Interaction of CO with an Au monatomic chain at different strains: electronic structure and ballistic transport

We study the energetics, the electronic structure, and the ballistic transport of an infinite Au monatomic chain with an adsorbed CO molecule. We find that the bridge adsorption site is energetically favored with respect to the atop site, both at the equilibrium Au-Au spacing of the chain and at larger spacings. Instead, a substitutional configuration requires a very elongated Au-Au bond, well above the rupture distance of the pristine Au chain. The electronic structure properties can be described by the Blyholder model, which involves the formation of bonding/antibonding pairs of 5{\sigma} and 2{\pi}* states through the hybridization between molecular levels of CO and metallic states of the chain. In the atop geometry, we find an almost vanishing conductance due to the 5{\sigma} antibonding states giving rise to a Fano-like destructive interference close to the Fermi energy. In the bridge geometry, instead, the same states are shifted to higher energies and the conductance reduction with respect to pristine Au chain is much smaller. We also examine the effects of strain on the ballistic transport, finding opposite behaviors for the atop and bridge conductances. Only the bridge geometry shows a strain dependence compatible with the experimental conductance traces

Ballistic conductance of magnetic Co and Ni nanowires with ultrasoft pseudo-potentials

The scattering-based approach for calculating the ballistic conductance of open quantum systems is generalized to deal with magnetic transition metals as described by ultrasoft pseudo-potentials. As an application we present quantum-mechanical conductance calculations for monatomic Co and Ni nanowires with a magnetization reversal. We find that in both Co and Ni nanowires, at the Fermi energy, the conductance of $d$ electrons is blocked by a magnetization reversal, while the $s$ states (one per spin) are perfectly transmitted. $d$ electrons have a non-vanishing transmission in a small energy window below the Fermi level. Here, transmission is larger in Ni than in Co.Comment: 9 pages, 6 figures, to appear in PR

Phonons Softening in Tip-Stretched Monatomic Nanowires

It has been shown in recent experiments that electronic transport through a gold monatomic nanowire is dissipative above a threshold voltage due to excitation of phonons via the electron-phonon interaction. We address that data by computing, via density functional theory, the zone boundary longitudinal phonon frequency of a perfect monatomic nanowire during its mechanical elongation. The theoretical frequency that we find for an ideally strained nanowire is not compatible with experiment if a uniformly distributed stretch is assumed. With the help of a semi-empirical Au-Au potential, we model the realistic nanowire stretching as exerted by two tips. In this model we see that strain tends to concentrate in the junctions, so that the mean strain of the nanowire is roughly one half of the ideal value. With this reduced strain, the calculated phonon softening is in much better agreement with experiment.Comment: 9 pages,3 figures, Surface Science, in pres

Characterization of the Novel Photosynthetic Protein PPP7 involved in Cyclic Electron Flow around PSI

Photosynthetic organisms are able to convert light energy into chemical energy by the operation of the two photosystems, the cytochrome b6/f complex and the ATPase. The two photosystems operate in series during linear electron flow to split H2O and to generate NADP+. During electron transport, a pH gradient is generated across the thylakoid membrane which is used for the generation of ATP. In addition to the linear electron transport mode, ATP can also be produced via cyclic electron flow around photosystem I (CEF). The physiological role of CEF in vascular plants with C3-type photosynthesis is still not solved. Potential functions of CEF are (i) the dissipation of excessive light energy by increasing non-photochemical quenching (NPQ); (ii) ATP synthesis during steady-state photosynthesis; (iii) the regulation of the stromal oxidation state under stress conditions and under conditions when the Calvin cycle is not available as a sink for NADPH. With exception of the thylakoid NADPH-dehydrogenase complex and the stromal protein PGR5, the components that contribute to CEF are still unknown. Obscure is also the regulation that controls the switch from linear to cyclic flow. We have identified a novel transmembrane protein, named PPP7, which is located in thylakoids of photoautotrophic eukaryotes. Mutants lacking PPP7 exhibit the same phenotype as plants missing PGR5. These mutants show reduced NPQ, decreased P700 oxidation and perturbation of ferredoxin-dependent CEF. The work described in this thesis demonstrates that PPP7 and PGR5 interact physically, and that both co-purify with photosystem I. PPP7 does also interact in yeast assays with the cytochrome b6/f complex, as well as with the stromal proteins ferredoxin (Fd) and ferredoxin-NADPH oxido-reductase (FNR), but PPP7 is not a constitutive component of any of the major photosynthetic complexes. In consequence, the existence of a PPP7/PGR5 complex integrated in the thylakoid membrane and facilitating CEF around PSI in eukaryotes, possibly by shuttling electrons together with ferredoxin and the FNR from photosystem I to the cytochrome b6/f complex, is proposed. Moreover, CEF is enhanced in the Arabidopsis psad1 and psae1 mutants with a defect in photosystem I oxidation in contrast to the cyanobacterial psae mutant which exhibits an decreased CEF, pointing to fundamental mechanistic differences in the cyclic electron flow of cyanobacteria and vascular plants. The Arabidopsis psad1 and psae1 mutants also show higher contents of ferredoxin and of the PPP7/PGR5 complex, supporting a role of PPP7 and PGR5 in the switch from linear to cyclic electron flow depending on the redox state of the chloroplast