22 research outputs found
Effect of stretching on the ballistic conductance of Au nanocontacts in presence of CO: a density functional study
CO adsorption on an Au monatomic chain is studied within density functional
theory in nanocontact geometries as a function of the contact stretching. We
compare the bridge and atop adsorption sites of CO, finding that the bridge
site is energetically favored at all strains studied here. Atop adsorption
gives rise to an almost complete suppression of the ballistic conductance of
the nanocontact, while adsorption at the bridge site results in a conductance
value close to 0.6 G0, in agreement with previous experimental data. We show
that only the bridge site can qualitatively account for the evolution of the
conductance as a function of the contact stretching observed in the
experimental conductance traces. The numerical discrepancy between the
theoretical and experimental conductance slopes is rationalized through a
simple model for the elastic response of the metallic leads. We also verify
that our conductance values are not affected by the specific choice of the
nanocontact geometry by comparing two different atomistic models for the tips
Interaction of a CO molecule with a Pt monatomic wire: electronic structure and ballistic conductance
We carry out a first-principles density functional study of the interaction
between a monatomic Pt wire and a CO molecule, comparing the energy of
different adsorption configurations (bridge, on top, substitutional, and tilted
bridge) and discussing the effects of spin-orbit (SO) coupling on the
electronic structure and on the ballistic conductance of two of these systems
(bridge and substitutional). We find that, when the wire is unstrained, the
bridge configuration is energetically favored, while the substitutional
geometry becomes possible only after the breaking of the Pt-Pt bond next to CO.
The interaction can be described by a donation/back-donation process similar to
that occurring when CO adsorbs on transition-metal surfaces, a picture which
remains valid also in presence of SO coupling. The ballistic conductance of the
(tipless) nanowire is not much reduced by the adsorption of the molecule on the
bridge and on-top sites, but shows a significant drop in the substitutional
case. The differences in the electronic structure due to the SO coupling
influence the transmission only at energies far away from the Fermi level so
that fully- and scalar-relativistic conductances do not differ significantly.Comment: 12 pages, 12 figures; figure misplacement and minor syntax issues
fixed, some references updated and correcte
Interaction of a CO molecule with a Pt monoatomic chain: the top geometry
Recent experiments showed that the conductance of Pt nanocontacts and
nanowires is measurably reduced by adsorption of CO. We present DFT
calculations of the electronic structure and ballistic conductance of a Pt
monoatomic chain and a CO molecule adsorbed in an on-top position. We find that
the main electronic molecule-chain interaction occurs via the and
orbitals of the molecule, involved in a donation/back-donation
process similar to that of CO on transition-metal surfaces. The ideal ballistic
conductance of the monoatomic chain undergoes a moderate reduction by about 1.0
G_0 (from 4 G_0 to 3.1 G_0) upon adsorption of CO. By repeating all
calculations with and without spin-orbit coupling, no substantial spin-orbit
induced change emerges either in the chain-molecule interaction mechanism or in
the conductance.Comment: 4 pages, 2 figures, in proceedings of Frontiers of Fundamental and
Computational Physic
Oxide Heterostructures from a Realistic Many-Body Perspective
Oxide heterostructures are a new class of materials by design, that open the
possibility for engineering challenging electronic properties, in particular
correlation effects beyond an effective single-particle description. This short
review tries to highlight some of the demanding aspects and questions,
motivated by the goal to describe the encountered physics from first
principles. The state-of-the-art methodology to approach realistic many-body
effects in strongly correlated oxides, the combination of density functional
theory with dynamical mean-field theory, will be briefly introduced. Discussed
examples deal with prominent Mott-band- and band-band-insulating type of oxide
heterostructures, where different electronic characteristics may be stabilized
within a single architectured oxide material.Comment: 19 pages, 9 figure
Giant apparent lattice distortions in STM images of corrugated sp2-hybridised monolayers
