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

Magnetic and orbital blocking in Ni nanocontacts

We address the fundamental question of whether magneto-resistance (MR) of atomic-sized contacts of Nickel is very large because of the formation of a domain wall (DW) at the neck. Using {\em ab initio} transport calculations we find that, as in the case of non-magnetic electrodes, transport in Ni nanocontacts depends very much on the orbital nature of the electrons. Our results are in agreement with several experiments in the average value of the conductance. On the other hand, contrary to existing claims, DW scattering does {\em not} account for large MR in Ni nanocontacts.Comment: 5 pages, 3 Figure

Effect of electron correlations in Pd, Ni, and Co monowires

We investigated the effect of mean-field electron correlations on the band electronic structure of Co, Ni, and Pd ultra-thin monatomic nanowires, at the breaking point, by means of density-functional calculations in the self-interaction corrected LDA approach (LDA+SIC) and alternatively by the LDA+$U$ scheme. We find that adding static electron correlations increases the magnetic moment in Pd monowires, but has negligible effect on the magnetic moment in Co and Ni. Furthermore, the number of $d$-dominated conductance channels decreases somewhat compared to the LDA value, but the number of $s$-dominated channels is unaffected, and remains equal to one per spin.Comment: to appear in PR

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

Orbital eigenchannel analysis for ab-initio quantum transport calculations

We show how to extract the orbital contribution to the transport eigenchannels from a first-principles quantum transport calculation in a nanoscopic conductor. This is achieved by calculating and diagonalizing the first-principles transmission matrix reduced to selected scattering cross-sections. As an example, the orbital nature of the eigenchannels in the case of Ni nanocontacts is explored, stressing the difficulties inherent to the use of non-orthogonal basis sets and first-principles Hamiltonians.Comment: 5 pages, 5 figurs; replaced with final version, introduction revised; to be published in PR

Selective d-state Conduction Blocking in Nickel Nanocontacts

The lowest conductance step for a Ni nanocontact is anomalously small in comparison with the large expected number of conducting channels. We present electronic structure calculations for an extremely idealized Ni nanobridge consisting of just a monatomic nanowire. Our calculations show that no less than eight single spin bands cross the Fermi level in a nonmagnetic Ni monatomic wire, dropping marginally to seven in the more stable, fully ferromagnetic state. However, when we build in the wire a magnetization reversal, or domain wall, by forcing the net magnetization to be zero, we suddenly find that d electrons selectively cease to propagate across the wall. s electron propagation remains, and can account for the small observed conductance steps.Comment: 9 pages, 4 figures, Surface Science, to appea

Anisotropic magnetoresistance in nanocontacts

We present ab initio calculations of the evolution of anisotropic magnetoresistance (AMR) in Ni nanocontacts from the ballistic to the tunnel regime. We find an extraordinary enhancement of AMR, compared to bulk, in two scenarios. In systems without localized states, like chemically pure break junctions, large AMR only occurs if the orbital polarization of the current is large, regardless of the anisotropy of the density of states. In systems that display localized states close to the Fermi energy, like a single electron transistor with ferromagnetic electrodes, large AMR is related to the variation of the Fermi energy as a function of the magnetization direction.Comment: 7 pages, 4 figures; revised for publication, new figures in greyscal

Kondo effect of magnetic impurities on nanotubes

The effect of magnetic impurities on the ballistic conductance of nanocontacts is, as suggested in recent work, amenable to ab initio study \cite{naturemat}. Our method proceeds via a conventional density functional calculation of spin and symmetry dependent electron scattering phase shifts, followed by the subsequent numerical renormalization group solution of Anderson models -- whose ingredients and parameters are chosen so as to reproduce these phase shifts. We apply this method to investigate the Kondo zero bias anomalies that would be caused in the ballistic conductance of perfect metallic (4,4) and (8,8) single wall carbon nanotubes, ideally connected to leads at the two ends, by externally adsorbed Co and Fe adatoms. The different spin and electronic structure of these impurities are predicted to lead to a variety of Kondo temperatures, generally well below 10 K, and to interference between channels leading to Fano-like conductance minima at zero bias

Kondo impurities in nanotubes: the importance of being "in"

Transition metal impurities will yield zero bias anomalies in the conductance of well contacted metallic carbon nanotubes, but Kondo temperatures and geometry dependences have not been anticipated so far. Applying the density functional plus numerical renormalization group approach of Lucignano \textit{et al.} to Co and Fe impurities in (4,4) and (8,8) nanotubes, we discover a huge difference of behaviour between outside versus inside adsorption of the impurity. The predicted Kondo temperatures and zero bias anomalies, tiny outside the nanotube, turn large and strongly radius dependent inside, owing to a change of symmetry of the magnetic orbital. Observation of this Kondo effect should open the way to a host of future experiments

Kondo conductance across the smallest spin 1/2 radical molecule

Molecular contacts are generally poorly conducting because their energy levels tend to lie far from the Fermi energy of the metal contact, necessitating undesirably large gate and bias voltages in molecular electronics applications. Molecular radicals are an exception because their partly filled orbitals undergo Kondo screening, opening the way to electron passage even at zero bias. While that phenomenon has been experimentally demonstrated for several complex organic radicals, quantitative theoretical predictions have not been attempted so far. It is therefore an open question whether and to what extent an ab initio-based theory is able to make accurate predictions for Kondo temperatures and conductance lineshapes. Choosing nitric oxide NO as a simple and exemplary spin 1/2 molecular radical, we present calculations based on a combination of density functional theory and numerical renormalization group (DFT+NRG) predicting a zero bias spectral anomaly with a Kondo temperature of 15 K for NO/Au(111). A scanning tunneling spectroscopy study is subsequently carried out to verify the prediction, and a striking zero bias Kondo anomaly is confirmed, still quite visible at liquid nitrogen temperatures. Comparison shows that the experimental Kondo temperature of about 43 K is larger than the theoretical one, while the inverted Fano lineshape implies a strong source of interference not included in the model. These discrepancies are not a surprise, providing in fact an instructive measure of the approximations used in the modeling, which supports and qualifies the viability of the DFT+NRG approach to the prediction of conductance anomalies in larger molecular radicals