We present theory of dc Josephson effect in contacts between Fe-based and spin-singlet s-wave superconductors. The method is based on the calculation of temperature Green's function in the junction within the tight-binding model. We calculate the phase dependencies of the Josephson current for different orientations of the junction relative to the crystallographic axes of Fe-based superconductor. Further, we consider the dependence of the Josephson current on the thickness of an insulating layer and on temperature. Experimental data for PbIn/Ba1−xKx(FeAs)2 pointcontact Josephson junctions are consistent with theoretical predictions for s± symmetry of an order parameter in this material. The proposed method can be further applied to calculations of the dc Josephson current in contacts with other new unconventional multiorbital superconductors, such as Sr2RuO4 and superconducting topological insulator CuxBi2Se3.

We present the derivation of boundary conditions on a wave function at the normal metal/superconductor (N/S) interface by extending the tight-binding approach developed for semiconducting heterostructures [Phys. Rev. 27 (1983) 3519]. Based on these boundary conditions, we formulate a quantitative theory for tunneling spectroscopy in N/S junctions, where a superconductor is characterized by complex non-parabolic energy spectrum beyond effective mass approximation. As an application to single-band unconventional superconductors, we re-derive the known conductance formula [Phys. Rev. Lett. 74 (1995) 3451] with generalized definition of a normal-state conductance. We further apply the model to junctions between normal metals (N) and multi-band iron-based superconductors (FeBS). Our calculations show that tunneling studies of (100) oriented N/FeBS junctions allow to distinguish between the s AE and the s þþ order parameter symmetry in FeBS. In low transparent N/FeBS junctions with the s þÀ symmetry in FeBS, finite energy subgap Andreev bound states are formed due to sign change of pair potential between different Fermi surface pockets. Another fingerprint of the s þÀ symmetry in FeBS is suppressed Andreev conductance in high transparent (100) N/FeBS junctions compared to the case of the s þþ symmetry. Our results may serve as a basis for quantitative tunneling spectroscopy of FeBS. KEYWORDS: superconductivity, multi-band iron-based superconductor, boundary condition, tunneling spectroscopy, Andreev bound state A. V. BURMISTROVA et al. J. Phys. Soc. Jpn. 82 (2013) 034716 FULL PAPERS 034716-2 #2013 The Physical Society of Japan A. V. BURMISTROVA et al.

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