12,121 research outputs found
Exact Kohn-Sham eigenstates versus quasiparticles in simple models of strongly correlated electrons
We present analytic expressions for the exact density functional and
Kohn-Sham Hamiltonian of simple tight-binding models of correlated electrons.
These are the single- and double-site versions of the Anderson, Hubbard and
spinless fermion models. The exact exchange and correlation potentials are
fully non-local. The analytic expressions allow to compare the Kohn-Sham
eigenstates of exact density functional theory with the many-body
quasi-particle states of these correlated-electron systems. The exact Kohn-Sham
spectrum describes correctly many of the non-trivial features of the many-body
quasi-particle spectrum, as for example the precursors of the Kondo peak.
However, we find that some pieces of the quasi-particle spectrum are missing
because the many-body phase-space for electron and hole excitations is richer
Universality in the transport response of molecular wires physisorbed onto graphene electrodes
We analyze the low-voltage transport response of large molecular wires
bridging graphene electrodes, where the molecules are physisorbed onto the
graphene sheets by planar anchor groups. In our study, the sheets are pulled
away to vary the gap length and the relative atomic positions. The molecular
wires are also translated in directions parallel and perpendicular to the
sheets. We show that the energy position of the Breit-Wigner molecular
resonances is universal for a given molecule, in the sense that it is
independent of the details of the graphene edges, gaps lengths or of the
molecule positions. We discuss the need to converge carefully the k-sampling to
provide reasonable values of the conductance.Comment: 6 pages, 6 figure
Impact of edge shape on the functionalities of graphene-based single-molecule electronics devices
We present an ab-initio analysis of the impact of edge shape and
graphene-molecule anchor coupling on the electronic and transport
functionalities of graphene-based molecular electronics devices. We analyze how
Fano-like resonances, spin filtering and negative differential resistance
effects may or may not arise by modifying suitably the edge shapes and the
terminating groups of simple organic molecules. We show that the spin filtering
effect is a consequence of the magnetic behavior of zigzag-terminated edges,
which is enhanced by furnishing these with a wedge shape. The negative
differential resistance effect is originated by the presence of two degenerate
electronic states localized at each of the atoms coupling the molecule to
graphene which are strongly affected by a bias voltage. The effect could thus
be tailored by a suitable choice of the molecule and contact atoms if edge
shape could be controlled with atomic precision.Comment: 11 pages, 20 figure
Boundary Effects in 2+1 Dimensional Maxwell-Chern-Simons Theory
The boundary effects in the screening of an applied magnetic field in a
finite temperature 2+1 dimensional model of charged fermions minimally coupled
to Maxwell and Chern-Simons fields are investigated. It is found that in a
sample with only one boundary -a half-plane- a total Meissner effect takes
place, while in a sample with two boundaries -an infinite strip- the external
magnetic field partially penetrates the material.Comment: revte
Symmetry-induced interference effects in metalloporphyrin wires
Organo-metallic molecular structures where a single metallic atom is embedded
in the organic backbone are ideal systems to study the effect of strong
correlations on their electronic structure. In this work we calculate the
electronic and transport properties of a series of metalloporphyrin molecules
sandwiched by gold electrodes using a combination of density functional theory
and scattering theory. The impact of strong correlations at the central
metallic atom is gauged by comparing our results obtained using conventional
DFT and DFT+U approaches. The zero bias transport properties may or may not
show spin-filtering behavior, depending on the nature of the d state closest to
the Fermi energy. The type of d state depends on the metallic atom and gives
rise to interference effects that produce different Fano features. The
inclusion of the U term opens a gap between the d states and changes
qualitatively the conductance and spin-filtering behavior in some of the
molecules. We explain the origin of the quantum interference effects found as
due to the symmetry-dependent coupling between the d states and other molecular
orbitals and propose the use of these systems as nanoscale chemical sensors. We
also demonstrate that an adequate treatment of strong correlations is really
necessary to correctly describe the transport properties of metalloporphyrins
and similar molecular magnets
Hydrotalcite/SBA15 composites for pre-combustion CO2 capture: CO2
Hydrotalcite-like compounds (HT) show potential as CO2 adsorbent materials for pre-combustion CO2 capture applications, but require improvements in stability, adsorption capacity and kinetics. In this study, HT/SBA15 hybrids (with different Mg/Al ratios varying from 0.3 to 3) have been synthesised using a two-stage grafting method to coat a mesoporous SBA15 with hydrotalcite layers. The HT/SBA15 hybrids showed significant improvement in intrinsic CO2 uptake (per mass of HT), initial uptake rate, and multicycle stability compared to unsupported HT. Compared to previously reported nanostructured carbon supports (e.g. CNF, MWCNTs), the HT/SBA15 hybrids were found to be more thermally stable and exhibit comparable adsorption uptake and rates. In particular, the use of SBA15 as a support is shown to prevent the gradual loss in weight from thermal decomposition observed for HT/MWCNT or HT/GO composites over extended cycling
Offline Signature Verification by Combining Graph Edit Distance and Triplet Networks
Biometric authentication by means of handwritten signatures is a challenging
pattern recognition task, which aims to infer a writer model from only a
handful of genuine signatures. In order to make it more difficult for a forger
to attack the verification system, a promising strategy is to combine different
writer models. In this work, we propose to complement a recent structural
approach to offline signature verification based on graph edit distance with a
statistical approach based on metric learning with deep neural networks. On the
MCYT and GPDS benchmark datasets, we demonstrate that combining the structural
and statistical models leads to significant improvements in performance,
profiting from their complementary properties
Beyond-Constant-Mass-Approximation Magnetic Catalysis in the Gauge Higgs-Yukawa Model
Beyond-constant-mass approximation solutions for magnetically catalyzed
fermion and scalar masses are found in a gauge Higgs-Yukawa theory in the
presence of a constant magnetic field. The obtained fermion masses are several
orders of magnitude larger than those found in the absence of Yukawa
interactions. The masses obtained within the beyond-constant-mass approximation
exactly reduce to the results within the constant-mass approach when the
condition is satisfied. Possible
applications to early universe physics and condensed matter are discussed.Comment: Revised numerical results. New figures. Several sections rewritte
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