high-temperature superconductor Ba–Bi–O also a very strong and anisotropic renormalization of the oxygen bond-stretching modes is predicted. In another investigation c-axis polarized infrared- and Raman-active modes of the HTSC's are calculated in terms of charge fluctuations and anisotropic dipole-fluctuations. Mode assignments discussed controversially in the literature are proposed. Finally, interlayer phonons propagating along the c-axis and their accompanying charge response are investigated. Depending on the strength of the interlayer coupling calculations are performed ranging from the static, adiabatic response regime to the non-adiabatic regime where dynamical screening of the bare Coulomb interaction and phonon–plasmon coupling becomes relevant within a certain region around the direction of the c-axis. A comparison with the experimental situation is given. Both, the oxygen bond-stretching modes calculated in adiabatic approximation and the non-adiabatic coupled c-axis phonon–plasmon modes are found to be important for pairing via lattice- and charge-degrees of freedom. Favouring aspects to achieve high-temperature superconductivity are also discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Critical state of YBCO superconductors with artificially patterned holes

Abstract: The magnetic behavior of melt-textured YBCO materials containing columnar holes has been systematically investigated with the help of a new "current-tracer" software based on the critical state model, and magnetic Hall probe imaging. The influence of the different geometrical distribution of holes, hole size and hole density on the remanent magnetization profiles and the effective-to-local critical current density ratio is discussed. For the single-crystalline YBCO samples with artificially patterned holes studied, the measured magnetization maps and J/sub c//sup eff//Jc ratios agree nicely with the simulations.

Microwave impedance of YBa2Cu3O7-δ high-temperature superconductor films in a magnetic field

Abstract: The temperature, field, and intensity (amplitude) dependences of the surface impedance of magnetron-sputtered YBa2Cu3O7−δ quasi-single-crystal films on a sapphire substrate with a CeO2 buffer layer were measured. The measurements were performed with a coplanar resonator at 5.25 GHz in a weak constant magnetic field 0

Superconducting State in Electron-doped High-Temperature Superconductors

Abstract: Magnetic and superconducting properties of electron-doped high-temperature superconductors are studied with fluctuation exchange approximation applied to the Hubbard model on a square lattice. In contrast to hole-doped cases, the hot spots, the intersections of the magnetic Brillouin zone boundary and the Fermi surface, are located near the diagonals of the Brillouin zone. This is found to result in (1) a weaker coupling between the antiferromagnetic spin fluctuations and quasiparticles and (2) a peculiar deviation of the angular dependence of the order parameter from a simple form of cos k x a -cos k y a . The latter is consistent with the implication from Raman scattering experiments, and can be direct evidence of superconductivity caused by antiferromagnetic spin fluctuations.

Adiabatic normal zone development in MgB/sub 2/ Superconductors

Abstract: A-priori knowledge of the normal zone development in MgB/sub 2/ conductors is essential for quench protection of applications. Therefore the normal zone propagation in a monofilament MgB/sub 2//Fe conductor under near-adiabatic conditions at 4.2 K has been measured and simulated. The results show normal zone propagation velocities up to several meters per second. In addition, by including the voltage-current relation into the computational model, the influence of the n-value on the normal zone propagation is determined. The simulations show that lower n-values suppress the normal zone propagation velocity due to lower heat generation in the MgB/sub 2/ filaments.

Magnetic Excitations in Bilayer High-Temperature Superconductors with Stripe Correlations

Abstract: The universal behaviour of the magnetic excitations in high-temperature superconductors is described in a model with static stripes retaining only the localized spin degrees of freedom. The stripes are represented by a model of coupled two-leg spin ladders. We start from the results obtained previously by continuous unitary transformations for an isolated spin ladder. A quantitative description of neutron scattering data is reached, using a model for a single cuprate layer with well established values of the exchange coupling constants. The neutron resonance peak is explained in terms of a saddle point in the dispersion of the magnetic excitations. Here we make predictions for bilayer systems with in-phase or out-of-phase stripe correlations. The results may serve as a guide for future experimental analyses.

