Probing the current-phase relation in Josephson point-contact junctions between Pb 0.6 In 0.4 and Ba 0.6 K 0.4 (FeAs) 2 superconductors

The Josephson effect in point contacts between an “ordinary” superconductor Pb 0.6In0.4 (Tc≈6.6K) and single crystals of the Fe-based superconductor Ba0.6K0.4(FeAs)2 (Tc≈38.5K), was investigated. In order to shed light on the order parameter symmetry of Ba0.6K0.4(FeAs)2, the dependence of the Josephson supercurrent Is on the temperature and on sin (dφ) with d= 1 , 2 was studied. The dependencies of the critical current on temperature Ic(T) and of the amplitudes of the first current steps of the current–voltage characteristic inexp(P)(n= 0 , 1 , 2) on the power of microwave radiation with frequency f=(1.5÷8)GHz were measured. It is shown that the dependencies Ic(T) are close to the well-known Ambegaokar–Baratoff (AB) dependence for tunnel contacts between “ordinary” superconductors and to the dependence calculated by Burmistrova et al. (Phys Rev B 91, 214501 (2015)) for microshorts between an “ordinary” superconductor and a two-band superconductor with s± order parameter symmetry at certain values of the transparency of boundaries and thickness of the transition layer. It is found that the dependencies inexp(P) cannot be approximated within the resistively shunted model using the normalized microwave frequencies Ω = 2 πf/ (2 eVc/ ħ) with characteristic voltages Vc= IcRN, (RN—normal resistance of the contact) found from the low-voltage parts of the current–voltage characteristics. The reasons for this failure are discussed and a method is proposed for accurately determining the value of Ω , which takes into account all the features of the point contact affecting the period of the dependence inexp(P). An analysis of the Ic(T) and inexp(P) dependencies shows that the superconducting current of the Josephson contacts under investigation is proportional to the sin of the phase difference φ, Is= Icsin(φ). The implications of these results on the symmetry of the order parameter are also discussed.