Abstract
The London penetration depth was measured in optimally doped Ba${}_{0.6}$K${}_{0.4}$Fe${}_{2}$As${}_{2}$ crystals, with and without columnar defects produced by 1.4 GeV ${}^{208}$Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap $\ensuremath{\Delta}{\phantom{\rule{1.0pt}{0ex}}}_{\mathrm{min}}/{k}_{B}{T}_{C}\ensuremath{\approx}1$. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a ${T}^{2}$ power law was observed below ${T}_{C}/3$, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. The data provide evidence for an ${s}_{+\ensuremath{-}}$ pairing state.
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