30 research outputs found
Multi-orbital Non-Crossing Approximation from maximally localized Wannier functions: the Kondo signature of copper phthalocyanine on Ag (100)
We have developed a multi-orbital approach to compute the electronic
structure of a quantum impurity using the non-crossing approximation. The
calculation starts with a mean-field evaluation of the system's electronic
structure using a standard quantum chemistry code. Here we use density
functional theory (DFT). We transformed the one-electron structure into an
impurity Hamiltonian by using maximally localized Wannier functions (MLWF).
Hence, we have developed a method to study the Kondo effect in systems based on
an initial one-electron calculation. We have applied our methodology to a
copper phthalocyanine molecule chemisorbed on Ag (100), and we have described
its spectral function for three different cases where the molecule presents a
single spin or two spins with ferro- and anti-ferromagnetic exchange couplings.
We find that the use of broken-symmetry mean-field theories such as Kohn-Sham
DFT cannot deal with the complexity of the spin of open-shell molecules on
metal surfaces and extra modeling is needed
Many-body effects in magnetic inelastic electron tunneling spectroscopy
Magnetic inelastic electron tunneling spectroscopy (IETS) shows sharp
increases in conductance when a new conductance channel associated to a change
in magnetic structure is open. Typically, the magnetic moment carried by an
adsorbate can be changed by collision with a tunneling electron; in this
process the spin of the electron can flip or not. A previous one-electron
theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009)] successfully explained both
the conductance thresholds and the magnitude of the conductance variation. The
elastic spin flip of conduction electrons by a magnetic impurity leads to the
well known Kondo effect. In the present work, we compare the theoretical
predictions for inelastic magnetic tunneling obtained with a one-electron
approach and with a many-body theory including Kondo-like phenomena. We apply
our theories to a singlet-triplet transition model system that contains most of
the characteristics revealed in magnetic IETS. We use two self-consistent
treatments (non-crossing approximation and self-consistent ladder
approximation). We show that, although the one-electron limit is properly
recovered, new intrinsic many-body features appear. In particular, sharp peaks
appear close to the inelastic thresholds; these are not localized exactly at
thresholds and could influence the determination of magnetic structures from
IETS experiments.Analysis of the evolution with temperature reveals that these
many-body features involve an energy scale different from that of the usual
Kondo peaks. Indeed, the many-body features perdure at temperatures much larger
than the one given by the Kondo energy scale of the system.Comment: 10 pages and 6 figure
Current-induced mechanical torque in chiral molecular rotors
A great endeavor has been undertaken to engineer molecular rotors operated by
an electrical current. A frequently met operation principle is the transfer of
angular momentum taken from the incident flux. In this paper we present an
alternative driving agent that works also in situations where angular momentum
of the incoming flux is conserved. This situation arises typically with
molecular rotors that exhibit an easy axis of rotation. For quantitative
analysis we investigate here a classical model, where molecule and wires are
represented by a rigid curved path. We demonstrate that in the presence of
chirality the rotor generically undergoes a directed motion, provided that the
incident current exceeds a threshold value. Above threshold, the corresponding
rotation frequency (per incoming particle current) for helical geometries turns
out to be , where is the ratio of the mass of an incident
charge carrier and the mass of the helix per winding number
Chirality-controlled spin scattering through quantum interference
Chirality-induced spin selectivity has been reported in many experiments, but
a generally accepted theoretical explanation has not yet been proposed. Here,
we introduce a simple model system of a straight cylindrical free-electron
wire, containing a helical string of atomic scattering centers, with spin-orbit
interaction. The advantage of this simple model is that it allows deriving
analytical expressions for the spin scattering rates, such that the origin of
the effect can be easily followed. We find that spin-selective scattering can
be viewed as resulting from constructive interference of partial waves
scattered by the spin-orbit terms. We demonstrate that forward scattering rates
are independent of spin, while back scattering is spin dependent over wide
windows of energy. Although the model does not represent the full details of
electron transmission through chiral molecules, it clearly reveals a mechanism
that could operate in chiral systems.Comment: 7 pages, 4 figure
Current-induced mechanical torque in chiral molecular rotors
A great endeavor has been undertaken to engineer molecular rotors operated by an electrical current. A frequently met operation principle is the transfer of angular momentum taken from the incident flux. In this paper we present an alternative driving agent that works also in situations where angular momentum of the incoming flux is conserved. This situation arises typically with molecular rotors that exhibit an easy axis of rotation. For quantitative analysis we investigate here a classical model, where molecule and wires are represented by a rigid curved path. We demonstrate that in the presence of chirality the rotor generically undergoes a directed motion, provided that the incident current exceeds a threshold value. Above threshold, the corresponding rotation frequency (per incoming particle current) for helical geometries turns out to be , where is the ratio of the mass of an incident charge carrier and the mass of the helix per winding number
Band selection and disentanglement using maximally-localized Wannier functions: the cases of Co impurities in bulk copper and the Cu (111) surface
We have adapted the maximally-localized Wannier function approach of [I.
