Correlation effects and spin-orbit interactions in two-dimensional hexagonal 5d transition metal carbides, Ta_n+1C_n (n = 1,2,3)

Density functional calculations are used to investigate the electronic structure of two-dimensional 5d tantalum carbides with honeycomb-like lattice structures. We focus on changes in the low-energy bands near the Fermi level with dimensionality. We find that the Ta 5d states dominate, and the extended nature of the wave functions makes them weakly correlated. The carbide sheets are prone to long-range magnetic order and we evaluate their stability to enhanced electron-electron interactions through a Hubbard U correction. Lastly, we find that the splitting of the bands near the Fermi level caused by spin-orbit interaction decreases with increasing dimensionality. In the lowest-dimensionality (n = 1) case, the band splitting pushes a conduction band above the Fermi level and leads to a semi-metallic state.