Iron-based lamellar superconducting materials

The hundred-year history of superconductivity has been marked by unexpected discoveries. When H. K. Onnes discovered superconductivity in mercury in 1911, he wanted to test Lord Kelvin's hypothesis that electron motion would freeze as it approached absolute zero. Just over 40 years later, with the development of the BCS theory by J. Bardeen, L. Cooper and J.R. Schrieffer in 1957, it was thought that all the main aspects of superconductivity had been explained and understood. Successive discoveries of numerous superconducting materials provided striking confirmation of the BCS theory. Superconductivity was then assumed to be confined to metals and metal alloys, and a record critical temperature of 23.3 K was reached in Nb ₃ Ge.

The unexpected discovery of superconductivity in Chevrel phases, organic conductors or heavy fermions had given the first indications of a possible inadequacy of the BCS theory, but it was the discovery in 1986 of superconductivity in cuprate-type oxides, with critical temperatures above the liquefaction temperature of nitrogen, that brought the main blow to its supposed universality. These successive discoveries have led to numerous technological developments, from high-speed electronics and electromagnets to magnetic detection using SQUID (Superconducting Quantum Interference Device). Nevertheless, it is interesting to note that, more than 20 years after the discovery of cuprates, there is still no consensus theory to explain the origin of their superconducting state. During this period, one of the only certainties to prevail was that the superconducting state could not appear in an iron-based material, this element being reputed to be a poison for superconductivity due to its magnetic properties. The discovery of superconductivity in LaFeAsO _0.88 F _0.12 in 2008, with a critical temperature of 26 K, which was soon to rise to 55 K by replacing lanthanum with samarium, then sounded like a thunderclap.

The aim of this article is to give an overview of the new family of superconductors constituted by these iron-based layered materials. We will focus both on the physical properties of these materials and on potential technological developments.