Observation of the Josephson Effect inPb/Ba1−xKxFe2As2Single Crystal Junctions

Abstract

We have fabricated $c$-axis Josephson junctions on single crystals of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ by using Pb as the counterelectrode in two geometries, planar and point contact. Junctions in both geometries show resistively shunted junction $I\mathrm{\text{−}}V$ curves below the $T_C$ of the counterelectrode. Microwave induced steps were observed in the $I\mathrm{\text{−}}V$ curves, and the critical currents are suppressed with an in-plane magnetic field with well-defined modulation periods indicating that the Josephson current is flowing in a manner consistent with the small to intermediate sized 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 ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ and a conventional superconductor suggests the existence of an $s$-wave symmetry in this class of iron pnictide superconductors.