Evidence for two-gap superconductivity inBa0.55K0.45Fe2As2

Abstract

Directionalpoint-contact Andreev-reflection spectroscopy measurements on the Ba0.55K0.45Fe2As2 single crystals are presented.The spectra show significant differences when measured in the ab plane in comparison with those measured in the c direction of the crystal. In the latter case only a reduced point-contact conductance around zero bias has been revealed persisting well above Tc and probably related to the structural and magnetic transitions in the system. Within the ab plane two superconducting energy gaps are detected below Tc. Here a reduced conductance above Tc could also be found. The fits of the ab-plane data to the superconducting s-wave two-gap model indicate that the smaller gap has a size below the BCS value while the large gap reveals much higher coupling strength. One of the important questions to be addressed in these multiband systems concerns the symmetry of the superconducting order parameter(s). There are already many theoretical predictions on this topic but also a body of experimental studies is emerging. Band structure calculations have shown disconnected sheets of Fermi surfaces with possibly different superconducting energy gaps. A minimal model has to include two bands: the hole band around the Γ point and the electron one around the M point [7]. In contrast to the multiband but conventional s-wave scenario in MgB 2 [8], here, the extending s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets was proposed by Mazin et al. In the following we present directional PCAR studies on iron pnictides, namely on the Ba 0.55 K 0.45 Fe 2 As 2 single crystals. The spectra have shown significant differences when measured in the ab plane and in the c direction. In the latter case no suitable conditions to observe superconducting energy gaps with coherent gap-like peaks and enhanced conductance in the PCAR spectrum have been found. A predominant feature has been revealed in the form of a reduced point-contact conductance persisting well above T c which can be attributed to a reduced density of states (DOS) due to a structural phase transition [18] and magnetism found in a partial volume fraction in these samples below 70 K [11]. On the other hand within the ab plane two superconducting energy gaps are found. The pair of peaks due to a small gap has been scattered between ±(3 − 5) mV while the humps indicating the presence of the second gap are placed at ±(10 − 12) mV. In the superconducting gap-like spectra the reduced conductance above T c could also be found. The ab-plane spectra have been successfully fitted to the s-wave two-gap model providing the conventional temperature dependence of the two gaps with the coupling strengths 2∆ 1 /kT c = 2.5 − 4 and 2∆ 2 /kT c = 9 − 10.The Ba 0.55 K 0.45 Fe 2 As 2 single crystals were grown out of a Sn flux as described in details in Ref. [18], where the following characteristics have been found: typical dimensions of the crystals are 2x2x0.1 mm 3 . The crystallographic c axis is perpendicular to the plane of the platel...

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