Magnetism and superconductivity are closely related in many ways. Superconductors can expel weak applied magnetic fields, but they lose their ability to carry electric current without resistance in strong magnetic fields, and it is generally assumed that magnetism and superconductivity cannot co-exist in the same material. Now Katharina Franke and co-workers at the Free University of Berlin have studied the interaction between these two phenomena at the level of single atoms, and shown that they can co-exist.
The Berlin team used scanning tunnelling microscopy and spectroscopy to study individual magnetic molecules of manganese phthalocyanine (MnPc) on a lead surface, which was superconducting at the cryogenic temperatures used in the experiments. In particular they explored the competition between a magnetic effect known as Kondo screening, which can co-exist with superconductivity, and magnetic interactions that are strong enough to break up the Cooper pairs of electrons that are essential for superconductivity.
Under certain conditions they found that two distinct states co-existed in a Moire pattern. In one state the Kondo screening effect was complete and the Cooper pairs remained intact, so the superconductivity was preserved. In the other state the screening was not strong enough to prevent the magnetic moment of the MnPc molecule breaking up the Cooper pairs and destroying the superconductivity.
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Living in harmony. Nature Nanotech (2011). https://doi.org/10.1038/nnano.2011.92