7,568 research outputs found
Non-equilibrium GW approach to quantum transport in nano-scale contacts
Correlation effects within the GW approximation have been incorporated into
the Keldysh non-equilibrium transport formalism. We show that GW describes the
Kondo effect and the zero-temperature transport properties of the Anderson
model fairly well. Combining the GW scheme with density functional theory and a
Wannier function basis set, we illustrate the impact of correlations by
computing the I-V characteristics of a hydrogen molecule between two Pt chains.
Our results indicate that self-consistency is fundamental for the calculated
currents, but that it tends to wash out satellite structures in the spectral
function.Comment: 5 pages, 4 figure
Developing the conditions for education for citizenship in higher education (CiCe network report)
The key questions surrounding education for citizenship in higher education are what is education for, what are universities for, and what values should education systems aim to develop in young people? These raise complex issues. As these are such challenging questions and asthere is little agreement about them, this raises for the public,students, academics and educationists, as many issues as definitiveanswers. Nearly all of us who chose education as a career did so tomake a difference; not just degree results, but to the lives andaspirations of young people and society as a whole. Thus, althoughwe champion the development of critical abilities, the skills ofenquiry and questioning, activity based approaches to learning andthe notion of rights as something to be cherished, this is not in itselfmerely a chronicle. It is to suggest that a mixture of creativecontent, ethos and a participatory, consultative, democratic approachin the framework of macro and micro improvements can lead tobetter, deeper learning and crucially a fairer and more just society
Conserving GW scheme for nonequilibrium quantum transport in molecular contacts
We give a detailed presentation of our recent scheme to include correlation
effects in molecular transport calculations using the GW approximation within
the non-equilibrium Keldysh formalism. We restrict the GW self-energy to the
central region, and describe the leads by density functional theory (DFT). A
minimal basis of maximally localized Wannier functions is applied both in the
central GW region and the leads. The importance of using a conserving, i.e.
fully self-consistent, GW self-energy is demonstrated both analytically and by
numerical examples. We introduce an effective spin-dependent interaction which
automatically reduces self-interaction errors to all orders in the interaction.
The scheme is applied to the Anderson model in- and out of equilibrium. In
equilibrium at zero temperature we find that GW describes the Kondo resonance
fairly well for intermediate interaction strengths. Out of equilibrium we
demonstrate that the one-shot G0W0 approximation can produce severe errors, in
particular at high bias. Finally, we consider a benzene molecule between
featureless leads. It is found that the molecule's HOMO-LUMO gap as calculated
in GW is significantly reduced as the coupling to the leads is increased,
reflecting the more efficient screening in the strongly coupled junction. For
the IV characteristics of the junction we find that HF and G0W0[G_HF] yield
results closer to GW than does DFT and G0W0[G_DFT]. This is explained in terms
of self-interaction effects and life-time reduction due to electron-electron
interactions.Comment: 23 pages, 16 figure
Throughput Maximization in Multiprocessor Speed-Scaling
We are given a set of jobs that have to be executed on a set of
speed-scalable machines that can vary their speeds dynamically using the energy
model introduced in [Yao et al., FOCS'95]. Every job is characterized by
its release date , its deadline , its processing volume if
is executed on machine and its weight . We are also given a budget
of energy and our objective is to maximize the weighted throughput, i.e.
