Symmetry-constrained low-energy effective Hamiltonian for topological RuC and OsC monolayers.

We derive a low-energyeffective Hamiltonian for monolayer osmium carbide (OsC) and ruthenium carbide (RuC) in a planar hexagonal configuration. First-principles calculations indicate that both monolayers are dynamically stable and exhibit features of a two-dimensional quantum spin Hall phase, characterized by a nontrivialtopological invariant. Using symmetry analysis at thepoint, we construct a multibandHamiltonian including spin-orbit coupling and reduce it to a four-band low-energy model through Löwdin partitioning. The effective Hamiltonian has a block-diagonal form, with two blocks related by time-reversal symmetry, analogous to the Bernevig-Hughes-Zhang (BHZ) model. In contrast to the standard BHZ form, the symmetry-allowed off-diagonal coupling contains quadratic momentum-dependent terms, which modify the low-energy dispersion near thepoint. The fitted parameters reproduce theab initioband structures in the low-energy region, yielding a compact model for analyzing the electronic and topological properties of monolayer OsC and RuC.