Trigonal Symmetry Reduction and Correlation Effects in 2D Transition Metal Dihalides MX2 and Trihalides MX3

<br/>The physics of van der Waals correlated dihalides and trihalides MX₂ and MX₃, with M a transition metal and X a halogen ligand, is determined by the triangular and hexagonal lattice formed by the transition metal ions, and each atom’s octahedral cage. Here we show that the 2D structure of these layered materials gives an inherent trigonal symmetry breaking to the transition metal d orbital manifold. This 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 as a result of further symmetry breaking.<br/><br/><br/>This research was supported in part by the National Science Foundation (NSF) under DMREF Award DMR1729303. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0001209.AJM is supported in part by Programmable Quantum Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under award DE-SC0019443. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The Flatiron Institute is a division of the Simons Foundation.