Superconducting materials

Discovered in 1986 by Bednorz and Müller the copper-oxide or cuprate superconductors have fascinated the condensed matter physics community for almost 40 years. The fundamental reason is that the physical mechanism causing Cooper pairing in these materials has restricted decades of efforts from thousands of scientist. That the mechanism behind superconductivity in copper-oxide superconductors is different from the BCS theory in its original version where electron are paired via the intervention of phonons (vibrations of the crystalline lattice, i.e. of the positive ions), is well established.

Instead, there is a substantial body of evidence suggesting that Cooper-pairing in copper-oxides and related materials is caused by magnetic correlations. However, these magnetic correlations are themselves manifestations of electronic degrees of freedom, i.e. pairing of electrons to produce high-temperature superconductivity appears to be caused by the electrons themselves. This situation is theoretically much more complicated that the known phonon-based scenario, not least because electrons might favor other ordered states, apart from superconductivity. Indeed, experiments have uncovered density waves of charge and spin degrees of freedom whose interrelation with superconductivity is complex. One class of theories posit that while superconductivity competes with spin and/or charge density wave order, the fluctuations of the density wave orders, which can be studied with x-rays and neutrons could nevertheless mediate superconductivity.

In MAGNET, we collaborate with researchers at the University of Copenhagen, the Paul Scherrer Institute and University of Zürich on studies of high-temperature copper-oxide superconductors. The efforts center in particular on the La2-xSrxCuO4 family of copper-oxides, and on the interplay of spin and charge density wave order with superconductivity (see figure). The experimental tools of choice are x-ray and neutron diffraction and spectroscopy. The team of collaborators overlaps with the construction team behind the BIFROST spectrometer at ESS, and we look forward to using this new instrument to cast new light on the spin-dynamics in copper oxides.

[1] Simutis et al, Commun Phys 5, 296 (2022). https://doi.org/10.1038/s42005-022-01061-4-