Flux motion, electrical resistance and noise in superconductors

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Abstract
The basic physics of the processes producing flux flow resistance in superconductors is reconsidered. It is shown that in the dynamic intermediate state (in which at low fields flux tubes move through the material) there is no potential difference between the contacts; the measured voltage is produced by magnetic flux outside the superconductor which changes as each of the tubes move. On the other hand, in a type 2 superconductor near Hc2 there is a potential difference between the contacts and no effect of changing magnetic flux. We make the conjecture that in general in the mixed state there are contributions of both types to the voltage measured (V), the fraction Vp/V produced by a potential difference in the superconductor decreasing as the field is lowered below Hc2. In all cases the measured voltage V is the same as if there were an electric field (ν × B)/c in the superconductor, where ν is the average velocity of the flux `entities' and B the flux density in the material. The low-frequency noise in superconductors corresponds to fluctuations in the component of voltage Vi produced by magnetic flux changes. This has the consequence that the noise spectrum should depend on the arrangement of the leads of the measuring circuit and also that the decrease in noise observed with increasing field in the mixed state corresponds to the decrease in the ratio Vi/V and does not necessarily represent a decrease in the size of the flux `entities' with increasing field.