Intrinsic atomic-scale modulations of the superconducting gap of 2H-NbSe₂

We present scanning tunneling microscopy and spectroscopy measurements at 100 mK in the superconducting material 2H-NbSe₂ that show well defined features in the superconducting density of states, with changes that follow atomic periodicity. Our experiment demonstrates that the intrinsic superconducting density of states can show atomic size modulations, which reflect the reciprocal space structure of the superconducting gap. In particular, we obtain that the superconducting gap of 2H-NbSe₂ has sixfold modulated components at 0.75 and 1.2 mV. Moreover, we also find related atomic size modulations inside vortices, demonstrating that the much discussed star shape vortex structure produced by localized states inside the vortex cores has a superposed atomic size modulation, which has hitherto been undetected. The tip substrate interaction in an anisotropic superconductor has been calculated, giving position dependent changes related to the observed gap anisotropy.