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  • Articles  (21)
  • YBCO  (11)
  • resistivity  (10)
  • Springer  (21)
  • 1990-1994  (18)
  • 1985-1989  (3)
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  • Electrical Engineering, Measurement and Control Technology  (21)
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  • Articles  (21)
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  • Springer  (21)
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  • 1
    ISSN: 1572-9605
    Keywords: Critical current ; high temperature superconductor ; thin film ; angular dependence ; magnetic field dependence ; YBCO ; metalorganic preparation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Values of the transport current have been obtained as a function of angle of the magnetic field to the plane of the thin film YBa2Cu3O7−d (YBCO) prepared by metalorganic deposition. Current flowed in thea, b planes at various angles to the applied magnetic fields to 4.2 T. Films with thicknesses near 350 nm were studied. For these thin films andH∥c a secondary maximum is observed whenJ c〉 1 MA/cm2. NearH∥a, b a large increase inI c is observed, and the sharpness, field dependence, and field directional dependence is discussed. The response to currents aboveI c is determined and the fit to a power lawV∼I n shows a strong correlation betweenI c andn.
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  • 2
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    Journal of superconductivity 7 (1994), S. 345-348 
    ISSN: 1572-9605
    Keywords: Tunneling ; Y1−xPrxBa2Cu3O7−δ ; resistivity ; energy gap
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Many different forms of superconducting tunneling have been attempted with the high-T c cuprates. For these studies to be convincing, it is important to assure that elastic tunneling is the dominant conduction mechanism in the structures considered. Some simple criteria to determine whether this is the case are reviewed, and then our results (which satisfy the criteria) on the system Y1−xPrxBa2Cu3O7−δ are discussed. In this study we show that with increasing Pr concentration, the material becomes more gapless.
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  • 3
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    Journal of superconductivity 7 (1994), S. 481-483 
    ISSN: 1572-9605
    Keywords: High-T c superconductors ; YBa2Cu3O7 ; infrared ; plasmon ; resistivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The temperature dependence observed in the mid- and near-infrared optical properties of YBa2Cu3O7 is explained in terms of the Drude model for free charge carriers. In the Drude model, the linear temperature dependence of the dc resistivity arises from the free charge carriers having a temperature-dependent mean free path. This temperature dependence results in the plasmon contribution to the dielectric constant having a damping coefficient which also varies linearly with temperature. We find that the temperature dependence which is observed in the absorption and reflection spectra of YBa2Cu3O7 is consistent with this simple model.
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  • 4
    ISSN: 1572-9605
    Keywords: YBCO ; high field ; critical field ; resistance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We have measured the upper critical fieldH C2(T) of YBa2Cu3O7 (YBCO) thin films in magnetic fields up to 140 T forH applied parallel to thec-axis of the film. The critical field in the zero-temperature limit is 138 T, and the temperature dependence does not fit any simple model.
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  • 5
    ISSN: 1572-9605
    Keywords: Critical current density ; Ag addition ; grain alignment ; flux pinning ; defect structure ; YBCO ; melt-texturing
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Silver has been previously added to the melt-textured YBa2Cu3O7−x in order to increase the critical current density (J c ) of these materials. However, the effect of this addition on theJ c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa2Cu3O7−x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJ c of the MTG YBCO/Ag samples is more than 104A/cm2 under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJ c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.
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  • 6
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    Journal of superconductivity 7 (1994), S. 159-164 
    ISSN: 1572-9605
    Keywords: Uniaxial pressure dependence ; YBa2Cu3O7−δ ; resistivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We present measurements of the uniaxial pressure dependence ofT c of untwinned YBa2Cu3O7−δ crystals with various oxygen stoichiometries. For all samples investigated,T c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La2−x Sr x CuO4 and YBa2Cu4O8 systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.
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  • 7
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    Journal of superconductivity 7 (1994), S. 251-255 
    ISSN: 1572-9605
    Keywords: Specific heat ; impurity ; defects ; YBCO ; superconducting volume fraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT c , on a number of YBa2Cu3O7 samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu2+ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.
