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  • Articles  (10)
  • flux pinning  (10)
  • Springer  (10)
  • 1990-1994  (10)
  • 1950-1954
  • Electrical Engineering, Measurement and Control Technology  (10)
  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 5 (1992), S. 47-53 
    ISSN: 1572-9605
    Keywords: High-T c superconductor ; fluxoid ; quasistatic magnetic field ; flux pinning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The quasistatic magnetic field responses (typically 0.8 Hz) of fluxoids in ac-axis oriented Y1Ba2Cu3O7−δ (Y123) film have been studied using a computerized mechanical pendulum at 77 K. The resonance frequence of the pendulum is significantly shifted to higher values due to the interaction between the Y123 film and a magnet mounted on the pendulum. The contribution of the magnet-superconductor interaction to the shift of the squared resonance frequency shows, in a certain range, a logarithmic dependence on the driving force (proportional to the amplitude). The present results are consistent with the previous studies on the bulk Y123 materials for different configuration.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 5 (1992), S. 127-135 
    ISSN: 1572-9605
    Keywords: High-T c superconductor ; fluxoid ; magnetic stiffness ; flux pinning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Using a computerized mechanical pendulum, we have studied the lateral magnetic flux motion of the Bi-based high-T c superconductors in the quasi-static magnetic field (0.1 Hz) for the range 1 μm to 0.35 mm. The effect of d.c. and a.c. transport currents on the flux motion has also been investigated. We argue that the lateral magnetic stiffness is a measure of the pinning potential of the fluxoids in the superconductors. The pinning energy barrierε is estimated to be about 0.3 eV based on the stiffness measurements, which is comparable with the data from the YBa2Cu3O7−χ superconductors.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 5 (1992), S. 313-318 
    ISSN: 1572-9605
    Keywords: Surface impedance ; complex penetration depth ; flux flow ; flux pinning ; flux creep
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The presence of vortices is known to have an important effect upon the rf surface impedanceZ s of type-II superconductors. In this paper we summarize a new theoretical approach that permits the influence of flux flow, flux pinning, and flux creep uponZ s =R s -iX s to be calculated over a wide range of angular frequenciesω, magnetic inductionsB, and temperaturesT. The normal-fluid excitations and the nonlocality of vortex-generated fields are also accounted for. The physics is conveniently described in terms of a complex penetration depthλ(ɛ, B, T), to which the surface impedance is related viaZ s = −iμ 0ɛλ(ω,B,T). We limit our attention to the linear response of the superconductor in the low-rf-power regime for which the surface impedance is power-independent.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 6 (1993), S. 161-171 
    ISSN: 1572-9605
    Keywords: granular superconductivity ; Josephson effect ; fluxons ; flux pinning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The granular high-T c superconductors can carry very low transport current, in comparison to that found in the bulk of the material. Magnetization and critical current measurements at very low field indicate that this low transport current behaves as expected from a critical state model. The presence of weak links between the grains in granular aggregates is firmly established, together with the Josephson character of such links. The existence of some kind of magnetic particles, and of a mechanism of pinning for them, is required to explain the critical state regime. In this paper we examine the flux structures which can be present in the granular systems, which can be described by an array of SQUIDs, and we show their similarities to fluxons in a continuous medium and to fluxons in uniform Josephson junctions. A simple model, based on a two-dimensional network of pointlike Josepson junctions, is adequate to demonstrate the existance of the IF (intergranular fluxon). Its characteristics depend on a single parameter, which gives the coupling strength between grains. The discreteness of the system is the cause of an intrinsic pinning of the intergranular fluxons.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 6 (1993), S. 185-189 
    ISSN: 1572-9605
    Keywords: Thallium cuprates ; superconductivity ; magnetic hysteresis ; irreversibility ; flux pinning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract dc magnetic hysteresis as well as flux penetration and flux expulsion were investigated in Tl2−y Ba2CaCu2O8−x polycrystals and monocrystals. All measurements were performed at 35 K and in the 0–5 T field range. Hysteresis measurements revealed an irreversibility field of about 2 T. Existing models predict identical field-cooled (fc) and zero-field-cooled (zfc) magnetizations and vanishing time dependence above this field. Although the identical fc and zfc magnetizations are in fact observed, the time dependence vanishes only for flux penetration after zero-field cooling; a remanence is preserved after field cooling and decays with a finite relaxation rate. Activation energies calculated on the basis of the thermal activation model display a pronounced field dependence, and arelower for flux penetration than for flux expulsion in high fields (H≥3 T) for all orientations. This behavior of extreme layered superconductors contradicts classical theoretical models and questions the original definition of the irreversibility line as well. All of our results are consistent with the recent theory of lock-in transition, and can be well interpreted by using those principles.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 6 (1993), S. 243-246 
