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We describe a method for calculating, within density functional theory, the electronic structure associated with typical defects which substitute for Cu in the CuO 2 planes of high-T c superconducting materials. The focus is primarily on Bi 2 Sr 2 CaCu 2 O 8 , the material on which most STM measurements of impurity resonances in the superconducting state have been performed. The magnitudes of the effective potentials found for Zn, Ni and vacancies on the in-plane Cu sites in this host material are remarkably consistent with phenomenological fits of potential scattering models to STM resonance energies. The effective potential ranges are quite short, of order 1 Å with weak long range tails, in contrast to some current models of extended potentials which attempt to fit STM data.For the case of Zn and Cu vacancies, the effective potentials are strongly repulsive, and states on the impurity site near the Fermi level are simply removed. The local density of states (LDOS) just above the impurity is nevertheless found to be a maximum in the case of Zn and a local minimum in case of the vacancy, in agreement with experiment. The Zn and Cu vacancy patterns are explained as due to the long-range tails of the effective impurity potential at the sample surface. The case of Ni is richer due to the Ni atom's strong hybridization with states near the Fermi level; in particular, the short range part of the potential is attractive, and the LDOS is found to vary rapidly with distance from the surface and from the impurity site. We propose that the current controversy surrounding the observed STM patterns can be resolved by properly accounting for the effective impurity potentials and wave-functions near the cuprate surface. Other aspects of the impurity states for all three species are discussed.2

Pt intermetallic line compounds, such as with Pb and Bi, have been observed to improve dramatically the anode carbon monoxide (CO) tolerance of fuel cells for oxidation of small organic molecules. We have used density functional theory to study the CO and H adsorption on different surfaces of these line compounds. Among different surface orientations of PtPb and PtBi, we find (100)B and (110) have much lower cleavage energies and CO adsorption energies than (100)A and (001) and also much lower CO adsorption energies than Pt(111). Thus, (100)B and (110) are the surfaces most relevant to experimental observations, and the increased CO tolerance is not attributable to the (001) surface of the line compounds as assumed experimentally, because it binds CO the strongest, even more strongly than Pt(111). We also find that CO is not likely to dissociate on these materials. Finally, we correlate d-band center and CO adsorption energy for these non close-packed systems by developing a more universal form of the original d-band center model that includes effects of symmetry of adsorption site and local relaxation. We find that the increased CO tolerance arises from a downward shift of Pt d-band center because of alloying, which also accounts for the difference between PtPb and PtBi.

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