Josephson effect and Andreev reflection in Ba(1-x)NaxFe2As2 (x=0.25 and 0.35) point contacts

Abstract

I(V) characteristics and their first derivatives of ScS and ScN-type (S-superconductor, c-constriction, N-normal metal) point-contacts (PCs) based on Ba{1-x}NaxFe2As2 ($x$=0.25 and 0.35) were studied. ScS-type PCs with S=Nb,Ta and Pb show Josephson-like resistively shunted I(V) curves with microwave induced Shapiro steps which satisfy relation 2eV=hw. The I_cR_N product (I_c-critical current, R_N-- normal state PC resistance) in these PCs is found to be up to 1.2mV. All this data with the observed dependence of the I_c on the microwave power of ScS PCs with Pb counterelectrode indicates the presence of the singlet s-wave type pairing in Ba(1-x)NaxFe2As2. From the dV/dI(V) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap ratio 2\Delta/k_BT_c = 3.6 \pm 1 for the compound with x=0.35 was evaluated. Analysis of these dV/dI(V) at high biases V, that is well above \Delta, testifies transition to the thermal regime in PCs with a voltage increase.