| The following compounds were studied primarily for their superconducting properties or their potential to exhibit superconductivity.;The layered ternary sp conductor NaAlSi, possessing the iron-pnictide "111" crystal structure, superconducts at 7 K. Using density functional methods, it is shown 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 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. The possibility of a non-phononic pairing mechanism is discussed.;The hexagonal Laves phase of CaLi2, nonsuperconducting at ambient pressure, is a strong candidate for superconductivity at higher pressures. Zero resistance has been observed in CaLi2 at pressures above 11 GPa and temperatures as high as 13 K. However, it hasn't been ruled out that superconducting elemental Li instead may be the source of the superconductivity. The crystallographic structure of CaLi2 has been studied via powder x-ray diffraction at high pressures and low temperatures in a diamond anvil cell, in an attempt to measure the structural phase diagram in the superconducting regime.;PuPt2In7 does not superconduct down to 2 K, despite its electronic and structural similarities with superconductor PuCoGa 5 and other Pu-based superconductors. Results of generalized gradient approximation (GGA)+U calculations on PuPt2In 7 and on hypothetical PuPt2Ga7 are discussed. The strength of the c-f hybridization of PuPt2In 7 is similar to that of PuCoIn5, while PuPt2Ga 7 is less localized and more like PuCoGa5. The bare and f-weighted susceptibility within the constant matrix element approximation is calculated, showing a maximum at qx = y = 0.5. A similar and slightly stronger maximum is also found in PuCoGa5 and PuCoIn5. The absence of superconductivity in PuPt2In7 is examined based on the results of the calculations. |
