NaAlSi: a self-doped semimetallic superconductor with free electrons and covalent holes H. B. Rhee, S. Banerjee, E. R. Ylvisaker, and W. E. Pickett Department of Physics, University of California, Davis, CA 95616 (Dated: November 12, 2018) 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 3 s 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. PACS numbers: I. INTRODUCTION The discovery of new superconductors in unex- pected materials brings the potential to understand something deeper, or perhaps something different, about the underlying properties that favor supercon- ducting pairing. The discovery of high-temperature superconductivity (to 56 K so far) in Fe-pnictides 1 is a recent spectacular example, and is also an ex- ample of close relationships between magnetism and superconductivity, though the connections are still far from clear. New superconductors where little or no mag- netic effects are present are also arising, and these clearly involve different physics from the cuprate or Fe-pnictide high-temperature superconductors. Electron-doped MNCl, where M = Zr or Hf, becomes superconducting immediately upon undergoing the insulator-to-metal transition, 2–5 which, in the case of M = Hf, is higher than 25 K. The similarly lay- ered, electron-doped, ionic insulator TiNCl super- conducts up to 16 K. Magnetic behavior in these materials is at most subtle, amounting to an en- hancement in Pauli susceptibility near the metal-to- insulator transition. 6 In this paper we address the ternary silicide NaAlSi (space group P4/nmm , Z = 2), another ionic and layered material that shows unexpected superconductivity, and does so in its native (with- out doping or pressure) stoichiometric state, at 7 K. 7 NaAlSi introduces new interest from several viewpoints. First, it is an sp electron supercon- ductor, with a high T$_{c}$ for such materials at am- bient pressure. Pb is an sp superconductor with Yet another closely related compound is CaAlSi, whose two different stacking polymorphs and par- Third, NaAlSi is the isovalent sister (one row down in the periodic table for each atom) of LiBC. LiBC itself is (in a sense) isovalent and also isostructural to MgB$_{2}$; however due to the B-C alternation around the hexagon in the honeycomb-structure layer, LiBC is insulating rather than conducting. When hole- doped while retaining the same structure, Li$_{1}$$_{−}$$_{x}$BC has stronger electron-phonon coupling than does MgB$_{2}$. 12 While NaAlSi has a substantially different structure than LiBC, its isovalence and its combina- tion of covalence with some ionic character is shared with LiBC. Second, the Al-Si layered substructure is like that of the FeAs layer in the Fe-pnictide superconductors, raising the possibility of some connections between their electronic structures. In fact, NaAlSi has the same structure as the Fe-pnictide “111” compounds, with Al being tetrahedrally coordinated by Si (anal- ogous to Fe being tetrahedrally coordinated by As). In spite of their structural similarities, these com- pounds have major differences; for example, the Fe pnictides are 3 d electron systems with magnetism, while NaAlSi is an sp electron system without mag- netism. comparable T$_{c}$ (7.2 K) but with simple metallic bonding and heavy atoms, making it very differ- ent. Doped Si 8 and doped diamond 11 superconduct in the same range, and are of course very different classes of materials. A more relevant example is the pseudo-ternary compound Ba$_{1}$$_{−}$$_{x}$K$_{x}$BiO$_{3}$ (BKBO), which undergoes an insulator-to-metal transition for x ≈ 0 . 4, beyond which its T$_{c}$ surpasses 30 K.[9,10] arXiv:1001.0665v1 [cond-mat.supr-con] 5 Jan 2010