Twofold role of columnar defects in iron based superconductors

We report on the introduction of columnar defects in Ba(1-x)K(x)Fe(2)As(2)and BaFe2(As1-xPx)(2)single crystals via 1.2 GeV Pb irradiation. Scanning transmission electron microscopy analysis proves the formation of continuous defects along the ion tracks, with a diameter of about 3 nm, and a planar density compatible with the irradiation fluence. The twofold role of such defects, i.e. as pair breakers as well as pinning centers, is investigated by a microwave technique, allowing us to determine critical temperatureT(c), surface impedance and penetration depth lambda(L), and by magneto-optical imaging and superconducting quantum interference device magnetometry to evaluate the critical current densityJ(c). The decrease ofT(c)is quite modest and, together with lambda(L)modifications, testifies the increase of pair-breaking scattering following irradiation. The dependence ofJ(c)on irradiation dose and temperature is due to the pinning landscape induced by the columnar defects, and shows the existence of an optimal irradiation dose to enhance the critical current.