Onset of flux penetration into a type-II superconductor disk

Кузнецов, А. В.; Eremenko, D. V.; Trofimov, V. N.

doi:10.1103/physrevb.56.9064

Cited by 28 publications

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“…[10,13], see also Refs. [14,15].In this letter the geometry-caused magnetic irreversibility of ideal pin-free type II superconductors is calculated and discussed for the two most important examples of circular disks (or cylinders) and long strips (or slabs) with rectangular profile of arbitrary aspect ratio b/a. I present flux-density profiles and magnetization loops and give explicit expressions for the entry field H en and for the reversibility field H rev above which the magnetization curve is reversible.…”

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confidence: 99%

Irreversible magnetization of pin-free type-II superconductors

1999

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The magnetization curve of a type II superconductor in general is hysteretic even when the vortices exhibit no volume or surface pinning. This geometric irreversibility, caused by an edge barrier for flux penetration, is absent only when the superconductor has precisely ellipsoidal shape or is a wedge with a sharp edge where the flux lines can penetrate. A quantitative theory of this irreversibility is presented for pin-free disks and strips with constant thickness. The resulting magnetization loops are compared with the reversible magnetization curves of ideal ellipsoids.PACS numbers: 74.60. Ec, 74.60.Ge, 74.55.+h The magnetic moment of most superconductors is well known to be irreversible. After Abrikosov's [1] prediction of quantized flux lines it became clear [2] that the magnetic hysteresis is caused by pinning of these vortex lines at inhomogeneities in the material. Flux-line pinning and the related critical state [3] were subsequently confirmed quantitatively in numerous papers [4]. However, similar hysteresis effects were also observed [5] in type I superconductors, which do not contain flux lines but normal conducting domains, and in type II superconductors with negligible pinning. In these two cases the magnetic irreversibility is caused by a geometric (specimen-shape dependent) barrier which delays the penetration of magnetic flux but not its exit. In this respect the geometric barrier behaves similar to the Bean-Livingston barrier [6,7] for vortices penetrating a parallel surface.The geometric irreversibility is most pronounced for thin films of constant thickness in a perpendicular field. It is absent only when the superconductor is of exactly ellipsoidal shape or is tapered like a wedge with a sharp edge where flux penetration is facilitated. In ellipsoids the inward directed driving force exerted on the vortex ends by the surface screening currents is exactly compensated by the vortex line tension [8], and thus the magnetization is reversible. In specimens with constant thickness (i.e. rectangular cross-section) this line tension opposes the penetration of flux lines at the four corner lines, thus causing an edge barrier; but as soon as two penetrating vortex segments join at the equator they contract and are driven to the specimen center by the surface currents, see Fig. 1 below. As opposed to this, when the specimen profile is tapered and has a sharp edge, the driving force even in very weak applied field exceeds the restoring force of the line tension such that there is no edge barrier. The resulting absence of hysteresis in wedge-shaped samples was nicely shown by Morozov et al. [9].An elegant analytical theory of the field and current profiles in thin superconductor strips with an edge barrier has been presented by Zeldov et al. [10], see also the extensions [11]. With increasing applied field H a , the magnetic flux does not penetrate until an entry field H en is reached; at H a = H en the flux immediately jumps to the center, from where it gradually fills the entire strip or disk. This ...

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Irreversible magnetization of pin-free type-II superconductors

1999

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“…by measuring the moment in zero field after successively applying higher external fields and searching for the first deviation from zero at H p [26]. This procedure is still influenced by a finite critical current density [27] and by geometry effects. (sample A) can be converted into H c1 (H p ) for a rectangular sample geometry [28], which leads to H c1 /H p ∼ = 17.7 for γ = 4.5, i.e.…”

