Understanding magnetism and superconductivity

Unlike conventional superconductors which must be chilled to below 93 K (roughly 200 °C below room temperature), high-temperature superconductors can operate at the relatively balmier 138 K (−135 °C).

As electrons flow through the positively charged lattice structure of the superconductor with zero resistance, the surrounding atomic nuclei pull in another electron, and the two become what is known as a Cooper pair. However, in high-temperature superconductors, magnetic interactions between electrons are thought to support Cooper pair formation.

To better understand the relationship between magnetism and superconductivity, a team of scientists led by Bernhard Keimer of the Max Planck Institute for Solid State Research in Stuttgart, Germany, including Soltan Soltan of Helwan University, Cairo, used superlattices — which are like nano sandwiches of a high-temperature superconductor and a ferromagnetic material.

The magnetic layer, composed of LCMO (lanthanum, calcium and manganese oxide), exhibits a form of magnetism in which all the magnetic moments align in one direction, breaking the Cooper pairs, hampering the superconductivity and decreasing the transition temperature of the superconductor made of YBCO (a compound of yttrium, barium and copper oxide).

The researchers cooled the superlattices until magnetic order appeared in LCMO and superconductivity appeared in YBCO. At the transition temperature, one vibration in the copper oxide group of YBCO changed frequency. This was mirrored by a similar change in the manganese oxide group in the LCMO layers. This 'electron-phonon coupling' didn't just occur at the boundaries between the materials, but throughout the layer of LCMO. How this vibration travels is not clear, but it appears to be triggered by the nascent superconductivity in the YBCO layer.

"[This] gives us the opportunity to influence other properties such as thermoelectricity and conventional superconductivity" says Bernhard Keimer, one of the senior authors.

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