Probing crystal-field modulations with magnetic adatoms on the incipient charge-density-wave superconductor $2H$-NbS$_2$

Abstract

The interplay between multiple quantum phases in layered materials may lead to incipient quantum behavior, where the material's ground state is close to a phase transition and sensitive to local disorder. The transition metal dichalcogenide material $2H$-NbS$_2$ exhibits incipient charge-density-wave behavior along with a well-developed superconducting state, creating a scenario where the local lattice instabilities play a crucial role. Here we present how an individual magnetic atom on $2H$-NbS$_2$ can be applied as a local sensor to reveal hidden crystal-field modulations. By manipulating the adatom across the surface with the tip of a scanning tunneling microscope, we measure variations in the Yu-Shiba-Rusinov (YSR) excitation spectra and map the local environment around an intrinsic point defect. We find that while the superconducting state is spatially uniform, the YSR excitation energy strongly depends on the position of the atom. We determine that the main contribution to this effect originates from variations in the local crystal-field environment. These results establish a new approach to investigate crystal-field modulations at the atomic scale and reveal how defects and lattice instabilities shape the atomic landscape of an incipient charge-density-wave material.