ABSTRACT
Discovery of iron-based superconductors paved the way to a competitor of high-temperature superconductors, easier to produce, better performing in high fields, and promising to be less expensive. Critical parameters are investigated by resistivity measurements as a function of temperature, field, and angle R(T,H,θ). This work presents a deep analysis of H-θ phase diagram of PLD-processed Fe(Se,Te) superconducting films, thus revealing material and pinning anisotropy at once. By selecting different thresholds along the R(T,H,θ) curves, all possible regimes emerge. Surprisingly, anisotropy arises moving from the upper critical field toward the irreversibility line: gradually a non-monotonous transition from 3D to 2D, and backward to 3D occurs. Although Fe(Se,Te) appears as a 3D superconductor, its anisotropic pinning landscape shows up similarities with an intrinsic layered superconductor and Fe(Se,Te) definitively mimics YBCO. We propose a general method to disentangle, in any other superconductor, material dimensionality and pinning anisotropy that are key constraints for applications.
ABSTRACT
The process of developing superconducting materials for large scale applications is mainly oriented to optimize flux pinning and the current carrying capability. A powerful approach to investigate pinning properties is to combine high resolution imaging with transport measurements as a function of the magnetic field orientation, supported by a pinning modelling. We carry out Transmission Electron Microscopy, Electron Energy Loss Spectroscopy and critical current measurements in fields up to 16 T varying the angle between the field and c-axis of Fe(Se,Te) epitaxial thin films deposited on CaF2 substrates. We find evidence of nanoscale domains with different Te:Se stoichiometry and/or rotated and tilted axes, as well as of lattice distortions and two-dimensional defects at the grain boundaries. These elongated domains are tens of nm in size along the in-plane axes. We establish a correlation between these observed microstructural features and the pinning properties, specifically strongly enhanced pinning for the magnetic field oriented in-plane and pinning emerging at higher fields for out-of-plane direction. These features can be accounted for within a model where pinning centers are local variations of the critical temperature and local variations of the mean free path, respectively. The identification of all these growth induced defects acting as effective pinning centers may provide useful information for the optimization of Fe(Se,Te) coated conductors.
ABSTRACT
The role of a layered structure in superconducting pinning properties is still at a debate. The effects of the vortex shape, which can assume for example a staircase form, could influence the interplay with extrinsic pinning coming from the specific defects of the material, thus inducing an effective magnetic field dependence. To enlighten this role, we analysed the angular dependence of flux pinning energy U(H,θ) as a function of magnetic field in FeSe0.5Te0.5 thin film by considering the field components along the ab-plane of the crystal structure and the c-axis direction. U(H,θ) has been evaluated from magneto-resistivity measurements acquired at different orientations between the applied field up to 16 T and FeSe0.5Te0.5 thin films grown on a CaF2 substrate. We observed that the U(H,θ) shows an anisotropic trend as a function of both the intensity and the direction of the applied field. Such a behaviour can be correlated to the presence of extended defects elongated in the ab-planes, thus mimicking a layered superconductor, as we observed in the microstructure of the compound. The comparison of FeSe0.5Te0.5 with other superconducting materials provides a more general understanding on the flux pinning energy in layered superconductors.
ABSTRACT
Anisotropy effects on flux pinning and flux flow are strongly effective in cuprate as well as iron-based superconductors due to their intrinsically layered crystallographic structure. However Fe(Se,Te) thin films grown on CaF2 substrate result less anisotropic with respect to all the other iron based superconductors. We present the first study on the angular dependence of the flux flow instability, which occurs in the flux flow regime as a current driven transition to the normal state at the instability point (I*, V*) in the current-voltage characteristics. The voltage jumps are systematically investigated as a function of the temperature, the external magnetic field, and the angle between the field and the Fe(Se,Te) film. The scaling procedure based on the anisotropic Ginzburg-Landau approach is successfully applied to the observed angular dependence of the critical voltage V*. Anyway, we find out that Fe(Se,Te) represents the case study of a layered material characterized by a weak anisotropy of its static superconducting properties, but with an increased anisotropy in its vortex dynamics due to the predominant perpendicular component of the external applied magnetic field. Indeed, I* shows less sensitivity to angle variations, thus being promising for high field applications.
ABSTRACT
We study the current-voltage characteristics of Fe(Se,Te) thin films deposited on CaF2 substrates in form of nanostrips (width w ~ λ, λ the London penetration length). In view of a possible application of these materials to superconductive electronics and micro-electronics we focus on transport properties in small magnetic field, the one generated by the bias current. From the characteristics taken at different temperatures we derive estimates for the pinning potential U and the pinning potential range δ for the magnetic flux lines (vortices). Since the sample lines are very narrow, the classical creep flow model provides a sufficiently accurate interpretation of the data only when the attractive interaction between magnetic flux lines of opposite sign is taken into account. The observed voltages and the induced depression of the critical current of the nanostrips are compatible with the presence of a low number ([Formula: see text]) magnetic field lines at the equilibrium, a strongly inhomogeneous current density distribution at the two ends of the strips and a reduced Bean Livingston barrier. In particular, we argue that the sharp corners defining the bridge geometry represent points of easy magnetic flux lines injection. The results are relevant for creep flow analysis in superconducting Fe(Se,Te) nanostrips.
