The spectroscopic signature of the Co magnetic state in CoxNbSe2 superconducting single crystals

The magnetic state of Co in NbSe2 has been studied with low temperature scanning tunneling microscopy and spectroscopy. The local density of states (DOS) at the surface of CoxNbSe2, for different dopings x, does not show bound states predicted in the case of superconductors with magnetic impurities. Only occasionally local asymmetries in the DOS are found at some impurity sites and they appear to be very local. These asymmetries, together with a very low magnetic moment obtained from susceptibility measurements, point towards a possible Kondo effect at play. Tunneling spectra recorded at the impurity site in the presence of a magnetic field of 7 T (i.e. when the sample is in the normal state) show a dip-like feature at the Fermi energy, reminiscent of the Fano resonance observed in magnetic adatoms on metallic surfaces.