The exchange-correlation functionals of the generalized gradient approximation ͑GGA͒ are still the most used for the calculations of the geometry and electronic structure of solids. The PBE functional ͓J. P. Perdew et al., Phys. Rev. Lett. 77, 3865 ͑1996͔͒, the most common of them, provides excellent results in many cases. However, very recently other GGA functionals have been proposed and compete in accuracy with the PBE functional, in particular for the structure of solids. We have tested these GGA functionals, as well as the local-density approximation ͑LDA͒ and TPSS ͑meta-GGA approximation͒ functionals, on a large set of solids using an accurate implementation of the Kohn-Sham equations, namely, the full-potential linearized augmented plane-wave and local orbitals method. Often these recently proposed GGA functionals lead to improvement over LDA and PBE, but unfortunately none of them can be considered as good for all investigated solids.

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The charge response of the ladders in Sr 14−x Ca x Cu 24 O 41 is characterized by dc resistivity, low frequency dielectric, and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x = 0, 3, and 6, in addition to the mode along the legs. For x = 8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.The physics of doped Mott-Hubbard insulators challenges conventional theories of metals and insulators. 1 The effect of strong Coulomb interactions produces a rich variety of exotic ordering phenomena, which have been the focus of intense scientific activity in recent years. The spin-chain and ladder self-doped compound Sr 14−x Ca x Cu 24 O 41 has attracted much attention since it is the first superconducting copper oxide with a nonsquare lattice. 2 Theoretically, in doped two-leg Cuu O ladders, superconductivity ͑SC͒ is tightly associated with the spin gap and in competition with charge-density wave ͑CDW͒. 3 While both the spin gap and CDW were established in the ladders of Sr 14−x Ca x Cu 24 O 41 , 4-6 the relevance of these objects to electronic properties and superconductivity is still subject of intensive discussion. Recently, it was shown, on the basis of dielectric response data, that substitution of Sr 2+ by Ca 2+ gradually suppresses the insulating CDW phase, which eventually vanishes for x Ͼ 9, Ref. 7. In contrast to these results, dynamical Raman response observed above RT for x = 0 was assigned to CDW fluctuations and found to persist in the metallic phase of x = 12, a system which becomes SC under pressure. 8 It is of particular interest to learn more about the nature of CDW order in the spin ladders, which presents a nice experimental system of strongly interacting electrons. Although the ground state for 0 ഛ x ഛ 9 reveals a number of well-known fingerprints of the conventional CDW, such as the pinned phason 9 and the broad dispersion at radio frequencies due to screening of the CDW by free carriers, 5-7 its origin is certainly more complicated, since the system does not undergo a metal-to-insulator but an insulator-to-insulator transition. The role of Ca substitution is another open issue. Suppression of the CDW phase was ascribed 7 to worsened nesting conditions 10 implying that the system becomes more 2D already at ambient pressure for large x. On the other hand, it was suggested that at ambient pressure ͑for all x͒ the charge dynamics is essentially one-dimensional 2,11 ͑1D͒ and that only the application of pressure induces the dimensional crossover from 1 to 2. 2 In this Report, we address these important questions concerning the charge-ordered ground state in the ladders of Sr 14−x Ca x Cu 24 O 41 . Our results give evidence that the CDW is two dimensional with an anisotropic dispersion: the longrange charge order develops only in ladder planes, leading to a screen...

The specialized exchange-correlation functionals of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] (WC) and Hammer et al. [Phys. Rev. B 59, 7413 (1999)] [revised Perdew, Burke, and Ernzerhof (RPBE)] yield good results for either the lattice constants of solids or the atomization energies of molecules, respectively, but are rather poor for the opposite quantity. By combining the WC and RPBE functionals, we construct a functional that performs equally well for both molecules and solids. Our proposed functional, which is still in the form of the simple generalized gradient approximation, can thus be applied efficiently to systems that involve both finite and infinite systems, a case that is crucial, for example, in heterogeneous catalysis studies. Therefore, the chemisorption of CO on transition-metal surfaces was considered, and it is shown that our functional gives improved results.

The charge response in the spin chain/ladder compound Sr14−xCaxCu24O41 is characterized by DC resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x = 0 to 10 K for x = 9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the inter-ladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.