NaAlSi: a self-doped semimetallic superconductor with free electrons and covalent holes

TL;DR

NaAlSi is a self-doped semimetallic superconductor with a unique electronic structure featuring covalent holes and free-electron-like carriers, with superconductivity likely arising from a non-phononic pairing mechanism.

Contribution

This study reveals the intrinsic low-carrier-density semimetallic nature and detailed electronic structure of NaAlSi, highlighting its self-doped features and potential unconventional pairing mechanism.

Findings

NaAlSi is a self-doped semimetal with a narrow Fermi level peak.

Superconductivity occurs at 7 K in NaAlSi.

NaAlGe has a similar band structure but does not superconduct.

Abstract

The layered ternary sp conductor NaAlSi, possessing the iron-pnictide "111" crystal structure, superconducts at 7 K. Using density functional methods, we show that this compound is an intrinsic (self-doped) low-carrier-density semimetal with a number of unusual features. Covalent Al-Si valence bands provide the holes, and free-electron-like Al 3s bands, which propagate in the channel between the neighboring Si layers, dip just below the Fermi level to create the electron carriers. The Fermi level (and therefore the superconducting carriers) lies in a narrow and sharp peak within a pseudogap in the density of states. The small peak arises from valence bands which are nearly of pure Si, quasi-two-dimensional, flat, and coupled to Al conduction bands. Isostructural NaAlGe, which is not superconducting above 1.6 K, has almost exactly the same band structure except for one missing piece of small Fermi surface. Certain deformation potentials induced by Si and Na displacements along the c-axis are calculated and discussed. It seems likely that the mechanism of pairing is related to that of several other lightly doped two-dimensional nonmagnetic semiconductors (TiNCl, ZrNCl, HfNCl), which is not well understood but apparently not of phonon origin.