We report a detailed study of the electronic and structural properties of the 39K superconductor MgB 2 and of several related systems of the same family, namely Mg 0.5 Al 0.5 B 2 , BeB 2 , CaSi 2 and CaBeSi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dimensional σ-bonding bands contribute significantly at the Fermi level.Similarities and differences between MgB 2 and BeB 2 (whose superconducting properties have not been yet investigated) are analyzed in detail. Our calculations for Mg 0.5 Al 0.5 B 2 show that metal substitution cannot be fully described in a rigid band model. CaSi 2 is studied as a function of pressure, and Be substitution in the Si planes leads to a stable compound similar in many aspects to diborides.

The experimental determination of the scaling of the superconducting critical temperature (T-c) vs the Fermi temperature (T-f) of the holes in the boron sigma subband is presented. The Fermi level has been tuned near the "shape resonance," i.e., the two- to three-dimensional crossover of the Fermi surface of the boron sigma subband by changing the Al/Mg content in Al1-xMgxB2. The product k(f)xi(0) of the Fermi wave vector (k(f)) times the superconducting Pippard coherence length (xi(0)), that is a measure of the pairing strength, remains constant, k(f)xi(0)=90 for x>0.66. This high-T-c phase occurs in the boron superlattice under a tensile microstrain in the range 3%

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We present ab-initio local density FLAPW calculations on non-reactive Nterminated [001] ordered GaN/Ag and GaN/Au interfaces and compare the results (such as metal induced gap states and Schottky barrier heights) with those obtained for GaN/Al, in order to understand the dependence of the relevant electronic properties on the deposited metal. Our results show that the density of gap states is appreciable only in the first semiconductor layer close to the interface. The decay length of the gap states in the semiconductor side is about 2.0 ± 0.1Å and is independent of the deposited metal, therefore being to a good extent a bulk property of GaN. Our calculated values of the Schottky barrier heights are Φ Bp (GaN/Ag) = 0.87 eV and Φ Bp (GaN/Au) = 1.08 eV; both values are smaller than the GaN/Al value (Φ Bp (GaN/Al) = 1.51 eV) and this quite large spread of values excludes the possibility of a Fermi level pinning within the GaN band gap. Because of the low screening in GaN, the potential barrier at the junction is strongly affected by the structural arrangement of the first metal layer at the interface. This leads to quite large variations of the Schottky barrier height as a function of the metal, in contrast with the behavior of GaAs/metal interfaces.