We report that the $\left(\text{Ba},\text{K}\right){\text{Fe}}_2{\text{As}}_2$ crystal with $T_c=32\text{K}$ shows a pinning potential, $U_0$, as high as ${10}^4\text{K}$, with $U_0$ showing very little field dependence. The $\left(\text{Ba},\text{K}\right){\text{Fe}}_2{\text{As}}_2$ single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to ${\text{H}}_{c2}$. The isotropic rigid vortices observed in the two-dimensional (2D) $\left(\text{Ba},\text{K}\right){\text{Fe}}_2{\text{As}}_2$ distinguish this compound from 2D high-$T_c$ cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the $\left(\text{Ba},\text{K}\right){\text{Fe}}_2{\text{As}}_2$ crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the $\left(\text{Ba},\text{K}\right){\text{Fe}}_2{\text{As}}_2$ crystal.

Flux-pinning mechanism of ${\text{MgB}}_2$ doped with $10\text{wt}\%$ silicone-oil sintered at low and high temperatures has been investigated by magnetic measurements. The field dependence of the critical current density, $j_c\left(B\right)$, was analyzed within the collective pinning model. A crossover field, $B_{\text{sb}}$, from the single vortex to the small vortex bundle-pinning regime was observed. For both types of sintered samples, the temperature dependence of $B_{\text{sb}}\left(T\right)$ at low temperature is in good agreement with the $\delta l$ pinning mechanism, i.e., pinning associated with charge-carrier mean free path fluctuation. At temperatures close to the critical temperature, however, there is evidence for $\delta T_c$ pinning, which is associated with spatial fluctuations of the transition temperature. These results provide strong evidence that the liquid precursor, silicone oil, produces very small pinning centers and enhances the $j_c\left(B\right)$.

Hybrid polypyrrole (PPy)–multi walled carbon nanotube (MWNT) yarns were obtained by chemical and electrochemical polymerization of pyrrole on the surface and within the porous interior of twisted MWNT yarns. The material was characterized by scanning electron microscopy, electrochemical, mechanical and electrical measurements. It was found that the hybrid PPy-MWNT yarns possessed significantly higher mechanical strength (over 740 MPa) and Young's modulus (over 54 GPa) than the pristine MWNT yarn. The hybrid yarns also exhibited substantially higher electrical conductivity (over 235 S cm⁻¹) and their specific capacitance was found to be in excess of 60 F g⁻¹. Measurements of temperature dependence of electrical conductivity revealed semiconducting behaviour, with a large increase of band gap near 100 K. The collected low temperature data are in good agreement with a three-dimensional variable range hopping model (3D-VRH). The improved durability of the yarns is important for electrical applications. The composite yarns can be produced in commercial quantities and used for applications where the electrical conductivity and good mechanical properties are of primary importance.

The vortex liquid-to-glass transition has been studied in Ba0.72K0.28Fe2As2 (BaK-122), Ba(Fe0.91Co0.09)2As2(BaCo-122), and Ba(Fe0.95Ni0.05)2As2(BaNi-122) single crystal with superconducting transition temperature, Tc = 31.7, 17.3, and 18 K, respectively, by magnetoresistance measurements. For temperatures below Tc, the resistivity curves were measured in magnetic fields within the range of 0 ≤ B ≤ 13 T, and the pinning potential was scaled according to a modified model for vortex liquid resistivity. Good scaling of the resistivity ρ(B, T) and the effective pinning energy U0(B,T) were obtained. The vortex state is three-dimensional at temperatures lower than a characteristic temperature T*. The vortex phase diagram was determined based on the evolution of the vortex-glass transition temperature Tg with magnetic field and the upper critical field, Hc2. We found that non-magnetic K doping results in a high glass line close to the Hc2, while magnetic Ni and Co doping causes a low glass line which is far away from the Hc2. Our results suggest that non-magnetic induced disorder is more favourable for enhancement of pinning strength compared to magnetic induced disorder. Our results show that the pinning potential is responsible for the difference in the glass states.