Ferromagnetic and nonmagnetic 1T′ charge density wave states in transition metal dichalcogenides: Physical mechanisms and charge doping induced reversible transition

The charge density wave (CDW) states of two-dimensional transition metal dichalcogenides (TMDs) originate from intrinsic couplings between the electronic structures and lattice distortion, inducing interesting physical and chemical properties. The observed TMDs CDW states are mostly nonmagnetic (NM) but with a few ferromagnetic (FM) cases. Physical mechanisms for the formation of FM CDW remain elusive. In this paper, we used density functional theory calculations to study a set of TMDs with magnetic transition metal elements (e.g., V, Cr, and Mn). We found that the FM state can stem from the direct exchange to superexchange transition (e.g., $\mathrm{Cr}{X}₂$) or the $M\text{\ensuremath{-}}M$ ($M$ is the metal atom) dimerization (e.g., $\mathrm{Mn}{X}₂$). A crystal structure distortion index is proposed to distinguish the different formation mechanisms of FM CDW states. Interestingly, $\mathrm{Cr}{X}₂$ has both NM and FM CDW states, which is not observed in other TMDs materials. We found that charge (electron or hole) doping could modulate the different formation mechanisms and induce a phase transition between these two CDW states in ${\mathrm{CrS}}₂$, leading to significant actuation strain output (i.e., 12.17% and 5.93% along $x$ and $y$ directions, respectively) and drastic change of magnetism, which could enable some multifunctionality applications of TMD materials.