Irreversible Flux Entry in Tin Superconducting Strips: Geometrical Metastability

The nucleation of the normal state in a superconducting tin strip is observed in real time with a low-noise pulse pre-amplifier. For flat strips, the magnetic field at which the flux entry occurs is higher than the thermodynamical critical-field value. This is due to an edge pinning effect against the penetration of normal zones into the bulk. In this letter we establish that using our technique, the determination of the first irreversible flux entry (FIFE) is possible and provides a direct evaluation of the height of the energy barrier against flux penetration. The irreversible nucleation process is discussed.