Flux trapping in Josephson tunnel junctions

Effects of flux trapping in the electrodes of Josephson tunnel junctions on junction characteristics are studied experimentally in the case of the field-cooling process. The suppression of the critical current and the increase of the quasiparticle current of the junction due to the trapped flux are measured. From the increase of the quasiparticle current, the amount of the flux trapped in junction electrodes is evaluated. The obtained amount of the trapped flux is discussed quantitatively on the basis of the pinning theory of nonideal type-II superconductors, which gives the parameter of the electrodes characterizing the flux trapping. The relation between the amount of the suppression of the critical current and that of the trapped flux estimated from the quasiparticle current is studied quantitatively. It is shown that each trapped flux quantum suppresses the critical current of the junction by the ratio of about π(6λ_L)²/A, where λ_L is the London penetration depth and A is the junction area.