Vortex Structure in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and Evidence for Intrinsic Pinning

Using the high-resolution Bitter-pattern technique, we have studied the vortex structure in crystals of the high-$T_c$ superconductor, Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, for fields parallel to the $c$ axis. We describe the vortex pinning by twin boundaries and show that a vortex lattice develops in twin-free regions. But the results also show directly that vortices are pinned at low temperatures even in crystals which are essentially defect free. We believe this pinning must be intrinsic to the new superconductors.