Impurity-pinned incommensurate charge density wave and local phonon excitations in 2H-NbS2

Here, we report a scanning tunneling microscopy (STM) and spectroscopy (STS) study in the superconducting state of 2H-NbS2. We directly visualize the existence of an incommensurate charge density wave (CDW) that is pinned by atomic impurities. In strong tunneling conditions, the incommensurate CDW is depinned from impurities by the electric field from the STM tip. To explore the possible mechanism of this incommensurate CDW, we perform STM-based inelastic tunneling spectroscopy (IETS) to detect phonon excitations in 2H-NbS2 and measure the influence of atomic impurities on local phonon excitations. In comparison with the calculated vibrational density of states in 2H-NbS2, we find two branches of phonon excitations which correspond to the vibrations of Nb ions and S ions, and the strength of the local phonon excitations is insensitive to the atomic impurities. Our results demonstrate the coexistence of impurity-pinned incommensurate CDW and superconductivity in 2H-NbS2, and open a way for detecting atomic-scale phonon excitations in transition metal dichalcogenides with STM-based IETS.