Magnetic microstructure and flux dynamics of high-Tcsuperconductors

We present high-resolution Bitter patterns which reveal the magnetic microstructure of the high-$T_c$ superconductor ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, allowing the direct observation of vortices, and their response to magnetic-field changes. By forming patterns in the presence of a time-varying magnetic field, or a transport current, we also study the dynamics of flux motion. Our results reveal a hierarchy of pinning sites and a remarkable anisotropy in the characteristics of neighboring twins, and allow the mapping of superconducting regions in the presence of circulating and transport currents.