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. Reasonable agreements result from further comparisons of ⟨f ²⟩_1/2 with K ₀ at B = 0 calculated from superconductivity parameters and also with K ₀s measured by Antesberger and Ullmaier (1974) on the alloy NbTa which has a precipitate structure similar to that of the Nb samples.