Pinning In Superconductors Research Articles

Intrinsic Pinning in Superconductors with Extremely Small Coherence Lengths

AbstractBy means of a Ginsburg‐Landau‐type theory which takes into account the discrete lattice structure the variation of the energy (“Peierls potential”) of an isolated flux line is calculated when shifted relatively to the crystal lattice. In particular, a primitive cubic lattice is considered with a straight flux line, aligned parallel to a cube axis. The resulting Peierls potential may lead to intrinsic pinning if the coherence length is smaller than about two nearest neighbour distances. The coherence lengths at low temperatures determined for the recently discovered high Tc superconductors of the YBa2Cu3O7 class come very close to this value so that intrinsic pinning might possibly be relevant for these superconductors.

超電導体における磁束ピンニングの実験研究に関する最近の問題点

A review is given on recent topics on experimental studies of flux pinning in superconductors, while theoretical problems were discussed in the preceding review article. Here we compare experimental results with theories on the elementary pinning force and the summation problem.Quantitative comparison of the elementary pinning force is not possible at this moment because of ambiguity in the summation theory. The dependence of the specific pinning force of grain boundaries on the impurity parameter shows a similar tendency to the calculations by Zerweck, Yetter et al., and Welch based on the electron scattering mechanism. This observed tendency is different from the prediction from the microscopic theory of Thuneberg. The summation theories are compared with experiments within a regime of strong pinning where conventional calculation of the elementary pinning force holds safely. Observed pinning forces seem to obey the linear summation derived from the mean field approximation theory.The other topics such as the saturation phenomenon and the strain scaling are also discussed.

超電導体における磁束ピンニングの理論に関する最近の問題点

A review is given on recent topics on theories of flux pinning in superconductors. It covers the problem of estimate of elementary pinning forces of individual pinning centers interacting with flux lines and the summation problem to calculate the global pinning force density.As pointed out by Thuneberg, the elementary pinning force of pinning centers much smaller than the BCS coherence length should be reestimated by taking account of the electron-scattering effect. And calculations of the elementary pinning force of grain boundaries from the electron-scattering effect are briefly reviewed. Their dependence on the impurity parameter is opposite to the theoretical result of Thuneberg.The summation theories are reviewed and a significance of instabilities of flux lines appeared in the Labusch theory is emphasized. Then, recent two theories, the Larkin-Ovchinnikov theory and the mean field approximation theory are compared and problems contained in two theories are discussed.

Effective volume of voids for flux pinning in superconductors

Flux pinning by voids in niobium and vanadium has been investigated. It is discussed how the collective pinning theory of Larkin and Ovchinnikov may be applied in the case where plastic defects mainly determine the disorder in the flux-line lattice. It is argued that the macroscopic pinning force above its maximum close to the upper critical field is described by the amorphous limit of the collective pinning theory. This provides a means for comparing the elementary interaction, deduced from experiment, with the recent nonlocal theory of Thuneberg et al. that predicts an appreciable enhancement of the effective void volume for pinning. Good agreement is obtained for niobium; the results of vanadium are satisfactory.

Summation curves for flux pinning in superconductors

Flux pinning in irradiated superconductors having either dislocation-loop or void microstrutures is reexamined. The elementary interaction force fp can be accurately computed for such defects if their size is known from transmission electron microscopy. Data exist for a range of defects for which fp varies by over a factor of 104. If the data for voids and dislocation loops are plotted as Q, the volume pinning force per defect, versus fp at a given reduced induction b, the sets of data for each defect species overlap to form a single summation curve. This curve is not correctly described by either of the two most widely used summation models, the direct summation or the statistical summation of Labusch. The empirical summation curve runs parallel to, but a factor ?4 below, the direct summation curve at high fp but bends to decrease approximately as f2p at low fp. Increasing b produces a shift in the summation curve, parallel to the direct summation Q=fp line, by a factor which is proportional to (1−b)2. This shift is the origin of the peak effect which appears at high b for low fp defect microstructures. The failure of the statistical theory is thought to result from the presence of flux-line dislocations in the flux-line lattice which produce both a large decrease and a dispersion in the local threshold force necessary for pinning centers to be effective.

Determination, by neutron diffraction, of basic interaction forces for flux pinning in superconductors

Abstract A new method has been developed for the determination of the basic interaction force exerted on the flux lines in type II superconductors. From neutron diffraction by flux line lattices in the two-phase system Nb with Nb2N precipitates of a well defined metallurgical structure, it is observed that the flux lines are bent due to the presence of pinning centres. A measure for the bending is the width of the rocking curve which is rather directly related to the mean basic interaction force ⟨f 2⟩1/2. For four specimens with different number densities of precipitates from 1·3 × 1017 to 1·3 × 1018 m−3 the force ⟨f 2⟩1/2 has been evaluated as a function of the flux density B from the angular widths of the measured rocking curves. These widths ranged from 6’ to 4°. The basic interaction forces were independent of the number density; at B/B c2=0·5 a value ⟨f 2⟩1/2=2 × 10−10 N was obtained. The maximum interaction force K 0 obtained from the pinning force densities for one sample agreed well with ⟨f 2⟩1/2....

Fluxoid pinning in superconductors by a periodic array of magnetic particles

We assume a two-dimensional array of ferromagnetic particles on the surface of a thin type-II superconductor in a transverse magnetic field. With a particle at the center of each fluxoid, the pinning forceP and critical current increase ΔJ c due to the particles are calculated. General expressions without arbitrary parameters are given forP and ΔJ c as functions of magnetic field strength and the characteristics of the superconductor and the particles. For broad ranges of conditions values ofP and ΔJ c greater than 10−7 dyn and 105 A/cm2 result and for certain physically realistic sets of conditions,P>10−6 dyn and ΔJ c>106 A/cm2 are predicted. The above effects should be readily observable. An experiment for setting up the assumed conditions is proposed and discussed in some detail.