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  • 8
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    Journal of superconductivity 7 (1994), S. 261-264 
    ISSN: 1572-9605
    Keywords: Overdoped Tl2Ba2CuO6+δ ; electronic heat capacity ; resistivity ; pair breaking
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Using a high-precision differential technique with a resolution of 1∶104, we have measured the heat capacity of Tl2Ba2CuO6+δ over a temperature range 2–300 K for 0≤δ≤0.1. Anomalies atT c are seen for all superconducting compositions measured, and the results are consistent with a temperature- andδ-independent normal-state electronic termγ n∼0.6 mJ/g-at. K2. The samples with the largerT c 's exhibit strong fluctuations in their specific heat, typical of a highly anisotropic 2D superconductor, but there is some evidence that these fluctuations become weaker atT c falls-consistent with an increase in the coherence length on hole doping. At temperatures below 5 K an upturn in the data is observed which appears to increase in magnitude asT c falls, correlating with the increase in the Curie term of published susceptibility data.
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  • 9
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    Journal of superconductivity 7 (1994), S. 599-605 
    ISSN: 1572-9605
    Keywords: Bipolarous ; spin ; NMR ; Hall effect ; resistivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We extrapolate the BCS theory to the strong electron-phonon and (or) electron-spin fluctuation interaction and show that in the strong-coupling limit the ground state is a charged Bose liquid of lattice and (or) spin bipolarons. Kinetic and thermodynamic properties of charged bosons on a lattice in the normal and superconducting states are discussed, and some evidence for the model is given from NMR, neutron scattering, near-infrared absorption, Hall effect, resistivity, thermal conductivity, isotope effect, heat capacity, and critical magnetic fields of high-T c oxides. The maximum attainableT c is estimated to be in the region of the transition from the Fermi liquid to a charged Bose liquid (polaronic superconductivity). The proposed theory is not restricted by low dimensionality and might be applied to cubic oxides such as the “old” BaPbBiO and to alkali-doped C60 as well.
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  • 10
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    Journal of superconductivity 7 (1994), S. 823-828 
    ISSN: 1572-9605
    Keywords: YBCO ; laser plume ; emission spectroscopy ; absorption spectroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The plasma produced during laser ablation deposition of thin film YBCO has been studied by optical emission spectroscopy. There is evidence of increased YO band emission in the range 590–625 nm as the ambient oxygen gas pressure confining the plume is increased in the range 30–200 m Torr. Temporal profiles show that close to the target the plume is insensitive to ambient oxygen pressure. It is deduced that the optical emission here is excited by electron impact excitation. Further away from the target there is evidence that two distinct processes are at work. One is again electron excitation; the emission from this process decreases with distance because the expanding plume cools and collisions become less frequent in the expanding gas. The second is driven by oxidation of atomic species expelled at high speeds from the target. The main region of this activity is in the plume sheath where a shock front ensures heating of ambient O2 and reaction of monatomic plasma species to form oxide in an exothermic reaction. Spatial mapping of the emission demonstrates clearly how increasing oxygen gas pressure confines the plasma and enhances the emission intensity from the molecular YO species ejected from the target in a smaller region close to the target. Ba+ is observed as a dominant species only very close to (within 1 mm of) the target. Absorption spectra have been taken in an attempt to examine ground state and cool species in the plume. They reveal the quite surprising result that YO persists in the chamber for periods up to 1 msec. This suggests an explanation for the recent report of off-axis laser deposition in terms of simple condensation. Previously, quasi-ballistic transfer of material from target to substrate has been considered the only significant process.
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  • 11
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    Journal of superconductivity 7 (1994), S. 865-869 
    ISSN: 1572-9605
    Keywords: Symmetry of the pairing ; gaplessness ; YBCO ; cuprates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract There are conflicting experimental data related to the symmetry of the order parameter in cuprate superconductors. Our previously developed concept, which contains unusuals-wave pairing with a two-gap spectrum, energy gap anisotropy, and gaplessness for a nonstoichiometric state, provides a unified and consistent description of experimental data related to the symmetry.