We report on the strengths and limitations of scanning tunnelling microscopy (STM) when used for characterising atomic-scale features of quasi two-dimensional materials, such as graphene and single layers of hexagonal boron nitride, which may present strong corrugations when grown epitaxially on a substrate with a lattice mismatch. As a paradigmatic test case, we choose single-layer and bilayer graphene on Ru(0001), because their STM images show both a long-range moire modulation and complex atomic-scale distortions of the graphene lattice. Through high-resolution STM measurements, we first determine with high accuracy the moire epitaxial relations of the single layer and the bilayer with respect to the metal substrate. In particular, we also provide direct evidence for the existence of AA-stacked bilayer graphene domains on Ru(0001). We then demonstrate that the local strain distribution, as inferred from the same STM images, can be affected by large errors, so that apparent giant strains arise in some regions of the moire as an imaging artefact. With the aid of density functional theory simulations, we track down the origin of these fictitious distortions in the high directionality of the graphene pi-orbital density combined with the large corrugation of the sample. The proposed theoretical model correctly accounts for the observed dependence of the apparent strain on the STM tip-sample separation and on the different degree of curvature of the second graphene layer with respect to the single layer
Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
Quantum ESPRESSO is an integrated suite of computer codes for
electronic-structure calculations and materials modeling, based on
density-functional theory, plane waves, and pseudopotentials (norm-conserving,
ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn
Source Package for Research in Electronic Structure, Simulation, and
Optimization". It is freely available to researchers around the world under the
terms of the GNU General Public License. Quantum ESPRESSO builds upon
newly-restructured electronic-structure codes that have been developed and
tested by some of the original authors of novel electronic-structure algorithms
and applied in the last twenty years by some of the leading materials modeling
groups worldwide. Innovation and efficiency are still its main focus, with
special attention paid to massively-parallel architectures, and a great effort
being devoted to user friendliness. Quantum ESPRESSO is evolving towards a
distribution of independent and inter-operable codes in the spirit of an
open-source project, where researchers active in the field of
electronic-structure calculations are encouraged to participate in the project
by contributing their own codes or by implementing their own ideas into
existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
Efficient DFT+U calculations of ballistic electron transport: Application to Au monatomic chains with a CO impurity
An efficient method for computing the Landauer-Buttiker conductance of an open quantum system within DFT + U is presented. The Hubbard potential is included in electronic-structure and transport calculations as a simple renormalization of the nonlocal pseudopotential coefficients by restricting the integration for the onsite occupations within the cutoff spheres of the pseudopotential. We apply the methodology to the case of an Au monatomic chain in the presence of a CO molecule adsorbed on it. We show that the Hubbard U correction removes the spurious magnetization in the pristine Au chain at the equilibrium spacing, as well as the unphysical contribution of d electrons to the conductance, resulting in a single (spin-degenerate) transmission channel and a more realistic conductance of 1 G(0). We find that the conductance reduction due to CO adsorption is much larger for the atop site than for the bridge site, so that the general picture of electron transport in stretched Au chains given by the local density approximation remains valid at the equilibrium Au-Au spacing within DFT + U. DOI: 10.1103/PhysRevB.87.08510
Role of components of frailty on quality of life in dialysis patients: a cross-sectional study.
BACKGROUND:
Many people on dialysis suffer a variety of conditions that can affect frailty (the condition or quality of being frail), such as comorbidities, disabilities, dependence, malnutrition, cognitive impairment and poor social conditions. Frailty is suspected to affect quality of life (QoL).
OBJECTIVES:
The study aimed to evaluate the effect of the different components of frailty on the QoL of people on dialysis.
METHODS:
We enrolled 203 out of 233 prevalent patients on dialysis in the Trieste area of Italy. We applied the Short-Form 36 (SF-36) questionnaire, Activities of Daily Living, Instrumental Activities of Daily Living, Subjective Global Assessment scales and Karnofsky Index. In addition we analysed their social conditions.
RESULTS:
Dependence, malnutrition and disability had a negative role on QoL. Living with family and good social-economic conditions were significantly related to a better QoL.
CONCLUSIONS:
Dependence, malnutrition, disability, poor social and economic conditions have a significant effect on life quality. The role of comorbidities appears to be less important. Screening of patients, nutritional and functional rehabilitation and prevention of social isolation appear to be indispensable in guaranteeing a satisfactory life qualit