New Insights Into High-Temperature Superconductors

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After 30 years of research, there are still many unsolved puzzles about high-temperature superconductors - among them is the magnetic “stripe order” found in some cuprate superconductors. A Danish research team has taken a closer look at these stripes, using high-resolution neutron scattering at the spectrometers FLEXX (HZB) and ThALES (ILL, Grenoble). Their results, now published in Physical Review Letters, challenge the common understanding of stripe order, and may contribute to unveil the true nature  of high-temperature superconductivity.

It has been known for about 30 years that cuprate superconductors become superconducting at surprisingly high temperatures – often above the boiling point of liquid nitrogen (-196 °C). This makes them particularly interesting for applications. Research has revealed that the mechanism which leads to the formation of the superconducting state is different for the cuprates than for conventional superconductors. However, despite intensive studies, this unusual mechanism is still not properly understood. Scientists hope that by understanding what makes high-temperature superconductors special, they will eventually be able to find a material that is superconducting at room temperature.

In the cuprates, superconductivity is intimately connected with the magnetic properties – in stark contrast to conventional superconductors, where magnetism destroys superconductivity. For several cuprate compounds, an unusual state is found where stripes of magnetic order alternate with stripes of charge, which are superconducting (see figure). Also magnetic excitations, apparently associated with the magnetic stripes, have been observed.

A team from Niels Bohr Institute, University of Copenhagen, Denmark, has performed neutron scattering experiments to take a closer look at the magnetic stripes. Using the spectrometers FLEXX (HZB) and ThALES (ILL, Grenoble), they were able to analyse the stripes with very high resolution. They deduced from their data that the magnetic stripe order and the magnetic excitations, although also stripe-like, are not related to each other, but actually originate from different regions in the sample. The comparison with other studies suggests that phase separation into a magnetic and a superconducting phase occurs, and that the striped magnetic excitations belong to the superconducting phase. This model requires a careful re-consideration of many other studies on cuprate superconductors which assume that the stripe order and excitations have the same origin. The results were now published in Physical Review Letters.