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

Lattice dynamics effects on the magnetocrystalline anisotropy energy: application to MnBi

Using a first-principles fully relativistic scheme based on ultrasoft pseudopotentials and density functional perturbation theory, we study the magnetocrystalline anisotropy free energy of the ferromagnetic binary compound MnBi. We find that differences in the phonon dispersions due to the different orientations of the magnetization (in-plane and perpendicular to the plane) give a difference between the vibrational free energies of the high-temperature and low-temperature phases. This vibrational contribution to the magnetocrystalline anisotropy energy (MAE) constant, $K_u$, is non-negligible. When the energy contribution to the MAE is calculated by the PBEsol exchange and correlation functional, the addition of the phonon contribution allows to get a $T = 0$ K $K_u$ and a spin-reorientation transition temperature in reasonable agreement with experiments.Comment: 6 pages, 4 figures, 2 table

Electric fields with ultrasoft pseudo-potentials: applications to benzene and anthracene

We present density functional perturbation theory for electric field perturbations and ultra-soft pseudopotentials. Applications to benzene and anthracene molecules and surfaces are reported as examples. We point out several issues concerning the evaluation of the polarizability of molecules and slabs with periodic boundary conditions.Comment: 10 pages, 7 figures, to appear in J. Chem. Phy

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

Ab-initio calculations for the beta-tin diamond transition in Silicon: comparing theories with experiments

We investigate the pressure-induced metal-insulator transition from diamond to beta-tin in bulk Silicon, using quantum Monte Carlo (QMC) and density functional theory (DFT) approaches. We show that it is possible to efficiently describe many-body effects, using a variational wave function with an optimized Jastrow factor and a Slater determinant. Variational results are obtained with a small computational cost and are further improved by performing diffusion Monte Carlo calculations and an explicit optimization of molecular orbitals in the determinant. Finite temperature corrections and zero point motion effects are included by calculating phonon dispersions in both phases at the DFT level. Our results indicate that the theoretical QMC (DFT) transition pressure is significantly larger (smaller) than the accepted experimental value. We discuss the limitation of DFT approaches due to the choice of the exchange and correlation functionals and the difficulty to determine consistent pseudopotentials within the QMC framework, a limitation that may significantly affect the accuracy of the technique.Comment: 13 pages, 9 figures, submitted to the Physical Review B on October 2

First-Principles Wannier Functions of Silicon and Gallium Arsenide

We present a self-consistent, real-space calculation of the Wannier functions of Si and GaAs within density functional theory. We minimize the total energy functional with respect to orbitals which behave as Wannier functions under crystal translations and, at the minimum, are orthogonal. The Wannier functions are used to calculate the total energy, lattice constant, bulk modulus, and the frequency of the zone-center TO phonon of the two semiconductors with the accuracy required nowadays in ab-initio calculations. Furthermore, the centers of the Wannier functions are used to compute the macroscopic polarization of Si and GaAs in zero electric field. The effective charges of GaAs, obtained by finite differentiation of the polarization, agree with the results of linear response theory.Comment: 12 pages, 2 PostScript figures, RevTeX, to appear in Physical Review

Magnetic phenomena, spin-orbit effects, and Landauer conductance in Pt nanowire contacts

Platinum monatomic nanowires were predicted to spontaneously develop magnetism, involving a sizable orbital moment via spin orbit coupling, and a colossal magnetic anisotropy. We present here a fully-relativistic (spin-orbit coupling included) pseudo-potential density functional calculation of electronic and magnetic properties, and of Landauer ballistic conductance of Pt model nanocontacts consisting of short nanowire segments suspended between Pt leads or tips, reprented by bulk planes. Even if short, and despite the nonmagnetic Pt leads, the nanocontact is found to be locally magnetic with magnetization strictly parallel to its axis. Especially under strain, the energy barrier to flip the overall spin direction is predicted to be tens of meV high, and thus the corresponding blocking temperatures large, suggesting the use of static Landauer ballistic electrical conductance calculations. We carry out such calculations, to find that inclusion of spin-orbit coupling and of magnetism lowers the ballistic conductance by about $15\div20$% relative to the nonmagnetic case, yielding $G\sim 2 G_0$ ($G_0=2e^2/h$), in good agreement with break junction results. The spin filtering properties of this highly unusual spontaneously magnetic nanocontact are also analysed.Comment: 10 pages, 5 figures, submitted to Phys. Rev.