The intermediate state in superconducting plates

Abstract

A powder technique has been used to study the equilibrium structure of the intermediate state, in flat plates of aluminium subjected to a perpendicular magnetic field. The characteristic domain spacing has been measured as a function of field strength and temperature for three plates of different thickness, and the results are interpreted in terms of a new theory, based on the original theory of Landau but modified to take account of the complicated way in which the individual normal domains are corrugated. Values are deduced for the surface tension parameter ∆ which are in reasonable agreement with those obtained previously by a different method. All the results now available for ∆, in tin as well as aluminium, are compared and discussed in § 6 of the paper. Some measurements have also been made using a slanting field, an ingenious method suggested by Sharvin. A slanting field has the effect of alining the domains in the intermediate state and suppressing their corrugations, so that the theory ought to be simpler for this case. A simple theory, again based on Landau’s work, is given here, but the experimental results indicate that it may not be adequate. Experiments with the field perpendicular are thought to provide a more reliable method for the determination of surface tensions. The kinetic effects that occur when equilibrium is disturbed by altering the strength of the field have also been observed, in tin plates as well as aluminium ones. Tin has the advantage for this purpose of a larger critical field, large enough to shift the powder as the lines of force migrate through the specimen. Theoretical explanations are suggested for a number of the effects; in particular, a formula is obtained for the rate of penetration of flux towards the centre of the specimen when the field is first switched on, which agrees adequately with the observations. Patterns have been obtained with aluminium showing how flux is trapped when the applied field is reduced to zero. They demonstrate convincingly that two superconducting regions are unable to coalesce across an intervening layer of the normal phase.