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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“The materials with the highest critical temperatures are far from the best metals. Indeed they are closely related to insulators and they become poor conductors above the critical temperature. Why is this?” are not superconductors, and they have low critical temperatures. New superconductors, which can often be termed “bad metals”, metals which are poorly conducting if their temperature is increased above critical temperature; in these materials it has been thought that phonon interaction is largely irrelevant to superconductivity and conductivity. Capone however believes that the two families of superconductors can be closer than was originally thought. By looking at fullerides, superconductive compounds formed by a solid of fullerene buckyballs (C60) where phonon interaction is directly responsible for superconductivity and a Mott insulating state is present, Capone identifies these compounds as a bridge between standard and hightemperature superconductors.
field of superconductive materials, and these developments have also been incorporated into the project. In March 2010, a new superconductor was discovered – potassium-doped picene, an organic based aromatic compound. This superconductor, similar to the fullerene family of superconductors, was included in Capone’s calculations and the results of his testing gained some interesting results. “This material works even more like the copper based superconductors, even if it is an organic one.” Capone proposes that this new material provides even stronger evidence that there is a link between organic compounds, like the fullerenes, and the inorganic compounds such as the copper or iron based superconductors.
Capone believes that a key point of the research is to find a different way to understand why certain materials have higher critical temperatures than others, and his proposal is that the key to unlocking the mystery of high temperature superconductors lies in “bad metal behaviour” more in the identification of the “pairing glue”.
Perhaps the most important aspect of Capone’s research is the possibility of creating a superconductive material through which electrical current flows without any dissipation at ambient temperature. Such a material could have far reaching implications in the development of more effective systems for practical applications. Among other things, superconductors are currently used in the magnetic imaging of the human brain, as well as in the production of magnetic levitation transport, such as the JR-Maglev train system being developed in Japan. Capone states that the SUPERBAD research project is not in a sense concerned with creating new superconductors, but is more concerned with improving the ones already available. “This is an improvement that is definitely needed.”
Capone says: “Bad metal behaviour is a direct consequence of strong correlations.” The essence of superconductivity is achieving within a given material the optimum flow of electrons; the electrical resistance ideally should be zero. Researchers into superconductivity are now faced with a “catch-22” situation; A high critical temperature seems associated with a “bad metallic state” with high resistance, but you can not increase the level of correlations too much, because that would kill superconductivity. Capone explains that these new superconductive materials, which have more in common with insulators, are evidence that superconductivity effectively “cures” these materials: “Superconductivity is sort of a cure for the disease that leads to a bad metallic state.” Since the project’s inception, there have been further developments within the
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The overarching idea of the project is to be able to look at a given material’s structure on an atomic level, taking account of all the atomic components within it. With this structural understanding, it would then be possible to alter specific atoms within the material’s structure to increase the potential superconductive capabilities. Essentially, the ultimate aim of such research is to reach a point where superconductivity can occur at room temperatures.
At a glance Full Project Title Understanding high-temperature superconductivity from the foundations: Superconductivity as a cure for bad metallic behaviour Project Funding €1 Million provided by European Research Council (ERC) through the Starting Independent Grant scheme within the IDEAS program of FP7/ERC. Contact Details Project Coordinator, Massimo Capone Istituto Officina dei Materiali Consiglio Nazionale delle Ricerche (IOM/CNR) International School for Advanced Studies (SISSA/ISAS) Via Bonomea 265, I-34016 Trieste, Italy T: +39-040-3787-374 T: +39-339-6620959 E: massimo.capone@sissa.it E: massimocapone@gmail.com W: http://superbadproject.wordpress.com
Massimo Capone
Project Coordinator
Currently Researcher at the SISSA Unit Democritos of the Institute for Materials Workshop of Italian National Research Council (CNR) and Assistant Professor at SISSA. Graduated in Rome Sapienza and PhD at SISSA in Trieste. His research activity focuses on condensed matter physics and the properties of strongly correlated fermions in high-temperature superconductors, other functional materials and cold-atom systems.
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