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  • 12
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    Journal of superconductivity 7 (1994), S. 943-946 
    ISSN: 1572-9605
    Keywords: YBCO ; optical constants ; attenuated total reflection ; ellipsometry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Otto configuration attenuated total reflection (ATR) measurements of the excitation of surface plasmons in the infrared have been carried out on YBCO films deposited on MgO (100) substrates. The dielectric constants for YBCO at 3.392μm are determined to be −10+15i forc-axis material. The anisotropic nature of the cuprate is seen from films with other orientations: nearlya-axis material has constants of 4.0+7.0i. It is thus not metallic in its optical response along thec-axis which lies parallel to the substrate plane. Ellipsometric measurements in the visible onc-axis material point to a maximum surface plasmon energy of 1 eV.
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  • 13
    ISSN: 1572-9605
    Keywords: YBCO ; dc SQUID ; high-T- c junction ; step edge junction ; Josephson effect
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We describe the fabrication and testing of dc SQUIDs (Superconducting QUantum Interference Devices) obtained by photolithographic patterning of YBa2Cu3O7−x thin films deposited both on SrTiO3 and MgO substrates. The Josephson junctions in the superconducting loop are of the “step-edge” type, where the weak link is obtained through the growth of grains with different orientations across suitably prepared steps previously etched on the substrate surface. TheI−V characteristics of the devices tested show multiple branches and instabilities having a weak dependence on the external magnetic field, probably due to formation of junction clusters on the substrate step. The SQUID devices showed quantum interference behavior and an easily detectable voltage modulation with the applied magnetic field at a temperature of 77 K.
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  • 14
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    Journal of superconductivity 6 (1993), S. 313-316 
    ISSN: 1572-9605
    Keywords: YBCO ; oxygen defects ; point defects ; superconductors ; tetragonal phase
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The manner in which oxygen is incorporated into YBa2Cu3O x (YBCO) at ∼800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O i x . The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O2), as a function of compositionx and temperatureT (5.99≤x≤ 6.35, 825≤T≤1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O i x are noninteracting forx ≃ 6. (b) The stoichiometric composition of YBCO isx ≃ 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O2. (d) The Fermi level changes by ≃ −0.2 eV asx increases from 6.05 to 6.35.
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  • 15
    ISSN: 1572-9605
    Keywords: LiF-doped TlCaBaCuO ; resistivity ; susceptibility
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Measurements of the superconducting resistance as a function of temperature were performed using the conventional four-probe method. The transition to complete superconductivity was recorded for samples of (Tl2Ca2Ba2Cu3O10)100−xLiF x (2223) mixed with different LiF ratiosx=0, 2, 4, 5, 6, 8, 10, and 12 wt.%. It was found that the transition temperatureT c was increased up to 5 wt.% of LiF. Further addition of LiF decreasesT c . Therefore, 5 wt.% LiF is the optimum concentration giving a transition temperature of 130 K. Measurements of the superconducting resistance of all the samples except the (1111) compound show that the addition of 5 wt.% LiF increasesT c and decreases the metastable phases. The real part of the a.c. magnetic susceptibilityϰ a.c. is studied using a zero-field cooled mechanism. The temperature dependence ofϰ a.c. for the prepared TlBaCaCuO having stoichiometric composition of (1111), (2223), (2234), and (3245) and that after doping with 5 wt.% LiF showed a broad feature. The transition to the complete diamagnetic state takes place in a broad transition region containing many transition steps, indicating the presence of metastable phases. The addition of LiF decreases the fluctuation in the transition region and its effect in reducing the number of multiphases.
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  • 16
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    Journal of superconductivity 5 (1992), S. 67-74 
    ISSN: 1572-9605
    Keywords: Flux flow ; phase slip ; Josephson junctions ; resistivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Several critical experiments have shown that the so-called “background” magnetoresistance which occurs with no Lorentz force activation is isotropic in thea-b plane. This isotropic resistivity is linked to paradoxical behavior of vortices which cannot be changed by pinning, a result which may imply intrinsic pinning. Nevertheless, a number of flux-flow observations indicate that vortices may also behave as if they were unpinned. An explanation is proposed which states that this behavior arises as a consequence of defects in oxygen ordering in the Cu-O planes of HTSC crystals; this explanation involves segmentation of the Cu-O planes, and Josephson coupling between adjacent segments. The effective junction areas are seen to be very small, with dimensions characteristic of the size of the unit cell. Thermal motion of nano-scale weak links resolves the pinning paradox by inducing phase-slip resistive dissipation. The resulting phase-slip resistivity is shown to have axial symmetry, that is, the resistivity is independent of the angle between the current and the applied field, with both in thea-b plane.
