Mapping the orbital structure of impurity bound states in a superconductor

Superconductors are characterized by their zero resistance to electrical currents and by their ability to expel magnetic fields. This last effect shows that superconductivity and magnetism are not compatible phenomena. This is because superconductors are made by the attractive interaction between two electrons of opposed magnetic moment thanks to the deformation of the superconductor itself. Because the electron pair contains opposite magnetic moments a magnetic field will act in different ways on the two electrons, eventually destroying superconductivity if the field is large enough. Due to the existence of electron pairs instead of individual electrons, no single electron can be injected at low bias. However, thanks to the local magnetic moment produced by magnetic atoms, it is possible to destroy the pairing of electrons and inject individual electrons at very low bias. In this way, by using a scanning tunneling microscope, Choi et al managed to locally inject electrons on a Cr atom on a superconductor made of a thin film of lead. The extraordinary locality of the scanning tunneling microscope, permitted these authors to map the places where the

The extraordinary locality of the scanning tunneling microscope, permitted these authors to map the places where the individual electrons could live in an otherwise forbidden territory for electrons. This work enhances our understanding of superconductivity and its behavior in the presence of local magnetic moments on the atomic scale.

Figure.- A Cr atom on a lead surface. The lead surface is soft and allows the Cr atom to dig in between the first and second surface layer. This leads to a special local environment of the Cr atom formed by the 7 nearest-neighboring Pb atoms. Pb is a superconductor at low temperature and Cr is a magnetic impurity with one of the largest magnetic moments. This is an ideal system to study the effect of local magnetic moments in the complex electronic structure of superconductors.