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

Temperature dependent elastic constants and thermodynamic properties of BAs: An ab initio investigation

We present an ab initio study of the temperature dependent elastic constants of boron arsenide, a semiconductor that exhibits ultra-high thermal conductivity and is under investigation for thermal management in electronics. We test the consistency of our predictions by computing the temperature dependent sound velocity of the longitudinal acoustic mode along the [111] direction and comparing with experiments. Furthermore, as a by-product, we present the room temperature phonon dispersions and the temperature dependent thermal expansion, isobaric heat capacity, and average Gr\ufcneisen parameter compared with the most updated experiments and previous calculations when available. Finally, we present the theoretical estimate of the temperature dependent mean square atomic displacements

Quasi-harmonic temperature dependent elastic constants: applications to silicon, aluminum, and silver

We present ab-initio calculations of the quasi-harmonic temperature dependent elastic constants. The isothermal elastic constants are calculated at each temperature as second derivatives of the Helmholtz free energy with respect to strain and corrected for finite pressure effects. This calculation is repeated for a grid of geometries and the results interpolated at the minimum of the Helmholtz free energy. The results are compared with the quasi-static elastic constants. Thermodynamic relationships are used to derive the adiabatic elastic constants that are compared with the experimental measurements. These approaches are implemented for cubic solids in the thermo_pw code and are validated by applications to silicon, aluminum, and silver

Finite-temperature atomic relaxations: Effect on the temperature-dependent C 44elastic constants of Si and BAs

The effect of atomic relaxations on the temperature-dependent elastic constants (TDECs) is usually taken into account at zero temperature by the minimization of the total energy at each strain. In this paper, we investigate the order of magnitude of this approximation on a paradigmatic example: the C-44 elastic constant of diamond and zincblende materials. We estimate the effect of finite-temperature atomic relaxations within the quasi-harmonic approximation by computing ab initio the internal strain tensor from the second derivatives of the Helmholtz free-energy with respect to strain and atomic displacements. We apply our approach to Si and BAs and find a visible difference between the softening of the TDECs computed with the zero-temperature and finite-temperature atomic relaxations. In Si, the softening of C-44 passes from 8.6% to 4.5%, between T = 0 K and T = 1200 K. In BAs, it passes from 8% to 7%, in the same range of temperatures. Finally, from the computed elastic constant corrections, we derive the temperature-dependent Kleinman parameter, which is usually measured in experiments.Published under an exclusive license by AIP Publishing

On-surface and Subsurface Adsorption of Oxygen on Stepped Ag(210) and Ag(410) Surfaces

The adsorption of atomic oxygen and its inclusion into subsurface sites on Ag(210) and Ag(410) surfaces have been investigated using density functional theory. We find that--in the absence of adatoms on the first metal layer--subsurface adsorption results in strong lattice distortion which makes it energetically unfavoured. However subsurface sites are significantly stabilised when a sufficient amount of O adatoms is present on the surface. At high enough O coverage on the Ag(210) surface the mixed on-surface + subsurface O adsorption is energetically favoured with respect to the on-surface only adsorption. Instead, on the Ag(410) surface, at the coverage we have considered (3/8 ML), the existence of stable terrace sites makes the subsurface O incorporation less favourable. These findings are compatible with the results of recent HREEL experiments which have actually motivated this work.Comment: 8 pages, 4 figures and 1 tabl

Experimental investigation of the elastoplastic response of aluminum silicate spray dried powder during cold compaction

Mechanical experiments have been designed and performed to investigate the elasto-plastic behaviour of green bodies formed from an aluminum silicate spray dried powder used for tiles production. Experiments have been executed on samples obtained from cold compaction into a cylindrical mould and include: uniaxial strain, equi-biaxial flexure and high-pressure triaxial compression/extension tests. Two types of powders have been used to realize the green body samples, differing in the values of water content, which have been taken equal to those usually employed in the industrial forming of traditional ceramics. Yielding of the green body during compaction has been characterized in terms of yield surface shape, failure envelope, and evolution of cohesion and void ratio with the forming pressure, confirming the validity of previously proposed constitutive models for dense materials obtained through cold compaction of granulates.Comment: 17 pages; Journal of the European Ceramic Society, 201

THE PROMOTER OF VACUOLAR METAL TRANSPORTER IN ARABIDOPSIS HALLERI: AN EXAMPLE OF EVOLUTION FOR METAL HYPERTOLERANCE/HYPERACCUMULATION

In the metal hyperaccumulator Arabidopsis halleri, the metal vacuolar transporter (VMT) isinvolved in hyperaccumulation and hypertolerance. Three VMT promoter sequences have beenidentified in the genome of A. halleri; expression analysis, conducted by GUS assay, showed thateach promoter sequence of A. halleri induced higher expression than the promoter sequence of theVMT orthologous gene in A. thaliana. Moreover, it was observed that A. halleri VTM members areexpressed in leaf trichomes, whereas VTM of A. thaliana is not present in these organs. In silicoanalysis revealed that the 5\u2019 UTR region of VTM is highly conserved between the two Arabidopsisspecies, although a dimer of MYB-binding motifs harboured by the A. halleri promoters is mutatedin a single nucleotide in the sequence of A. thaliana. Site-specific mutagenesis of these motifs in thepromoter of A. halleri indicates that they are likely involved in trichome-specific expression. Therole of these MYB-binding motifs in the A. halleri VMT promoter was tested in A. thaliana,considering their effect in both metal tolerance and accumulation. Transgenic A. thaliana plantsexpressing VMT under the control of the native A. halleri VMT promoter are more tolerant, in termsof root length, biomass production and chlorophyll content, to high metal concentrations, than theones bearing the same construct with the mutated MYB-binding motifs. Differences in shoots andin roots were also observed regarding metal accumulation. These results underline the divergentevolution of the VMT promoter, conferring higher metal tolerance, accompanied by expression intrichomes, in the hyperaccumulator A. halleri and protection to heavy metal stress in transgenic A.thaliana. Furthermore, a possible biotechnological approach might be applied in phytoremediationor biofortification programs to modulate metal content in plants