In 2008, the team of the Japanese Hideo Hosono at the Tokyo Institute of Technology discovered superconductivity in iron-based compounds, pnictides, at temperatures almost as “high” as cuprates, which surprised the physicists’ community. We thought that “high” temperature superconductivity only worked for cuprates, composed of oxygen and copper layers, and now, we find it in semimetals composed of iron and arsenic. After cuprates, they are the “hottest” superconductors with a maximum critical temperature of 56 K (-217°C) (in 2010).
Their lamellar structure is really close to that of cuprates, and their properties depending on the number of electrons also seem very similar: they can be magnetic or superconducting (FIGURE). But many differences have also been highlighted. When they are magnetic, pnictides are metals, whereas cuprates are insulators. This proves that in pnictides, electrons repel from each other less. The very nature of superconductivity could also be different from that of cuprates.
For instance, it seems to be less stuck inside the layers and a lot more three-dimensional in pnictides than in cuprates. Its origin could be linked to the magnetism of the electrons forming it. All these singularities can be explained by the fact that the electrons per iron belong to different "bands", meaning different types of behavior which are all at play here, whereas there was only one type of electron behavior per copper in cuprates.
These various types of electrons belong to different orbitals, meaning they have a different geometrical “shape”. This makes the problem even more complicated and interesting. These pnictides are today at the heart of many research subjects all around the world.
As far as applications are concerned, pnictides are promising: they have good mechanical properties and their superconductivity can resist strong magnetic fields and electric currents.
