13,835 research outputs found
Impact of Fano and Breit-Wigner resonances in the thermoelectric properties of nanoscale junctions
We show that the thermoelectric properties of nanoscale junctions featuring
states near the Fermi level strongly depend on the type of resonance generated
by such states, which can be either Fano or Breit-Wigner-like. We give general
expressions for the thermoelectric coefficients generated by the two types of
resonances and calculate the thermoelectric properties of these systems, which
encompass most nanoelectronics junctions. We include simulations of real
junctions where metalloporphyrin molecules bridge gold electrodes and prove
that for some metallic elements the thermoelectric properties show a large
variability. We find that the thermopower and figure of merit are largely
enhanced when the resonance gets close to the Fermi level and reach values much
higher than typical values found in other nanoscale junctions. The specific
value and temperature dependence are determined by a series of factors such as
the strength of the coupling between the state and other molecular states, the
symmetry of the state, the strength of the coupling between the molecule and
the leads and the spin filtering behavior of the junction.Comment: 9 pages, 11 figure
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
Entropy Rate of Diffusion Processes on Complex Networks
The concept of entropy rate for a dynamical process on a graph is introduced.
We study diffusion processes where the node degrees are used as a local
information by the random walkers. We describe analitically and numerically how
the degree heterogeneity and correlations affect the diffusion entropy rate. In
addition, the entropy rate is used to characterize complex networks from the
real world. Our results point out how to design optimal diffusion processes
that maximize the entropy for a given network structure, providing a new
theoretical tool with applications to social, technological and communication
networks.Comment: 4 pages (APS format), 3 figures, 1 tabl
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
Effects of Bose-Einstein Condensation on forces among bodies sitting in a boson heat bath
We explore the consequences of Bose-Einstein condensation on
two-scalar-exchange mediated forces among bodies that sit in a boson gas. We
find that below the condensation temperature the range of the forces becomes
infinite while it is finite at temperatures above condensation.Comment: 10 pages, 2 figure
Demographic growth and the distribution of language sizes
It is argued that the present log-normal distribution of language sizes is,
to a large extent, a consequence of demographic dynamics within the population
of speakers of each language. A two-parameter stochastic multiplicative process
is proposed as a model for the population dynamics of individual languages, and
applied over a period spanning the last ten centuries. The model disregards
language birth and death. A straightforward fitting of the two parameters,
which statistically characterize the population growth rate, predicts a
distribution of language sizes in excellent agreement with empirical data.
Numerical simulations, and the study of the size distribution within language
families, validate the assumptions at the basis of the model.Comment: To appear in Int. J. Mod. Phys. C (2008
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