Emerging symmetry in cuprate superconductors The phenomenon of superconductivity was discovered over a century ago, yet it was long thought to be confined to the realm of extremely low temperatures. This changed with the discovery in 1986 of a material which acted as a superconductor at 35 Kelvin, now researchers aim to understand the underlying basis of high temperature superconductivity, as Dr Catherine Pépin explains The phenomenon of superconductivity was discovered in 1911, and it remains a source of fascination to scientists, who continue to build the theoretical foundations to underpin future research. A superconducting material shows exactly zero electrical resistance, enabling perpetual motion of electrons without any energy loss. “This means conductivity is infinite, superconductivity is a perfect phenomenon,” explains Dr Catherine Pépin, the coordinator of the Champagne project. While superconductivity was discovered in 1911, it was not until 1957 and the publication of the Bardeen-CooperSchrieffer theory that the phenomenon was more fully understood. “The electrons pair two-by-two, forming Cooper pairs, which condense into the quantum vacuum. It’s a macroscopic quantum phenomenon,” says Dr Pépin. “When you accelerate an electric charge, it should emit some electro-magnetic radiation, but this doesn’t happen in a superconducting material. Remarkably, in a conducting ring using superconducting materials, there is no energy loss and the conduction of the current is eternal. This idea of perpetual motion was a dream of the Greek philosophers, and nature has given us a realisation of it.” The original carrier in the conduction of electricity is an elementary particle – an electron – yet in a superconductor it’s not an electron that conducts electricity, but rather a Cooper pair. “This is two electrons together, so it becomes more like a wave. So each particle knows exactly what the other is doing, and then the whole field behaves as if everything is in phase, in synchronicity,” explains Dr Pépin. The quantum vacuum protects the phenomenon of superconductivity in these materials. The Maxwell equations that constrain the movement of electrons in metals are not valid in
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The SU(2) theory predicts the emergence of a variety of phases with varying temperature (T) and the concentration of carriers (p). In particular, it predicts the arising of topological defects: skyrmions in a pseudo-spin space, which can be ordered, as in the red phase, disordered as in the yellow phase. In the green region, these defects flatten and form a superconducting phase, depicted in green, in which the electrical resistivity vanishes. these situations and are replaced by London equations, under which Cooper pairs can conduct electricity without any loss of energy. Perfecting this phenomenon at room temperature could therefore revolutionise energy provision, greatly improving efficiency, while Dr Pépin says it could also hold implications beyond the energy sector. “A perfect conductor repels electromagnetic fields. A corollary of that is that you can create very strong magnetic fields, which is a realm that is just starting to be studied,” she outlines. For 70 years it was thought that superconductivity was confined to the regime of very low temperatures, but this changed with the discovery in 1986 of a material called lanthanum barium copper oxide (LBCO), the first high temperature superconductor. “This material showed superconductivity at a much higher temperature than the other superconductors, at 35 Kelvin,” says Dr Pépin. “More high-temperature superconducting ceramic materials were discovered in 1987, including yttrium barium copper oxide (YBCO) and others.
Researchers were able to raise the temperature at which this phenomenon is observable by more than 120 degrees, to 136 Kelvin at the end of 1987.” There are significant differences between these materials and standard superconductors however. In standard superconductors, the pairing mechanism between the two particles in a Cooper pair is due to the vibrations of the lattice, to the interaction between electrons and phonons, but this is not the case with high-temperature superconductors. “In this new class of superconductors, phonons are not the main driving force for this pairing mechanism,” says Dr Pépin. “The underlying mechanism behind superconductivity in this class of materials is different to the standard metallic superconductors.”
Cuprates This area forms a central part of Dr Pépin’s agenda, with researchers aiming to develop a new theory to describe a specific type of superconducting material called cuprates. The notion of elementary excitations is fundamental to this research; in particular, it is not clear
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