Using two different experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field Hc1(T ) of a Ca0.32Na0.68Fe2As2 (Tc ≈ 34 K) single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic s-wave-like gap or by a two-gaps model, while a d-wave approach cannot be used to fit the London penetration depth data. The intrinsic multiple Andreev reflection effect (IMARE) spectroscopy was used to detect bulk gap values in single crystals of the intimate compound Ba0.65K0.35Fe2As2, with the same Tc. We estimate the range of the large gap value ∆L = (6-8) meV (depending on small variation of Tc) and its k-space anisotropy about 30%, and the small gap is about ∆S ≈ (1.7±0.3) meV. This clearly indicates that the gap structure of our investigated systems is more likely to be of the two-gap nodeless s-wave type.

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Nematic order often breaks the tetragonal symmetry of iron-based superconductors. It arises from regular structural transition or electronic instability in the normal phase. Here, we report the observation of a nematic superconducting state, by measuring the angular dependence of the in-plane and out-of-plane magnetoresistivity of Ba0.5K0.5Fe2As2 single crystals. We find large twofold oscillations in the vicinity of the superconducting transition, when the direction of applied magnetic field is rotated within the basal plane. To avoid the influences from sample geometry or current flow direction, the sample was designed as Corbino-shape for in-plane and mesa-shape for out-of-plane measurements. Theoretical analysis shows that the nematic superconductivity arises from the weak mixture of the quasi-degenerate s-wave and d-wave components of the superconducting condensate, most probably induced by a weak anisotropy of stresses inherent to single crystals.

We investigated the critical current density (Jc) of Ba0.5K0.5Fe2As2 single-crystalline microbridges with thicknesses ranging from 276 to 18 nm. The J c of the microbridge with thickness down to 91 nm is 10.8 MA/cm 2 at 35 K, and reaches 944.4 MA/cm 2 by extrapolating Jc(T ) to T = 0 K using a two-gap s-wave Ginzburg-Landau model, well in accordance with the depairing current limit. The temperature, magnetic field, and angular-dependence of Jc(T, H, θ) indicated weaker field dependence and weakly anisotropic factor of 1.15 (1 T) and 1.26 (5 T), which also yielded the validity of the anisotropic Ginzburg-Landau scaling.The newly discovered Fe-based superconductors caused increasing attention due to their potential for applications, owing to high superconducting (SC) transition temperature (T c ), high upper critical fields (H c2 ), and a low critical current (J c ) anisotropy [1]. Compared with the layered cuprate family, although Fe-based superconductors have lower T c , they demonstrate nearly isotropic transport behavior [1]. Investigation of anisotropic J c is essential, from the view point of basic physics parameters and potential applications [2][3][4]. Generally, there are two main methods to explore the J c . The first one is to study patterned thin film micro-devices. The thin films have been fabricated successfully for several systems, including Fe(Se,Te) [2], Ba(Fe,Co) 2 As 2 [3], and LaFeAsO 1−x F x [4]. However, one can hardly obtain an ideal single-crystalline film, due to the presence of twins and low-angle grain boundaries resulting from the island-like growth process [5]. Although the existence of grain boundaries can enhance the J c by increasing pinning, the critical current exhibits exponential decay in the weak-link regime which poses a serious obstacle for practical applications. Up to now, the J c of Fe-based thin films was reported at the level of a few MA/cm 2 at 4.3 K [3][4][5]. The other method is to measure the J c indirectly from magnetization hysteresis loop (MHL) of single crystals. By using the Bean's model the J c can be evaluated from the vortex penetration profile [6], which is determined by pining force owing to the defects, grain boundary, and geometry of the materials. The J c value estimated from MHL remains as well of the order of a few MA/cm 2 at 4.3 K [7]. * Author to whom correspondence should be addressed. Electronic mail: Jun.Li@fys.kuleuven.beWe emphasize that both methods for J c determination are limited by the vortex motion in presence of a finite pining force, but not the intrinsic transport capability of the material, for which an exploration of the GinzburgLandau (GL) depairing current density (J GL dp ) is of great importance for understanding the existing limits for increasing J c [8][9][10]. The J GL dp corresponds to the critical pair-breaking current, which was theoretically estimated to be as high as 200 MA/cm 2 at 0 K for (Ba,K)Fe 2 As 2 [11]. This value is two orders of magnitude larger than previously reported J c 's. Generally, it is rather difficult t...

The determination of the pairing symmetry is one of the most crucial issues for the iron-based superconductors, for which various scenarios are discussed controversially. Non-magnetic impurity substitution is one of the most promising approaches to address the issue, because the pair-breaking mechanism from the non-magnetic impurities should be different for various models. Previous substitution experiments demonstrated that the non-magnetic zinc can suppress the superconductivity of various iron-based superconductors. Here we demonstrate the local destruction of superconductivity by non-magnetic zinc impurities in Ba0.5K0.5Fe2As2 by exploring phase-slip phenomena in a mesoscopic structure with 119 × 102 nm2 cross-section. The impurities suppress superconductivity in a three-dimensional ‘Swiss cheese'-like pattern with in-plane and out-of-plane characteristic lengths slightly below ∼1.34 nm. This causes the superconducting order parameter to vary along abundant narrow channels with effective cross-section of a few square nanometres. The local destruction of superconductivity can be related to Cooper pair breaking by non-magnetic impurities.