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

We present scanning tunneling microscopy and spectroscopy measurements at 100mK in the superconducting material 2H-NbSe2 that show well defined features in the superconducting density of states changing in a pattern closely following 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 six fold modulated components at 0.75 mV 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, hitherto undetected, superposed atomic size modulation. The tip substrate interaction in an anisotropic superconductor has been calculated, giving position dependent changes related to the observed gap anisotropy.