Observation of the Josephson effect in Pb/Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystal junctions

We have fabricated Josephson junctions using single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ and Pb (or PbIn) as the counter electrode in two geometries. The $c$-plane single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ were synthesized by the Sn-flux method with the nominal composition of $x$ = 0.4. In one junction geometry, Ag (30 nm) and PbIn (200 nm) were evaporated on the surface of the crystals. In the other geometry, a Pb point contact was used. Both geometry junctions show resistively shunted junction $I-V$ curves below the $T_{C}$ of the counter electrode. Microwave induced steps were observed in the $I-V$ curves, and the critical currents are completely suppressible with applied magnetic field in a manner consistent with a small junction limit. $I_{C}R_{N}$ products of up to 0.3 mV have been observed in these junctions at 4.2 K. The observation of Josephson coupling along the $c$-axis between an iron pnictide superconductor and a conventional superconductor suggests the existence of a non-d-wave superconducting order parameter in iron pnictide superconductors.