Flux Pinning/Heat Generation in Type II Superconductors

[DukeLuke]

I don't understand the mechanism by which "flux tubes" create heat, and are pinned by impurities in a type II superconductor. When Bc1<B<Bc2 I understand magnetic flux will penetrate the material in the form of flux tubes. Then the current in the superconductor exerts a force on these tubes driving them through the material and generating heat. My first question is how does this movement create heat. It's not apparent to me why this is the case.

I also understand these tubes are "pinned" by impurities in the material (stopping their movement). I don't understand how these impurities keep the flux from moving. I know in the case of NbTi the Ti precipitates are responsible for blocking the flux movement, but I'm not sure how they do it. I'm having a hard time finding resources to answer these questions, so suggestions on papers/books would be great if this post is too much to answer.

 

[BigJDubs]

The mechanism by which flux tubes create heat in type II superconductors is through the Joule heating effect. When a current flows through a material, it creates an electrical resistance, which results in a conversion of some of the energy from the current into thermal energy. In type II superconductors, the current is confined to the flux tubes due to the strong repulsive force between them and the superconductor. The movement of these flux tubes within the superconductor generates eddy currents, which results in an additional conversion of energy into thermal energy. The impurities in the material act as pinning centers for the flux tubes, preventing them from moving and dissipating energy. The pinning forces are generated by the interaction between the impurities and the magnetic field created by the flux tubes. This interaction creates a potential energy barrier that prevents the flux tubes from moving. For more information on this topic, please see the following papers: 1. S. B. Fields, “Flux Pinning in Type II Superconductors”, Journal of Low Temperature Physics, vol. 19, pp. 301-325 (1975).2. W. M. Visscher, “Flux Pinning in Type II Superconductors”, Reviews of Modern Physics, vol. 65, pp. 883-919 (1993).