Observation of nucleation and annihilation of flux-lines with opposite sign in high-T_c superconductors

Using the high-resolution Faraday effect (HRF), we observed nucleation of domains in high-T_c single crystals consisting of flux-lines with opposite signs while reducing the applied external magnetic field to zero. This nucleation of domains with reversed flux-lines is found in YBa₂Cu₃O_7-δ single crystals as well as in Bi₂Sr₂CaCu₂O_7-δ single crystals. The resulting domain pattern in the remanent state is then composed of domains with opposite polarity which are seperated by an annihilation zone. An experimental proof for the existence of opposite signs of the vortices has been given by observing the behaviour of the domains on the direction of the applied external magnetic field. We also show the dependence of the width of the annihilation zone on the external magnetic field, indicating the role of the pinning forces on this behaviour.