En las fronteras de la Materia Condensada

The scope of this work is to study the change of symmetry induced in the hexagonal vortex lattice of high T_c superconductors by engineered nanoscopic structures. A pinning potential with square symmetry was generated by e-beam lithographied Fe dots on the top surface of bulk BSCCO-2212 samples. We use the magnetic decoration technique to visualize the vortex structure in the sample surfaces. A square vortex lattice in the top surface is observed to match the Fe patterned sites when the ratio between vortex and dot density is one. Incommensurability effects varying this ratio are also studied. Taking profit from the continuous character of the vortex lines, the propagation of the square symmetry induced at the top surface was investigated by imaging the vortices at the bottom surface for different sample thicknesses. The square lattice induced at the top surface propagates in a thickness of few microns. At the bottom surface of thick enough samples the natural hexagonal symmetry is recovered. Imaging the vortex lattice in samples of different intermediate thickness allows us to detect with microscopic resolution the interface between phases with symmetries that can not be interconnected through a continuous phase transition.