Nuclear Quantum Effects on the Charge-Density Wave Transition in NbX 2 (X = S, Se).

Understanding the origin of charge-density wave (CDW) instability is important for manipulating novel collective electronic states. Many layered transition metal dichalcogenides (TMDs) share similarity in the structural and electronic instability, giving rise to diverse CDW phases and superconductivity. It is still puzzling that even isostructural and isoelectronic TMDs show distinct CDW features. For instance, bulk NbSe 2 exhibits CDW order at low temperature, while bulk NbS 2 displays no CDW instability. The CDW transitions in single-layer NbS 2 and NbSe 2 are also different. In the classic limit, we investigate the electron correlation effects on the dimensionality dependence of the CDW ordering. By performing ab initio path integral molecular dynamics simulations and comparative analyses, we further revealed significant nuclear quantum effects in these systems. Specifically, the quantum motion of sulfur anions significantly reduces the CDW transition temperature in both bulk and single-layer NbS 2 , resulting in distinct CDW features in the NbS 2 and NbSe 2 systems.