Effect of Heavy-ion Irradiation in Co-doped BaFe$_{2}$As$_{2}$*

The critical current density, $J_{c}$, at low temperatures and fields reaches 1x10$^{6}$ A/cm$^{2}$ in the optimally doped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ with $T_{c}\sim $24 K. In order to further improve the current carrying capability, we have irradiated swift heavy-ions into Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. Even with the same ion species and energy as those formed columnar defects in cuprate superconductors, columnar defects are not always created. By irradiating heavier ions like Au and Xe, we have successfully confirmed the creation of columnar defects and resulting enhancement of $J_{c}$. up to 5x10$^{6}$ A/cm$^{2}$ in the case of Au irradiation with a matching field of 20 kG. High-resolution TEM observations reveal the presence of discontinuous columnar defects with diameters of 2-5 nm, which are much smaller that those in cuprate superconductors. Interestingly, the lattice image is sustained in the defect region in contrast to amorphous formation in cuprates. Vortex dynamics in pristine and irradiated Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ will also be discussed.