14,307 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
Can simple models explain Zipf’s law for all exponents?
H. Simon proposed a simple stochastic process for explaining Zipf’s law for word frequencies. Here we introduce two similar generalizations of Simon’s model that cover the same range of exponents as the standard Simon model. The mathematical approach followed minimizes the
amount of mathematical background needed for deriving the exponent, compared to previous approaches to the standard Simon’s model. Reviewing what is known from other simple explanations of Zipf’s law, we conclude there is no single radically simple explanation covering the whole range of variation of the exponent of Zipf’s law in humans. The meaningfulness of Zipf’s law for word frequencies remains an open question.Peer ReviewedPostprint (published version
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
Research on WASH sector, environment and water resources in the Central Rift Valley of Ethiopia
Peer ReviewedPostprint (published version
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
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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