Oscillator studies of intragrain and intergrain dissipation in Y-Ba-Cu-O and Bi-Pb-Sr-Ca-Cu-O ceramic superconductors

Baar, D.J.; Franck, J. P.; Harrison, John P.; Lacroix, Y.; Yu, M.K.

doi:10.1016/0921-4534(90)90316-7

Cited by 11 publications

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“…Such ac measurements should be performed in a constant applied magnetic field B a in order to allow a linear-response interpretation based on moving Abrikosov vortices and Meissner currents [1][2][3][4][5][6][7][8][9][10]. A related, very sensitive type of experiment measures the damping and the enhancement of the resonance frequency of vibrating superconductors, either in the form of reeds performing flexural vibrations [11] or platelets glued on a vibrating tongue [12] or suspended on wires [13]. Periodic tilting of type-II superconductors is equivalent to the application of an alternating magnetic field perpendicular to B a since the magnetic flux inside the specimen is partly tilted together with the specimen when the vortices are pinned.…”

Section: Penetration Of Magnetic Ac Fields Into Type-ii Superconductomentioning

confidence: 99%

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Penetration of magnetic ac fields into type-II superconductors

1991

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The complex ac penetration depth, surface impedance, and susceptibility are calculated for a superconducting half space and film containing a vortex lattice of arbitrary orientation with respect to the surface. Within linear response theory, full account is taken of the correct boundary conditions (image vortices), diffuse driving force and nonlocal elasticity of the vortex lattice, collective or individual elastic pinning, flux flow (viscous drag), and thermally activated depinning (creep). PACS numbers: 74.30.Ci, 74.30.Gn, 74.60.Ge The discovery of high-7V superconductors has revived the interest in experimental methods which probe dissipation and screening by applying an alternating magnetic field or transport current [1-6]. Such ac measurements should be performed in a constant applied magnetic field B a in order to allow a linear-response interpretation based on moving Abrikosov vortices and Meissner currents [1-10]. A related, very sensitive type of experiment measures the damping and the enhancement of the resonance frequency of vibrating superconductors, either in the form of reeds performing flexural vibrations [11] or platelets glued on a vibrating tongue [12] or suspended on wires [13]. Periodic tilting of type-II superconductors is equivalent to the application of an alternating magnetic field perpendicular to B a since the magnetic flux inside the specimen is partly tilted together with the specimen when the vortices are pinned. All these experiments have in common that the applied ac field interacts with the penetrated vortices only at the surface of the specimen. The alternating field, transport current, or mechanical tilt generate a surface shielding current which exerts forces on the vortices or vortex ends. The resulting deformation of the vortex lattice at the surface then propagates into the interior, pushed forward by the elasticity of the vortex lattice and slowed down by pinning and viscous drag. If the ac perturbation is small, only the elastic interaction of vortices with the pins has to be considered. The elastic restoring force per unit volume OIL, the Labusch parameter [14], even as the critical current density J c or volume pinning force J C B (B denotes flux density), is the result of a collective interaction between the elastic vortex lattice and more or less random pins, e.g., oxygen vacancies. As shown by Campbell [4], for weak pinning and B parallel to the surface, the ac penetration depth is Xc = (C\\/(IL) ]/2 where cn«5 2 /^o is the compressional modulus of the vortex lattice. Similarly, for B perpendicular to the surface, tilt waves of vortices penetrate to a depth (c44/az,) 1/2 -X c where C44 = BBa/po~ B 2 //*o is the tilt modulus of the vortex lattice [14-17]. In the opposite limit, for completely rigid pinning (a L -* <*>), the vortices cannot adjust to an external field change. In this case the superconductor behaves as if it were in the Meissner state, and the ac penetration depth reduces to the London penetration depth X or to X ,=!S X/(\ -B/B C 2) {/2 where B C 2 is the...

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Section: Penetration Of Magnetic Ac Fields Into Type-ii Superconductomentioning

confidence: 99%

“…This pinning nonlocality may become important for BWx since the longitudinal correlation length L C^> R C is large for weak pinning; Eq. (12) with A, ac =^ac(&) then still applies but (13) and (14) are only approximate. Letting k^\^\ one then estimates X 2 C «\c(\ c /L c ) 2^ for L c >\c^>X.…”

Section: Sumed If Ftllj\ Only //// Waves Are Excited; Thus B 2 In (9mentioning

confidence: 99%