Understanding High-Temperature Superconductivity in correlated materials SUPERBAD, an ERC Starting Grant in theoretical condensed-matter physics identifies the cause of high-temperature superconductivity in a “cure” of the bad matallic behaviour caused by the strong electron-electron interaction. The project leader is Massimo Capone The SUPERBAD research project, headed by principal investigator Massimo Capone, hopes to gain a better understanding of high temperature superconductivity from the ground up by analysing theoretically and testing a broad range of superconductive materials, and also by studying the relationship between high temperature superconductivity and bad metallic behaviour. The project started in October 2009 and is funded by the European Research Council. Focussing on the newer superconductors discovered within the last twenty five years, the project’s aim is to investigate how superconductivity can be seen as a “cure” for bad metallic states. The origins of the SUPERBAD project can be found with the discoveries made in the mid-1980’s, where the critical temperature, the temperature at which electrical resistance in a given material becomes zero, reached never before seen levels thanks to the discovery of a new surprising family of materials. Before this time, only standard superconductors were known, as explained by the Bardeen, Cooper, and Schrieffer Theory (BCS Theory), which determined that superconductivity could not occur above 20 Kelvin. Capone explains: “Suddenly, we moved from a point where superconductivity could occur at temperatures below a few Kelvin’s to an entirely new world in which it could occur at temperatures close to 100 Kelvin, which makes a huge difference because that is above nitrogen’s boiling point; therefore, the cost of cooling these new materials is greatly reduced.”
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The discovery of these new materials was doubly significant when considering their properties. Standard superconductors are just normal metals, which have been cooled to their critical temperature, at which point their electrical resistance
A superconducting sample levitates in a magnetic field due to the Meissner effect. The same principle is at the basis of the magnetic levitation trains
drops to zero. Capone explains that the new superconductors, all based on layers of copper and oxygen atoms, like e.g. La -x SrxCuO2, were strange compounds: 2 “They are insulators if taken in their stoichiometric composition. However, it was found that when carriers were doped inside them, they become metal and these metals had the highest critical temperatures that had ever been observed. So, you have the best superconductor you’ve ever had, and it is closely related to an insulator”. This is a surprise because insulators by their very nature are the opposite of conductive materials. Moreover, the insulating behaviour is not of standard type, but it is an effect of the interaction between the electrons, as understood by Sir N. Mott. At the beginning of research into superconductivity, the search was focussed on conductive materials. However, with the discoveries made around the mid-eighties, researchers had to change their way of thinking. To begin with, researchers thought that there was nothing in common between standard superconductors and the new superconductors and soon the buzz word became “Strong Correlations”; as Capone explains: “This means that there are strong interactions between electrons, which are responsible not only for the insulating behaviour of the non-doped compounds, but also for superconductivity.” After 1986 it appeared as if two essentially separated families of superconductors existed. Standard superconductors, as found in BCS Theory where phonons are the mechanism of pairing, are metals when they
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