Pinning energetic analysis of different artificial defects in YBCO using the Bitter decoration technique

R. F. LUCCAS; A. PALAU; X. GRANADOS; T. PUIG; X. OBRADORS

Barcelona

Congreso; International Workshop on Coated Conductors for Applications (CCA2009); 2009

ICMAB

YBCO is a superconductor with a broad field of applications especially since the appearance of second generation coated conductors and all its power applications. Consequently, there have been a lot of efforts to control vortex pinning and vortex dynamics in this type of material. Natural defects like twin boundaries, stacking faults, dislocations, secondary phase,... may act as vortex pinning centers, however one would desire to introduce artificial defects in a controlled manner to tune and manipulate the superconducting properties. In this work we study vortex-defect interaction using the Bitter decoration technique for visualizing the flux line lattice (FLL) obtaining an estimation of the pinning energy from direct measure of vortex positions. Results on YBCO single crystals and films with different intrinsic pinning properties will be shown. In those samples we generated artificial nano-defects using two different methodologies: indentations at nano-scale and irradiation with focus ion beam. From Bitter decoration results we observe that when the artificial defects are generated in single crystals (ordered systems), the FLL symmetry is lost and an increase of vortex density occurs at the nano-defects. Instead, for the case of thin films (disordered systems), an increase in FLL ordering and symmetry is obtained. Finally, an energetic model based on London theory as an approximation of the Ginzburg-Landau solution, with a general analytic expression for the energy of arbitrary arrangements of vortex position[1] has been applied to decorated images to quantify the FLL energy. We observe that the FLL energy increases (decreases) in agreement with the increase of disorder (order) in the system. The vortex pinning energies associated to the introduced nano-defects have consequently been inferred from the energy variation occurring between artificial/natural systems analyzed. [1] E.H. Brandt, Supercond. Sci. Technol. 22, 034019 (2009).Supercond. Sci. Technol. 22, 034019 (2009).