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Mixed-state properties of superconductingMgB2single crystals

et al. 2002

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We report on measurements of the magnetic moment in superconducting MgB2 single crystals. We find µ0H c c2 (0) = 3.2 T, µ0H ab c2 (0) = 14.5 T, γ = 4.6, µ0Hc(0) = 0.28 T, and κ(Tc) = 4.7. The standard Ginzburg-Landau and London model relations lead to a consistent data set and indicate that MgB2 is a clean limit superconductor of intermediate coupling strength with very pronounced anisotropy effects.PACS numbers: 74.25. Ha, 74.60.Ec, 74.70.Ad The recent discovery of superconductivity in MgB 2 [1] has attracted a lot of attention. Especially the rather high transition temperature of nearly 40 K in such a simple compound is of interest for applications, but also for an analysis of the physical mechanism leading to superconductivity. Several experiments indicate a phonon mediated s -wave BCS mechanism [2,3]. Different models are proposed to explain the particular properties of MgB 2 [4,5]. Their correctness has to be checked by experiments, but only a few results are available on single crystals [6,7,8,9,10,11,12].We report in this Letter on magnetization measurements on single crystalline MgB 2 in magnetic fields applied parallel and perpendicular to the uniaxial crystallographic (≡ c) axis. A detailed evaluation allows us to obtain the temperature dependence of the most important reversible mixed state parameters, such as the critical magnetic fields, the characteristic lengths, the Ginzburg-Landau (GL) parameter and the anisotropy. We will show that MgB 2 is a clean limit superconductor of intermediate coupling strength with very pronounced anisotropy effects.Several single crystals of MgB 2 were grown using high pressure cubic anvils. Details of the process will be published elsewhere [13]. Two crystals (sample A:3 ) were investigated by magnetic methods. The transition temperature (T c ) of each sample was obtained from the ac -susceptibility measured in a 1 T quantum interference device (SQUID) magnetometer. Sample A shows an onset of T c at 38 K and a rather broad transition of about 1 K. A linear fit of H c c2 vs. T near T c indicates a "bulk transition temperature" of 37.5 K (see inset of fig. 1a). In sample B we find T c = 38.3 K, ∆ T c = 0.3 K and a "bulk T c " of 38.2 K. A simple analysis [14] of the slope of the magnetic moment after reversing the applied field demonstrates that the size of the domain, in which the supercurrents flow without impedance, is identical to the sample size. Furthermore, a comparison of the calculated and the measured magnetization in the Meissner regime indicates a superconducting volume fraction of about 100 %. The further evaluation of the mixed state parameters did not show significant differences between these two crystals.The measurements of the magnetic moment were carried out in the 1 T and in an 8 T (SHE) SQUID magnetometer (for details, cf. [15]). Fig. 1a shows the upper critical field of MgB 2 for applied fields H a c (H c c2 ) and H a ab (H ab c2 ). H c2 (T ) is determined either from the onset of the superconducting signal in the m(T ) curve ("T c (H a )") or ...

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“…Field H g corresponds to the penetration field. 18 Note that, similarly to the Bean-Livingston barrier, 26 one may expect the avalanche-type 27 surmounting of the geometrical barrier by pancake vortices in a layered superconductor. To calculate the probability of this process is the subject of further study.…”

Section: Discussionmentioning

confidence: 98%

“…14 Such a barrier, called the geometrical barrier, is also observed in thin flat samples of a type-II superconductor with weak pinning. [15][16][17][18] The geometrical barrier prevents the penetration of the normal domains into the inner part of the sample when the field increases up to some field H g called the penetration field of the geometrical barrier. 19 Below H g a local field at the sample equator is lower than the critical field, therefore the normal domains cannot cross the equator.…”

Section: Introductionmentioning

confidence: 99%

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Onset of flux penetration into a type-I superconductor disk

Кузнецов¹,

Eremenko²,

Trofimov

1998

Phys. Rev. B

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Virgin magnetization of a superconducting disk in a transverse magnetic field is discussed. A field in which a metastable state arises in samples due to the geometrical barrier is calculated on the base of both the Landau theory of the intermediate state and the Hao-Clem description of the mixed state. It was found that this field agrees well with the penetration field measured for thin flat samples of type-I superconductors with weak pinning.

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Onset of flux penetration into a type-II superconductor disk

Cited by 28 publications

References 44 publications

Irreversible magnetization of pin-free type-II superconductors

Irreversible magnetization of pin-free type-II superconductors

Mixed-state properties of superconductingMgB2single crystals

Onset of flux penetration into a type-I superconductor disk

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