    ISSN: 1572-9605
    Keywords: High-temperature superconductivity ; flux pinning ; flux motion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Measurements on the temperature and frequency dependence of the complex ac susceptibility of sintered Y1.01Ba1.95Cu2.97O x are reported. The data are used to obtain the flux-creep activation energies at the grain boundaries in the characteristic frequency band. The found nonlinear frequency dependence of these energies can be approximated differently in the low, characteristic, and high frequency bands. The relaxation frequency of thermal fluctuation of the vortex lattice (2πΓ ∼ 3×104 Hz) and the surface current density (J s=0–10 A/cm2) have been estimated.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 7 (1994), S. 675-677 
    ISSN: 1572-9605
    Keywords: Organic superconductor ; anisotropic superconductor ; flux pinning ; lower critical fieldH c1;κ-(BEDT-TTF)2Cu(NCS)2 ; κ-(BEDT-TTF)2Cu(NCS)2
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract The magnetic torque ofκ-(BEDT-TFF)2Cu(NCS)2 was measured as a function of field directionθ with respect to thea *-axis under constant magnetic fields,H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C1, in the irreversible region was found atθ c1 near theH∥bc-plane between 1.3 and 7 K. In addition, extra cusps, C2 and C3, were observed atθ c2 andθ c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component ofH giving each cusp is kept constant as $$H\cos \theta _{cn} = const \equiv H_{cp_n } (n = 1,2,3),$$ i.e., cusps C1, C2, and C3 are ruled by the characteristic field perpendicular to thebc-planeH cp1,H cp2, andH cp3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi2Sr2CaCu2O8. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors.
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  • 8
    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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  • 9
    ISSN: 1572-9605
    Keywords: Vibrating reed ; type-II superconductors ; organic superconductors ; flux pinning ; vortex lattice melting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Vibrating reed measurements were performed on single-crystal samples ofκ-(ET)2- Cu[N(CN)2]Br in an applied magnetic fieldH oriented either parallel (“longitudinal”) or perpendicular (“transverse”) to the highly conducting ac-plane. Field-cooling data taken for the longitudinal orientation andH〈0.7 T revealed a peak with a low-temperature shoulder in the reed dissipation 1/Q located at temperatureT Q below the superconducting transition temperatureT c (≈11.6 forH=0). The shoulder disappeared nearH≈0.7 T, accompanied by an abrupt change in the slope ofT Q (H), corresponding to a similar change in the slope of the upper critical magnetic fieldH c2 measured by Kwoket al. The existence of the shoulder in the dissipation peak bears on a number of current explanations for the exotic superconducting properties ofκ-(ET)2Cu[N(CN)2]Br.T Q data taken for the transverse orientation fell far below estimates ofH c2., indicating the existence of a substantial region of flux line (FL) mobility belowH c2(T). The location of a peak in NMR relaxation observed by De Sotoet al. lies close toT Q for the transverse orientation.
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  • 10
    ISSN: 1572-9605
    Keywords: Vibrating reed ; type-II superconductors ; oxide superconductors ; flux pinning ; vortex lattice melting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Vibrating reed (VR) and dc and ac magnetic susceptibility measurements were performed on a nominal single crystal of composition (Ba0.64K0.36)BiO3 in an applied magnetic fieldH. Field-cooling and field-sweep data revealed multiple peaks in the reed dissipation 1/Q located below the superconducting transition temperatureT c (≈29.6 K forH=0). A shoulder or onset (with increasingT) of dissipation appears forT≤18 K, which may be a signature of a flux lattice melting transition. VR data for dense ceramic samples of composition (Ba0.625K0.375)BiO3 (T c=28.6K) exhibit a relatively narrow and smooth peak in 1/Q that corresponds well to a broad, intermediate-temperature peak in the crystal data. Resistivity experiments demonstrate that the single ceramic peak occurs well below the temperature at which the electrical resistanceR≈0, suggesting that the higher-temperature crystalline peaks are positioned close to the upper critical field line and may be strongly dependent upon grain size or surface properties. Both ac and dc susceptibility data show no clear evidence for multiple phases or gross compositional inhomogeneities in the crystalline sample. Our results demonstrate that the VR technique is an extremely sensitive method to probe sample inhomogeneities and their role in flux pinning phenomena.
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