New Fe-based superconductors: properties relevant for applications

TLDR

Since the discovery of high‑temperature superconductivity in LaFeAs(O,F), several Fe‑layered families (1111, 122, 11, 111) have emerged, sharing cuprate‑like challenges such as layered structure, short coherence length, and unconventional pairing, yet offering advantages like metallic parents, lower electronic anisotropy, and s‑wave symmetry that may ease grain‑boundary current flow. The study aims to investigate and compare the superconducting properties of Fe‑based families to distinguish intrinsic from extrinsic behavior and identify the most promising material for practical applications, presenting an overview of key properties. The authors will discuss and compare upper critical field, electronic anisotropy, intragranular and intergranular critical current density across Fe‑based superconductor families. The 1111 family exhibits the highest Tc and most anisotropic upper critical field with fan‑shaped resistive transitions, whereas the 122 family has lower Tc, reduced anisotropy, and sharper transitions similar to low‑temperature superconductors.

Abstract

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length, and unconventional pairing, On the other hand the Fe-based superconductors have metallic parent compounds, and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, the supposed order parameter symmetry is s wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviours and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates, while the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.