Electron-Lattice Coupling in 2D Transition Metal Dihalides MX₂ and Trihalides MX₃

The range of phenomena allowed in a correlated material is determined by an interplay between the lattice structure and electronic physics, with the roles of the two often the topic of debate.

Here we examine the role of  trigonal symmetry breaking and its electronic effects on correlated 2D van der Waals dihalides and trihalides MX₂ and MX₃, with M a transition metal and X a halogen ligand. We show that the 2D structure of these layered materials gives an inherent trigonal symmetry breaking to the transition metal d orbital manifold [1]. It is at most a 2-2-1 degeneracy, which can be controlled via changes in the ligand-ligand bond lengths, and inter-layer distances. We show that this physics is key to understanding the insulating state of materials in this class and discuss possible new exotic states in this class of materials.