Magnetic interactions in a high-temperature superconductor

For over twenty years physicists have been baffled by high-temperature superconductivity — the flow of electricity with zero resistance in some materials at temperatures well above 30 K (−243.2 °C), far beyond the limit for ordinary superconductivity. Electrons in these materials gather in pairs which then, by the principles of quantum theory, condense into a superconducting fluid. But what causes the pairing?

The pairing mechanism of standard superconductors — vibrations in the crystal lattice that can sometimes cause attraction between electrons — is known to be too weak. Now a team of physicists, including Shankar Kunwar from the King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia, has added further proof that the mechanism of high-temperature superconductivity involves interactions between electrons and magnetic order in materials.

They studied Pr_0.88LaCe_0.12CuO₄, an antiferromagnetic compound which gradually alters its electrical and magnetic properties, as well as its microscopic structure, when annealed in an oxygen-poor environment to remove a small amount of oxygen. It starts out with strong 'anti-ferromagnetic' organization — a checkerboard-like patterning of atomic magnetic moments, where atoms act like tiny magnets that cancel out each other's magnetism. Yet after oxygen annealing, superconducting regions started to expand in the material.

Using neutron scattering and scanning tunnelling spectroscopy, the group showed that electrons and neutrons interact with the compound in a similar manner — and the behaviour of each changes identically upon annealing. Since neutrons have no charge and only a magnetic moment, the researchers suggest the similar behaviour implies that electrons also interact with the material primarily through magnetic excitations, rather than anything associated with crystal vibrations.

"Our data suggest that magnetic excitations are the glue for electron pairing and superconductivity, at least in these materials," says Kunwar.

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