Flux Creep in Hard Superconductors

Abstract

Resistive states of hard superconductors have been investigated by tube magnetization and resistance measurements. The flux-creep theory of Anderson is very effective in accounting for the experimental observations reported herein. Resistive phenomena were observed in the presence of transport current density $J$ and magnetic field $B$ perpendicular to $J$. It is found that the whole spectrum of resistive states can be represented in terms of a single parameter $\alpha =J(B+B_0)$, where $B_0$ is a constant of the material. This parameter represents essentially the Lorentz force or the magnetic pressure gradient in the material. While a wide range of $\alpha$ values is possible, under given experimental conditions superconductivity usually can not be maintained above a critical value. In tube magnetization, the critical value ${\alpha }_c$ is determined primarily by the rate with which the persistent current $J$ decays. If $\alpha$ is raised beyond ${\alpha }_c$, $J$ decays rapidly and $\alpha$ quickly falls near to ${\alpha }_c$. $\alpha$ continues to decrease slowly, but proportional to the logarithm of time as predicted by the theory. The observed temperature dependence of ${\alpha }_c$ is accounted for by the theory. Discrete, stochastic changes in field anticipated from the motion of flux bundles have been detected through pickup coils placed in close proximity to the superconducting tube. In resistance measurements, voltages appearing across 3Nb-Zr wire samples were measured by supplying $J$ externally in the presence of a perpendicular field $H$. The voltage observed is interpreted as a manifestation of an uncompensated emf arising from flux creep. At a given temperature, voltage readings obtained over a wide range of $J$ and $H$ are found to be a function of $\alpha =J(H+B_0)$ only. $V(\alpha , T)$ follows qualitatively a form expected from the theory. In resistance measurements, the critical value ${\alpha }_p$ is determined by the power dissipation in the material. If $\alpha$ is raised beyond ${\alpha }_p$, thermal conduction lags the power dissipation and the sample undergoes a catastrophic transition to the normal state.