NaAlSi: An Unusual Self-Doped Semimetal With Free Electrons and Covalent Holes

The ternary compound NaAlSi is a layered $sp$ conductor that superconducts at a relatively high $T_c$ of 7 K. Using first principles electronic structure calculations (the FPLO code), we find that NaAlSi is a self-doped semimetal with several unusual characteristics (in addition to the interesting fact that its structure is the same as those of the Fe-pnictide 111 coupounds). Na gives up its electron, allowing the Si-Al sublayer to form a distinct set of covalent valence bands, which are nearly filled. A strongly directional, Al-derived free electron band overlaps the valence bands, providing the electron carriers. The $k_z$ dispersion is small, but it occurs at the Fermi level, leading to unusual Fermi surfaces. Wannier functions will be provided to assist in the understanding of the bonding, and comparison with the CaAlSi compound, with one additional electron and a different structure (the MgB$_2$ structure) will be presented.