P.H. Kes

Calculations on the reversible and irreversible magnetic behavior of low-kappa type-II superconductors

A simple algebraic expression for the reversible magnetization curve in low-κ type-II superconductors is proposed. This model has been checked on the experimental results of very pure Nb and V, as measured by several authors. A quite good agreement is found forT/Tc>0.4; for lower temperatures the agreement becomes worse, but this model still seems better than any other model known thus far from the literature. Using this relation together with a critical-state model it is possible to calculate the irreversible magnetization curves in increasing and decreasing fields betweenHc1 andHHcH2. The results are compared with the experimental data we obtained on a rectangular Nb sample. It turns out that Goedemoeds pinning model gives a good description. Also, a method is described to obtain the flux density gradient as a function ofB directly from magnetization measurements. This provides a more sensitive method to compare the experiments with critical-state models.

The irreversible behaviour of superconducting niobium wires

The magnetic moment of an impure niobium sample was measured simultaneously with the heat development during variation of an external magnetic field along the axis. This was done at several temperatures. The results were interpreted by means of a tentative relation in which the pinning force is assumed to be proportional to (Hc2-B), the coefficient being proportional to (1 − t4). The heat development and the enthalpy, which are derived from the experiment, can be described using the same relation and an additional assumption about “local reversibility”. The flux jump region, which is observed below 3.5°K, is discussed in terms of the occurrence of a critical current or force density. No satisfactory interpretation of the formulae could be proposed on the basis of a model for the mixed state.

Thermal conductivity of niobium in the purely superconducting and normal states

AbstractThe thermal conductivity λ of four niobium samples has been measured between 1 and 10 K, both in the superconducting and normal states. The specimens differed in their crystal defect structures due to annealing at different temperatures (dislocations, grain boundaries) and, in one case, to subsequent fast neutron irradiation (dislocation loops). A procedure has been developed with which the electron and phonon contributions to the thermal conductivity can be separated with an accuracy not hitherto obtainable. All the samples proved to have the same energy gap at 0K:δ(0)=(1.95±0.02)kTc. The phonon conductivity in the superconducting stateλps has been compared with the formula of Bardeen, Rickayzen, and Tewordt extended for scattering mechanisms other than phonon-electron interaction. For the unirradiated samples at

Computations on the dimensional crossover in collective pinning

{\text{T}} \lesssim 0.15T_{\text{c}}

Influence of oxygen diffusion profiles on the surface barrier of superconducting niobium

, λps is proportional toT2, showing that dislocations are mainly responsible for the phonon scattering. The results are qualitatively in agreement with the theory of Klemens, giving a rough indication that the grain boundaries may be considered as arrays of line dislocations. Dislocation loops introduced by the neutron irradiation turn out to behave like clusters of point defects. A second consequence of the irradiation is an enhancement of the original dislocation scattering term.

The effect of neutron irradiation damage on the magnetic behavior of superconducting niobium in alternating fields

Abstract Xeutron irradiation at reactor ambient temperature gives rise to a peak in the magnetization curve of superconducting niobium near the upper critical field HC 1. It is correlated to a maximum in the critical current density Jc and it is referred to as the ‘peak effect’. The magnitude depends strongly on the irradiation dose. The peak effect can be explained qualitatively by a theory proposed by Pippard but quantitatively the agreement is not very good. A modification of Pippards theory in which the pinning is assumed to be caused by point defects does not give any improvement, but a modification introduced by Campbell and Evetts, based on pinning by line defects, leads to results in very close agreement with our experimental data.

SOME DIFFICULTIES ENCOUNTERED WITH SUPERCONDUCTING NIOBIUM--ZIRCONIUM COIL MAGNETS.

The displacement correlation function introduced by Larkin and Ovchinnikov for pinned flux lines in a superconducting layer is evaluated explicitly in order to study the thickness dependence. Expressions for the transverse and longitudinal correlation lengthsRc andLcare derived and a criterion for the crossover from two- to three-dimensional collective pinning is obtained. Using these expressions, computations are carried out for amorphous superconductors.Lcturns out to drop exponentially fast in three dimensions, whileRcchanges gradually through the crossover remaining close to its 2D value. The explicit thickness dependence is logarithmic, so that the pure 3D case is only accessible in a very limited experimental regime.

Influence of voids on the superconducting properties of vanadium

Abstract From the simultaneous measurements of the magnetization and the heat development during the magnetization process of a second type superconductor it is possible to calculate the enthalpy. This is done for an impure niobium sample. As a result Gorters criterion for flux jumping can be checked. It appears that this criterion holds in the first quadrant of the hysteresis loop.