70 research outputs found
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 electrons is blocked by a magnetization
reversal, while the states (one per spin) are perfectly transmitted.
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+ 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 -dominated conductance
channels decreases somewhat compared to the LDA value, but the number of
-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
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