Cuprates were discovered in 1986 by Georg Bednorz and Alex Müller and are the “kings” of superconductors. Today, they are the materials that become superconducting at the highest temperatures: -135°C or 138 K. These materials become superconducting at the temperature of liquid nitrogen: all the videos you can see on this website use cuprates, which are often called “high critical temperature superconductors” or “High Tc superconductors”.
However, the origin of their superconductivity has not been found yet and remains one of the most important and difficult mysteries of modern physics. Yet, their composition and structure is quite simple: they are composed of piled up atom layers. In all cuprates, we find copper and oxygen layers with a square structure (FIGURE). The number of electrons in these layers can be modified by oxidizing the material or modifying its chemical composition; this is called “doping”.
Physicists and chemists like to illustrate what happens to these oxides depending on their doping on a “phase diagram” (FIGURE). At a low temperature, the same compound is both insulating and magnetically orderly (the yellow area), but when the number of electrons is changed by a few percents, it becomes the best superconductor!
Physicists hoped that studying the behaviour of this material at a high temperature would help them understand why it becomes superconducting. This is when another peculiarity of these oxides appeared: the movement of their electrons in the copper and oxygen layers are completely different from the movement of the electrons in a piece of metal copper. Instead of being able to move freely, the electrons have trouble moving and they avoid one another because they are electrically pushed away from one another and are confined to the edges of the squares forming the layers: we say that the electrons are strongly correlated. And it might be in these complicated correlated electron movements that lies the origin of the best superconductivity ever observed!
More than twenty years after they were discovered, the highly original properties of these compounds still fascinate and surprise physicists. These materials and their peculiar properties have forced scientists to develop new theoretical tools and always more sophisticated and innovative experiments in order to understand these compounds better. It led to a revival of physics of oxide and solid surfaces. When will we find the solution?