Thermally activated depinning of the flux-line lattice in high Tc superconductors

This paper reports that as known since Abrikosov's prediction, magnetic flux can penetrate a type-II superconductor in the form of quantized flux lines. An applied current tends to drive these vortices, but moving vortices dissipate energy. In order to achieve zero resistivity, the flux lines have to be pinned at structural defects of the material. In High-Tc superconductors, thermally activated depinning leads to flux creep, thermally assisted flux flow (TAFF), and above an irreversibility line or depinning line Td(B), to completely unimpeded viscous flux motion. Numerous experiments may be explained by TAFF, which means a strongly B and T dependent flux diffusivity D(B,T). The depinning line corresponds to a line D(B,T) = constant, where the constant depends on the length and time scale of the experiment. For small specimens and low frequencies, Td(B) shifts to lower T and B. A still puzzling observation is that in many experiments this depinning line is close to what would follow from a Lindemann melting criterion of the flux-line lattice, though the depinning problem is certainly more complex than melting of a (hypothetical) pin-free vortex lattice