New insights into high temperature superconductors

Constant-energy images of spin excitations of iron pnictides

Constant-energy images of spin excitations of iron pnictides and its comparison with RPA/DMFT calculations. From: Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides, Nature Communications 4, Article number: 2874
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Superconductors are materials where below a certain critical temperature (Tc) all electrical resistance disappears. Discovered in 1911, superconductors have the potential to revolutionise the ways in which we store and transport electricity. However, for seven decades the highest critical temperature for a superconductors was 23K (-230°C), impractical for most applications. In 1986 the discovery of high temperature superconductors by IBM researchers provided hope for room temperature superconductivity, with a barium-lanthanum-copper oxide that had a critical temperature of 35K. Subsequently a host of so-called cuprate superconductors have been identified, but the holy grail of room temperature superconductivity has yet to be realised.

Iron based superconductors, discovered in 2008, have superconducting transition temperatures up to 55K – higher again than conventional superconductors. They are interesting because they show that the famous cuprate superconductors are not the only possible route to high superconducting transition temperatures. The iron based superconductors have similarities with the cuprates (e.g. the superconductivity takes place in sheets) and differences (e.g. the starting materials which need to be ‘doped’ to push in or pull out a small fraction of the electrons from the sheets to make them superconductors are metals rather than insulators). Studying a second, rather different, class of high-Tcmaterials offers the opportunity to find what really matters in the quest to push Tc higher in these or a future family of superconductors.

What both cuprate and iron-based superconductors appear to have in common is that the origin of the force that causes the electrons to pair up and produce the superconducting state almost certainly arises from the fluctuation of the magnetic moments in the materials. Systematic studies are key to understanding the nature of these fluctuations and their connection to superconductivity.

Pengcheng Dai and co-workers at ISIS including Toby Perring and E. Goremychkin used the MERLIN and MAPS instruments to study the magnetic fluctuations in hole doped (electrons removed) and electron doped (electrons added) variants of the most widely studied iron-based superconductor.

Prof Dai says “From the mass of data we were able to pull out two key common elements: superconductivity appears to require both high energy magnetic fluctuations (much more than an order of magnitude larger that the characteristic energy scale associated with the superconducting transition temperature), and low energy magnetic fluctuations that couple to metallic-like electrons. It is only because of the comprehensive mapping of the magnetism in absolute units we could perform on MAPS and MERLIN that we could reach this conclusion.”

 

Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides, Meng Wang, Chenglin Zhang, Xingye Lu, Guotai Tan, Huiqian Luo, Yu Song, Miaoyin Wang, Xiaotian Zhang, E.A. Goremychkin,   T.G. Perring, T.A. Maier, Zhiping Yin, Kristjan Haule, Gabriel Kotliar & Pengcheng Dai, Nature Communications 4 Article no. 2874 (2013).

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