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Superconductors: the role they play today and why is it so cool to study them

Superconductors are the electronic equivalent of the laser. In both cases the origin of the phenomenon is the quantum coherence. For the Laser, photons resulting from the stimulated emission are “synchronized”, thus conferring to the laser beam power, focus and a spectrum incomparable to that of an electric bulb. Thanks to all those properties the laser became an essential tool in industry as well as in fundamental research in order to handle light and matter. In a superconductor, quantum coherence appears at the electronic level : electrons bound in Cooper pairs condense into a coherent state below the superconducting critical temperature Tc. This electronic quantum coherence is responsible for two spectacular phenomena. The first one has given its name to those materials : superconductivity, electrical transport without losses, or in the case of a current loop, perpetual motion. This property is already exploited in the MRI (Magnetic Resonance Imaging) machines in which a big ring shelters a superconducting coil in which an electrical current runs perpetually in order to generate a magnetic field between 2 and 6 T.

The second property is even more spectacular : the Meissner effect. When magnetic is applied to a superconductor, supercurrents are spontaneously generated inside the superconductor, screening out the magnetic field lines in the bulk material. The superconductor hence becomes a magnet, carrying the exact opposite magnetization of the applied external magnetic field. The repelling forces between the superconductor and the external field can overcome gravity and levitation can occur. The Meisner effect can be exploited in order to produce levitation trains, like the Japanese Maglev train connecting Tokyo to Osaka at a cruise speed of 310 mph.

 

Figure 1 : Conventionnal magnet levitating above a superconductor cooled below the critical temperature Tc

Superconducting materials provide a new technological solution to the problems of energy distribution and efficiency. First of all, the densification of the urban areas and the increase of energy consumption is a severe problem for the electricity supply grid organisation. Thanks to their ability to carry high current densities, superconductors offer a much more compact technological alternative to the conventional copper lines. Superconductors can carry as much power as the regular copper grid in a much denser way but also more efficiently. This proves to be crucial properties when electricity is produced far away from the places it is used, in order to reduce losses. With the increase of amount of electricity produced with alternative energies such as wind, tide and sun light, superconductors will be more and more integrated in the electrical distribution grid design. Several companies such as American Superconductors in the US and Nexan in Europe already manufacture superconducting cables. Some of those cables are used today in the electrical distribution grid in Long Island. This small scale demonstration project shows that the technology exists and is fully operating.

Figure 2 : Comparison of the volume occupied by a 5GW overhead power line and a superconducting cable carrying the same power (from American Superconductor - talk IREQ 2009).

Superconductor based technologies however feature an inconvenient : superconductivity only appears below the critical temperature Tc. The superconducting cable of Long Island hence need to be cooled down at liquid nitrogen temperature (T = 77 K). Those cables are made of the materials with the highest Tc known so far. Those materials are called high-Tc cuprate superconductors. Cuprates were discovered 25 years ago and are considered since then as one of the biggest mystery of science. The microscopic mechanism responsible for superconductivity in those materials is still unknown despite those 25 years of intense research. The scientific activity in the area of high-Tc superconductivity will one day allow to determine the mechanism of high-Tc superconductivity. With that kind of knowledge, human scientists will be able to design new materials with the exact ingredients required to enhance the critical temperature and on the long term to produce a room temperature superconductor. Our group is contributing to this research effort with experiments in high magnetic fields. Please check other sections to find out more about research on high-Tc superconductivity in high magnetic fields.