263 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
Sesgos en la obtención de estimas de densidad obtenidas por medio de transectos lineales en ambientes estepáricos de las Islas Canarias
We studied bias in density estimations derived from strip transects in dry open–country in the Canary Islands. We also present some critical remarks on GarcĂa–del–Rey’s (2005) paper regarding strip transects and the validity of comparisons based on population densities of birds in scrublands on Tenerife island using two different methods: territory mapping and strip transect sampling. Although strip transects with census belts of 25 m do not account for detectability, this method only slightly undervalues true density estimates, and allowed to detect more than 85% of birds present in poorly vegetated environments in the Canary Islands. Previously published works on distribution and abundance of terrestrial birds in the Canary Islands using the strip transect sampling with belts of 25 m on both sides of the observer, thus provide reliable information that only slightly underestimates true densities.
Key words: Birds, Canary Islands, Census methods, Strip transects, Open–country environments, Population
density, Territory mapping.We studied bias in density estimations derived from strip transects in dry open–country in the Canary Islands. We also present some critical remarks on GarcĂa–del–Rey’s (2005) paper regarding strip transects and the validity of comparisons based on population densities of birds in scrublands on Tenerife island using two different methods: territory mapping and strip transect sampling. Although strip transects with census belts of 25 m do not account for detectability, this method only slightly undervalues true density estimates, and allowed to detect more than 85% of birds present in poorly vegetated environments in the Canary Islands. Previously published works on distribution and abundance of terrestrial birds in the Canary Islands using the strip transect sampling with belts of 25 m on both sides of the observer, thus provide reliable information that only slightly underestimates true densities.
Key words: Birds, Canary Islands, Census methods, Strip transects, Open–country environments, Population
density, Territory mapping.Se estudian los sesgos derivados del empleo del mĂ©todo del taxiado (transectos lineales con bandas de 25 m a cada lado del observador) para obtener densidades en ambientes estepáricos de las Islas Canarias. TambiĂ©n se presentan algunos comentarios crĂticos al trabajo de GarcĂa–del–Rey (2005) que compara estimas de densidad obtenidas en Tenerife utilizando dos mĂ©todos diferentes: mapeo de territorios y transecto lineal. Aunque el mĂ©todo del taxiado estima densidades relativas no corregidas por la detectabilidad de las especies, este mĂ©todo proporciona valores de densidad muy parecidos a los reales, ya que permite detectar en ambientes con poca cobertura vegetal a más del 85% de los individuos dentro de bandas de 25 m a cada lado del observador. Por tanto, los trabajos previamente publicados sobre densidades de aves en Canarias proporcionan estimas fiables sĂłlo ligeramente infravaloradas.
Palabras clave: Aves, Islas Canarias, Métodos de censo, Transecto lineal, Ambientes estepáricos, Densidad
de poblaciĂłn, Mapeo de territorios
Linear response time-dependent density functional theory of the Hubbard dimer
The asymmetric Hubbard dimer is used to study the density-dependence of the
exact frequency-dependent kernel of linear-response time-dependent density
functional theory. The exact form of the kernel is given, and the limitations
of the adiabatic approximation utilizing the exact ground-state functional are
shown. The oscillator strength sum rule is proven for lattice Hamiltonians, and
relative oscillator strengths are defined appropriately. The method of Casida
for extracting oscillator strengths from a frequency-dependent kernel is
demonstrated to yield the exact result with this kernel. An unambiguous way of
labelling the nature of excitations is given. The fluctuation-dissipation
theorem is proven for the ground-state exchange-correlation energy. The
distinction between weak and strong correlation is shown to depend on the ratio
of interaction to asymmetry. A simple interpolation between carefully defined
weak-correlation and strong-correlation regimes yields a density-functional
approximation for the kernel that gives accurate transition frequencies for
both the single and double excitations, including charge-transfer excitations.
Many exact results, limits, and expansions about those limits are given in the
appendices.Comment: 22 pages, 14 figure
Full analytical solution of finite-length armchair/zigzag nanoribbons
Finite-length armchair graphene nanoribbons can behave as one dimensional
topological materials, that may show edge states in their zigzag-terminated
edges, depending on their width and termination. We show here a full solution
of Tight-Binding graphene rectangles of any length and width that can be seen
as either finite-length armchair or zigzag ribbons. We find exact analytical
expressions for both bulk and edge eigen-states and eigen-energies. We write
down exact expressions for the Coulomb interactions among edge states and
introduce a Hubbard-dimer model to analyse the emergence and features of
different magnetic states at the edges, whose existence depends on the ribbon
length. We find ample room for experimental testing of our predictions in N = 5
armchair ribbons. We compare the analytical results with ab initio simulations
to benchmark the quality of the dimer model and to set its parameters. A
further detailed analysis of the ab initio Hamiltonian allows us to identify
those variations of the Tight-Binding parameters that affect the topological
properties of the ribbons
Towards a better understanding of fire performance assessment of façade systems: Current situation and a proposed new assessment framework
This manuscript presents tools and data that serve to enable an evaluation of the risk associated with vertical fire spread on buildings. A highly detailed context to cladding fires is described to unveil the complexity and magnitude of the problem and to identify gaps of information. An engineering framework is then developed which delivers required information that fills some of those gaps and that needs to be used towards achieving quantified fire performance. The data itself has been published as a publicly available database, entitled the Cladding Materials Library (www.claddingmaterialslibrary.com.au). This data can be used to support building fire risk assessments or as the basis for more in-depth research into façade fires. This paper presents the context of the data together with the competency framework necessary for upskilling building professionals to have the capacity to implement the engineering framework
Thermal characterization of building assemblies by means of transient data assimilation
The quantification of the overall “R-value” of building components is commonly achieved by using numerical models which are generally validated using the standardized Hot Box test. This test set-up follows a complex methodology specifically designed to deliver only the R-value. Modern building assemblies are of a level of complexity that many times a single parameter is insufficient to improve the design of the assembly. This paper proposes a simple thermal test set-up to analyse both transient and steady state heat flow processes, allowing for effective numerical fitting of parameters that describe all internal heat flow processes. As a result, the contribution of each element of an assembly can be evaluated on its overall insulating capabilities, thereby allowing for a truly optimised design solution. Two wall systems including significant thermal bridges have been chosen to illustrate this methodology. The proposed method, not only delivers a steady state thermal assessment as reliable as the standardized Hot Box procedure, but also allows a precise quantification of internal heat flows and the capability to conduct realistic transient state thermal assessments
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