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  • 17
    ISSN: 1572-9605
    Keywords: YBCO ; Ho addition ; critical current density
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract (YHo)Ba2Cu3O7−y is prepared by the powder melting process and its properties are investigated. It is found that Ho addition can change the melting temperature of a Y-system superconductor. TheJ c values of YBCO can be improved by Ho substitution, which is due to the effect of Ho addition on the number and quality of grain boundaries and the size and distribution of 211 particles. The highestJ c value was found to be 50,000 A/cm2 (77 K, 1T,H⊥I) in Y0.8Ho0.2Ba2Cu3O7−y. Our results indicate that the number and quality of grain boundaries have a great influence on the superconducting properties. A discussion is given on the relationship betweenJ c and the size and phase proportion of 211 particles. There are a lot of dense twins in the sample, and many crystal defects exist around the 211 phase.
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  • 18
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    Journal of superconductivity 5 (1992), S. 445-449 
    ISSN: 1572-9605
    Keywords: YBCO ; microbridge ; lithography ; etching
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Microbridges of YBa2Cu3O7 thin films have been fabricated by conventional photolithography and wet chemical etching using EDTA, and by the lift-off lithography technique. The variation of etch rate with etch time, etchant temperature, and post-deposition sintering temperature has been studied. It has been shown that both techniques are useful for film patterning. However, an additional sintering step is necessary for the chemically etched sample to regain the original film properties. An order of increase in critical current density is observed for the patterned film.
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  • 19
    ISSN: 1572-9605
    Keywords: high-T c superconductivity ; resistivity ; melting ; oxygen desorption ; metal-insulator transition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract We have investigated the temperature dependence of the electrical transport and the thermogravimetric properties, from -200°C to +1000Ć, of the Bi-Sr-Ca-Cu-O high-temperature superconductor. We conclude that this system has a cooperative, simultaneous-melting/oxygen desorption/metal-insulator transition that occurs near 900°C. We speculate on its nature and on its relationship to phenomena found in other high-T c superconductors.
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  • 20
    ISSN: 1572-9605
    Keywords: YBa2Cu3O7−δ ; resistivity ; thermoelectric power
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Four-probe resistivity (ρ) and thermoelectric power (TEP) measurements were carried out on samples of YBa2Cu3O7−δ up to 950°C, in air and in flowing oxygen at 1 bar. Below 700 K the TEP is small and increases rapidly above it, reaching, at 1200 K, +140μV/K in air and +120μV/K in oxygen. At the changeover temperature (700 K) the slope of logρ vs.T changes abruptly. These results are interpreted in terms of a model of transport of carriers in a narrow band, which is full forδ = 1 and half-filled forδ = 0. Possible origins for such a narrow band are discussed in detail.
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  • 21
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    Journal of superconductivity 1 (1988), S. 451-461 
    ISSN: 1572-9605
    Keywords: Y-Ba-Cu-O ; BaCuO2 ; Ba2Cu3O5+δ ; resistivity ; superconducting phases ; baseline hysteresis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Series of Y-Ba-Cu-O compositions were prepared from barium cuprates and Y2O3, using a two-step synthesis route. It has been shown that Ba2Cu3O5+δ is essential in formation of the YBa2Cu3O7−δ superconducting phase while BaCuO2 is not an appropriate starting material. A wide composition range fromx=2 to 17 was prepared from Ba2Cu3O5+δ in the YBa x Cu1.5x O z series without disappearance of macroscopic superconductivity atT c〉77 K. Resistivity measurements hint at the existence of two superconducting phases. ESR investigations revealed a baseline hysteresis, depending on the actual value ofx.
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