Coherent long-range magnetic bound states in a superconductor

Using low temperature scanning tunneling spectroscopy we accessed to localized states called Shiba states associated to magnetic impurities as well as their spatial dependence [1]. We studied samples of superconducting NbSe2 containing a really small concentration of native magnetic impurities. We observed the appearance of star-shaped structures around individual impurities with a size of the order of the coherence length of the superconductor ($\simeq $10 nm). The fine study of our data revealed an oscillation of the density of states along the star branches. To further analyze our results, we performed tight-binding calculations which reproduced the observed spatial symmetry. A semi-analytical calculation also enabled us to establish a relation between the different spatial scales observed and the physical quantities associated to superconductivity, and to understand the role of dimensionality in such systems. [1] Nature Physics (2015), DOI:10.1038/NPHYS3508