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-doped͒ low-carrierdensity semimetal with a number of unusual features. Covalent Al-Si valence bands provide the holes, and free-electronlike 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 carriers͒ 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, HfNCl͒, which is not well understood but apparently not of phonon origin.

Density functional theory calculations of the electronic structure of Ce-and Pu-based heavy fermion superconductors in the so-called 115 family are performed. The gap equation is used to consider which superconducting order parameters are most favorable assuming a pairing interaction that is peaked at (π,π,qz) -the wavevector for the antiferromagnetic ordering found in close proximity. In addition to the commonly accepted d x 2 −y 2 order parameter, there is evidence that an extended s-wave order parameter with nodes is also plausible. We discuss whether these results are consistent with current observations and possible measurements that could help distinguish between these scenarios.PACS numbers: 74.20. Mn, 71.27.+a, 74.70.Tx CeCoIn 5 is a heavy fermion superconductor [1], which has been shown to lie in close proximity to an antiferromagnetic quantum critical point [2][3][4]. Its structure consists of layers of square planar Ce atoms, similar in that respect to the cuprate superconductors. Soon after the discovery of superconductivity in CeCoIn 5 many measurements were performed that were consistent with lines of nodes in the superconducting gap, including specific heat, thermal conductivity [5], spin-lattice relaxation rate [6], and penetration depth [7]. Ideally, ARPES measurements could reveal the anisotropic gap structure in k-space, but current equipment has neither the energy resolution nor the temperature range to perform such a study. In its absence probes which are directionally weighted averages over the Fermi surface have been used to identify the location of the nodes, including field-angle dependent specific heat and thermal conductivity [8][9][10][11], H c2 (θ) [12] point contact Andreev reflection measurements [13], and vortex lattice structure [14,15] all of which now argue that nodes lie along the Γ → (π,π,q z ) direction in the tetragonal Brillouin zone. In addition, a neutron scattering resonance has been observed at (π,π,π) which can be interpreted as consistent with d x 2 −y 2 symmetry [16]. Taken together the experimental evidence appears fairly compelling that CeCoIn 5 is a superconductor with a d x 2 −y 2 gap structure. Measurements on other members in this family are relatively scarce due to the fact that either pressure is required for superconductivity to occur (as in CeRhIn 5 , CeIn 3 , Ce 2 RhIn 8 , and On the theoretical side, the BCS theory of superconductivity allows one to calculate the superconducting gap structure at T = 0 K by solving the gap equation:where ∆(k) is the superconducting gap function, k is the electronic dispersion, and Γ(k, k ) is the pairing interaction.To solve this equation the low energy electronic structure (ie. the Fermi surface, and effective masses) and the pairing interaction Γ(k, k ) must be known. This would appear to be a formidable if not impossible task for such strongly correlated materials such as the cuprates or the heavy fermion materials which we wish to study [31]. It is well known that for strongly correlated materials there are no well...

Stoichiometric LiFeAs at ambient pressure is an 18 K superconductor while isostructural, isoelectronic MgFeGe does not superconduct, despite their extremely similar electronic structures. To investigate possible sources of this distinctively different superconducting behavior, we quantify the differences using first principles density functional theory. Total Fe 3d occupations are identical, with individual 3d orbital occupations differing by no more than 0.015. However, a redistribution of bands just above the Fermi level ε F provides an important distinction, with more Fe-derived states within 0.5 eV of the Fermi level and a higher N(ε F ) in MgFeGe. For many mechanisms these features would enhance the tendency toward superconductivity by providing more Cooper pairs (in MgFeGe), but the tendency toward magnetic instability might be more important. Two of the five Fermi surfaces differ between LiFeAs and MgFeGe, but still lead to similar q-dependencies of susceptibilities χ 0 (q) including the familiar broad peak at (π, π). The larger χ 0 (q) in MgFeGe, by 10-15% throughout the zone, leads us to tentatively identify this proximity to magnetic instability as the feature underlying the absence of superconductivity in MgFeGe. Another significant difference is the 2.5% difference of the in-plane lattice constant, positioning LiFeAs as a chemically compressed version of MgFeGe. This has possible significance since certain Fe pnictides display pressure-induced superconductivity.

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