Topological theory of electron-phonon interactions in high-temperature superconductors

Abstract: There are large isotope effects in the phonon kinks observed in angle-resolved photoemission spectroscopy of optimally doped cuprate high-temperature superconductors (HTSC), but they are quite different from those expected for a nearly free-electron metal (Fermi liquid) with strong electron-phonon interactions (Eliashberg model). These differences, together with many other anomalies in infrared spectra, seem to suggest that other particles (such as magnons) must be contributing to HTSC. Here we use topological methods to discuss the data, emphasizing nanoscale phase separation and the importance of a narrow band of quantum percolative states near the Fermi energy that is spatially pinned to a self-organized filamentary dopant array, resulting in a filamentary glass. Topological discrete, noncontinuum, nonperturbative methods have previously explained the form of HTSC phase diagrams without involving detailed microscopic assumptions, and they are especially useful in the presence of strong nanoscale glassy disorder. These methods also explain the ``miracle'' of an ideal nearly free-electron (gas or liquid) phonon kink in sharply defined nodal quasiparticle states in LSCO at the metal-insulator transition. Careful study of the data reveals anharmonic phonon interactions. Finally, the universality of the kink energy and Fermi velocities below the Debye cutoff in different cuprates is the result of the marginally elastic nature of these configurationally glassy materials, and specifically the isostatic character of the ${\mathrm{CuO}}_{2}$ planes.

Control of porosity and composition in large-area YBCO films to achieve micrometer thickness and high J/sub c/ on sapphire substrates

Abstract: Relatively thick YBCO thin films (thickness d>0.5 /spl mu/m) ideal for fault current limiter as well as microwave applications have been successfully prepared by large-area pulsed laser deposition (PLD) on CeO/sub 2/-buffered sapphire substrates. The attainment of an unusually high film thickness (up to 2.0 /spl mu/m) without microcracking is attributed in part to the presence of pores correlated with yttrium-rich composition in the films. The effect of using YBCO targets with varied Y:Ba:Cu ratios on film porosity and surface morphology was investigated in detail. Using the optimum target composition, uniform critical current densities (J/sub c/) ranging from /spl sim/4.5 MA/cm/sup 2/ at d/spl sim/0.23 /spl mu/m to /spl sim/1.75 MA/cm/sup 2/ at d/spl sim/0.6 /spl mu/m have been achieved. Characterization of a film with d/spl sim/0.5 /spl mu/m showed low microwave surface resistance values [R/sub s/(77.3 K)/spl ap/2.09 m/spl Omega/ and R/sub s/(20 K)/spl ap/227 /spl mu//spl Omega/ at 22 GHz] comparable to the best YBCO films reported by other studies.

Superconducting and mechanical properties of RE-Ba-Cu-O/Ag bulk superconductors

Abstract: Melt-processed REBaCuO (RE: rare earth) superconductors have a high J/sub c/ at 77 K and a high magnetic field, which are expected to be used for high field applications such as superconducting permanent magnets with liquid nitrogen refrigeration, flywheels, current leads and so on. In this study, we discuss Ag-doped YBaCuO and SmBaCuO bulks melt-processed in air and their characteristic superconducting and mechanical properties. Isothermal undercooling growth in air with oxygen annealing and the Nd123/MgO thin film cold seeding technique were applied to seek a highly efficient process, homogeneity of composition, and feasibility of batch production. Single-domain growth of superconducting phases was achieved. The results of mechanical properties, e.g. flexural stress of REBaCuO bulk superconductors show the beneficial effect of Ag particles in the RE123 superconducting phase on strengthening and toughening the material. The results imply that the melt process is feasible for producing larger bulks in large-scale applications. Applications of superconducting technology to railways are also discussed.

