Traces of Charge Density Waves in NbS$_2$

Abstract

Among Transition Metal Dichalcogenides (TMD), NbS$_2$ is often considered as the archetypal compound that does not have a Charge Density Wave (CDW) in any of its polytypes. By comparison, close iso-electronic compounds such as NbSe$_2$, TaS$_2$ and TaSe$_2$ all have CDW in at least one polytype. Here, we report traces of CDW in the 2H polytype of NbS$_2$, using diffuse x-ray scattering measurements at 77\,K and room temperature. We observe 12 extremely weak satellite peaks located at $\pm$13$^\mathrm{o}$ 54' from $\vec{a}^*$ and $\vec{b}^*$ around each Bragg peaks in the $(h,k,0)$ plane. These satellite peaks are commensurate with the lattice via $3\vec{q}-\vec{q}\,'=\vec{a}^*$, where $\vec{q}\,'$ is the 120$^{\circ}$ rotation of $\vec{q}$, and define two chiral $\sqrt{13}\,a\times\sqrt{13}\,a$ superlattices in real space. These commensurate wavevectors and tilt angle are identical to those of the CDW observed in the 1T polytype of TaS$_2$ and TaSe$_2$. To understand this similarity and the faintness of the peaks, we discuss possible sources of local 1T polytype environment in bulk 2H-NbS$_2$ crystals.