Souza, N. Marzari and D. Vanderbilt, Phys. Rev. B 65, 035109 (2002)] to the
density functional theory based Siesta method [J. M. Soler et al., J. Phys.:
Cond. Mat. 14, 2745 (2002)] and applied it to the study of Co substitutional
impurities in bulk copper as well as to the Cu (111) surface. In the Co
impurity case, we have reduced the problem to the Co d-electrons and the Cu
sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined
density functional parameters in a fully orthonormal basis set. In order to
test the quality of the Wannier approach to surfaces, we have studied the
electronic structure of the Cu (111) surface by again transforming the density
functional problem into the Wannier representation. An excellent description of
the Shockley surface state is attained, permitting us to be confident in the
application of this method to future studies of magnetic adsorbates in the
presence of an extended surface state
Impact of electrode density of states on transport through pyridine-linked single molecule junctions
We study the impact of electrode band structure on transport through
single-molecule junctions by measuring the conductance of pyridine-based
molecules using Ag and Au electrodes. Our experiments are carried out using the
scanning tunneling microscope based break-junction technique and are supported
by density functional theory based calculations. We find from both experiments
and calculations that the coupling of the dominant transport orbital to the
metal is stronger for Au-based junctions when compared with Ag-based junctions.
We attribute this difference to relativistic effects, which results in an
enhanced density of d-states at the Fermi energy for Au compared with Ag. We
further show that the alignment of the conducting orbital relative to the Fermi
level does not follow the work function difference between two metals and is
different for conjugated and saturated systems. We thus demonstrate that the
details of the molecular level alignment and electronic coupling in
metal-organic interfaces do not follow simple rules, but are rather the
consequence of subtle local interactions
Real space manifestations of coherent screening in atomic scale Kondo lattices
The interaction among magnetic moments screened by conduction electrons drives quantum phase transitions between magnetically ordered and heavy-fermion ground states. Here, starting from isolated magnetic impurities in the Kondo regime, we investigate the formation of the finite size analogue of a heavy Fermi liquid. We build regularly-spaced chains of Co adatoms on a metallic surface by atomic manipulation. Scanning tunneling spectroscopy is used to obtain maps of the Kondo resonance intensity with sub-atomic resolution. For sufficiently small interatomic separation, the spatial distribution of Kondo screening does not coincide with the position of the adatoms. It also develops enhancements at both edges of the chains. Since we can rule out any other interaction between Kondo impurities, this is explained in terms of the indirect hybridization of the Kondo orbitals mediated by a coherent electron gas, the mechanism that causes the emergence of heavy quasiparticles in the thermodynamic limit.Financial support was provided by the Spanish Plan Nacional de I+ D+ i (grants MAT 2013-46593-C6-3-P, MAT2016-78293-C6-6-R, MAT2015-66888-C3-2-R, and FIS2015-64886-C5-3-P), Charles University (programme PRIMUS/Sci/09) and the European Union through programmes Interreg-POCTEFA (grant TNSI/EFA194/16) and H2020-EINFRA-5-2015 MaX Center of Excellence (grant no. 676598). M. M.-L., M. P., and D. S. acknowledge the use of SAI at Universidad de Zaragoza. R. R. acknowledges The Severo Ochoa Centers of Excellence Program (grant no. SEV-2017-0706) and Generalitat de Catalunya (grant no. 2017SGR1506 and the CERCA Programme)
Atomically wired molecular junctions: Connecting a single organic molecule by chains of metal atoms
Using a break junction technique, we find a clear signature for the formation
of conducting hybrid junctions composed of a single organic molecule (benzene,
naphthalene or anthracene) connected to chains of platinum atoms. The hybrid
junctions exhibit metallic-like conductance (~0.1-1G0), which is rather
insensitive to further elongation by additional atoms. At low bias voltage the
hybrid junctions can be elongated significantly beyond the length of the bare
atomic chains. Ab initio calculations reveal that benzene based hybrid
junctions have a significant binding energy and high structural flexibility
that may contribute to the survival of the hybrid junction during the
elongation process. The fabrication of hybrid junctions opens the way for
combining the different properties of atomic chains and organic molecules to
realize a new class of atomic scale interfaces