Possible Depinning Transition of a Single Flux Line Near a Columnar Defect in Type II Superconductors

Abstract

We use the Feynman--Kleinert variational method [R.P. Feynman and H. Kleinert, Phys. Rev. A {\bf 34}, 5080, (1986)] to calculate the partition function and effective pinning energy of a single flux line near a columnar pin in type II superconductors. It is found that there is a phase transition between a low temperature phase where the flux line is localized near the columnar pin and where its internal modes fluctuations are bounded, and a high temperature, depinned phase where the flux line is delocalized and its internal fluctuations are those of a free line.