Ab-initio Insights on the Fermiology of d¹ Transition metals in Honeycomb lattice : Hierarchy of hopping pathways and spin-orbit coupling

Motivated by the intriguing suggestion of realizing SU(8) Dirac semi-metal with $J=3/2$ electrons on a honeycomb lattice, we provide a systematic study of the interplay of various hopping pathways and atomic spin-orbit coupling for the low energy electrons in candidate d$^1$ transition metal halides MX$_3$ (M=Ti, Zr, Hf; X=F, Cl, Br). By combining first principle calculations and minimal hopping Hamiltonian, we uncover the role of dominant direct metal-metal hopping on top of indirect metal-halide-metal hopping. This sets up a hierarchy of hopping pathways that centrally modify the SU(8) picture for the above materials. These hopping interactions, along with the spin-orbit coupling, lead to a plethora of exactly compensated metals instead of the SU(8) Dirac semi-metal. Remarkably the same can be understood as descendants of a topological insulator obtained by gapping out the SU(8) Dirac semi-metallic phase. The resultant compensated metals have varied Fermi surface topology and are separated by Lifshitz phase transitions. We discuss the implications of the proximate Lifshitz transition, which may be accessed via strain, in the context of the relevant materials.