Superconducting properties of ultrathin Bi2Sr2CaCu2O8+x single crystals

Abstract: We use Ar-ion milling to thin Bi2Sr2CaCu2O8+x (Bi2212) single crystals down to a few nanometers or one to two (CuO2)2 layers. In decreasing the thickness, the superconducting transition temperature gradually decreases to zero and the in-plane resistivity increases to large values indicating the existence of a superconductor-insulator transition in ultrathin Bi2212 single crystals

Scaling of the vortex-liquid resistivity in high temperature superconductors

Abstract: By considering the competition of thermal activation and effective pinning, a general description of temperature dependent resistivity under magnetic fields has been deduced in high temperature superconductors. The temperature dependent resistivity can be expressed as ρ = ρn(1−x)s, with the energy ratio x = Ueff(T,B)/kBT. This model is a consistent description of vortex glass theory. In the experiments of both single-phase Hg-1223 and Tl-2212 thin film superconductors, the temperature dependent resistivity under various magnetic fields can be scaled onto a straight line by plotting lnρ versus ln((T− Tg)/(Tc− Tg)) with the slope s. This scaling behaviour is striking evidence of the proposed model.

Macroscopic magnetic flux motion in Y-Ba-Cu-O bulk superconductor during pulsed field magnetization

Abstract: Pulsed Field Magnetization (PFM) is one of the key technologies for utilizing RE-Ba-Cu-O bulk superconductors as a strong flux source for various applications. However, the performance of PFM is dependent on the conditions of applied magnetic field, cooling method, etc. and therefore, efficient and effective magnetization is an essential issue for practical use of PFM for high-performance bulk superconductors. In this work, macroscopic magnetic flux motion in Y-Ba-Cu-O bulk superconductor during PFM was observed using five co-axial pick-up coils.

Modulated-bath ac calorimetry using modified commercial Peltier elements

Abstract: We developed a type of ac microcalorimeter based on a modulated-bath technique for measuring the specific heat of small microgram samples in the temperature range from 30 to 300 K, and tested it in magnetic fields up to 14 T. The device is built from a modified commercial Peltier element. The temperature of its top plate can be modulated periodically by the Peltier effect, so that the oscillation is symmetrical about the temperature of the main bath. This avoids the problem of dc offsets which plague conventional ac calorimeters. The sample is attached to a thin thermocouple cross, acting as a weak thermal link to a platform. The absence of a heater reduces the background heat capacity (“addenda”) to a minimum. As an illustrative example of the performance of our device, the specific heat in fields up to 14 T of a small single crystal of the high-temperature superconductor Bi2.12Sr1.71Ca1.22Cu1.95Oy is determined.

Flux pinning enhancement in melt-processed YBa2Cu3O7 with extended nanodefects

Abstract: Melt-processed YBa2Cu3O7 (YBCO), containing a random array of Y–Ba–Cu–Ir–O nanostructures (about 20–150nm in diameter and several μm in length) has been grown. The pinning efficiency of the resulting YBCO nanocomposite is superior to Li-doped or undoped melt-processed YBCO at higher magnetic fields. At 77K, the irreversibility field increases more than 8T in the presence of the extended nanodefects. The increase of the effective activation energy of the YBCO nanocomposite is nearly twice that of the Li-doped material.

Topological Theory of Electron-Phonon Interactions in High Temperature Superconductors

Abstract: There are large isotope effects in the phonon kinks observed in angle-resolved photoemission spectroscopy of optimally doped cuprate high-temperature superconductors (HTSC), but they are quite different from those expected for a nearly free-electron metal (Fermi liquid) with strong electron-phonon interactions (Eliashberg model). These differences, together with many other anomalies in infrared spectra, seem to suggest that other particles (such as magnons) must be contributing to HTSC. Here we use topological methods to discuss the data, emphasizing nanoscale phase separation and the importance of a narrow band of quantum percolative states near the Fermi energy that is spatially pinned to a self-organized filamentary dopant array, resulting in a filamentary glass. Topological discrete, noncontinuum, nonperturbative methods have previously explained the form of HTSC phase diagrams without involving detailed microscopic assumptions, and they are especially useful in the presence of strong nanoscale glassy disorder. These methods also explain the ``miracle'' of an ideal nearly free-electron (gas or liquid) phonon kink in sharply defined nodal quasiparticle states in LSCO at the metal-insulator transition. Careful study of the data reveals anharmonic phonon interactions. Finally, the universality of the kink energy and Fermi velocities below the Debye cutoff in different cuprates is the result of the marginally elastic nature of these configurationally glassy materials, and specifically the isostatic character of the ${\mathrm{CuO}}_{2}$ planes.