the total weight of jobs that are completed between their respective release
dates and deadlines. We propose a polynomial-time approximation algorithm where
the preemption of the jobs is allowed but not their migration. Our algorithm
uses a primal-dual approach on a linearized version of a convex program with
linear constraints. Furthermore, we present two optimal algorithms for the
non-preemptive case where the number of machines is bounded by a fixed
constant. More specifically, we consider: {\em (a)} the case of identical
processing volumes, i.e. for every and , for which we
present a polynomial-time algorithm for the unweighted version, which becomes a
pseudopolynomial-time algorithm for the weighted throughput version, and {\em
(b)} the case of agreeable instances, i.e. for which if and only
if , for which we present a pseudopolynomial-time algorithm. Both
algorithms are based on a discretization of the problem and the use of dynamic
programming
Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe
We investigate the interactions of photoexcited carriers with lattice
vibrations in thin films of the layered transition metal dichalcogenide (TMDC)
WSe. Employing femtosecond electron diffraction with monocrystalline
samples and first principle density functional theory calculations, we obtain a
momentum-resolved picture of the energy-transfer from excited electrons to
phonons. The measured momentum-dependent phonon population dynamics are
compared to first principle calculations of the phonon linewidth and can be
rationalized in terms of electronic phase-space arguments. The relaxation of
excited states in the conduction band is dominated by intervalley scattering
between valleys and the emission of zone-boundary phonons.
Transiently, the momentum-dependent electron-phonon coupling leads to a
non-thermal phonon distribution, which, on longer timescales, relaxes to a
thermal distribution via electron-phonon and phonon-phonon collisions. Our
results constitute a basis for monitoring and predicting out of equilibrium
electrical and thermal transport properties for nanoscale applications of
TMDCs
Excitons in boron nitride nanotubes: dimensionality effects
We show that the optical absorption spectra of boron nitride (BN) nanotubes
are dominated by strongly bound excitons. Our first-principles calculations
indicate that the binding energy for the first and dominant excitonic peak
depends sensitively on the dimensionality of the system, varying from 0.7 eV in
bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the
hypothetical (2,2) tube. The strongly localized nature of this exciton dictates
the fast convergence of its binding energy with increasing tube diameter
towards the sheet value. The absolute position of the first excitonic peak is
almost independent of the tube radius and system dimensionality. This provides
an explanation for the observed "optical gap" constancy for different tubes and
bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].Comment: 5 pages, 2 figure
Prime numbers, quantum field theory and the Goldbach conjecture
Motivated by the Goldbach conjecture in Number Theory and the abelian
bosonization mechanism on a cylindrical two-dimensional spacetime we study the
reconstruction of a real scalar field as a product of two real fermion
(so-called \textit{prime}) fields whose Fourier expansion exclusively contains
prime modes. We undertake the canonical quantization of such prime fields and
construct the corresponding Fock space by introducing creation operators
--labeled by prime numbers -- acting on the vacuum. The
analysis of our model, based on the standard rules of quantum field theory and
the assumption of the Riemann hypothesis, allow us to prove that the theory is
not renormalizable. We also comment on the potential consequences of this
result concerning the validity or breakdown of the Goldbach conjecture for
large integer numbers.Comment: 20 pages in A4 format, 2 figure
Are there really cooper pairs and persistent currents in aromatic molecules?
arXiv.orgOver 20 years ago, one of us suggested the title was affirmative. In 2012, Cooper pairs were identified in several, but not all >aromatic> compounds tested, benzene being one. This manuscript discusses the formation of three time-reversed pairs of states forming pseudobosons (high energy Cooper pairs) in benzene at room temperature. The large stabilization in energy that results is the additive effect of energy gaps of an s wave state and a charge density wave permitting the pseudobosons to exist at room temperature. The overall result of these interactions is three pseudobosons occupying the lowest boson state and the positions of the carbon nuclei are optimum by forming a perfect hexagon. The possibility of a persistent current exists; detection might not be easy.Peer Reviewe
The exact Hohenberg-Kohn functional for a lattice model
Trabajo presentado al: "Deutsche Physikalische Gesellschaft Spring Meeting" celebrado en Dresden (Alemania) del 30 de marzo al 4 de Abril de 2014.Standard local exchange-correlation and semi-local functionals in ground-state density functional theory are known for their shortcomings in describing correct charge transfer, dissociation energies of
molecular ions, and barriers of chemical reactions.
To understand the failures of approximate functionals and to gain insight into the behavior of the exact functional, we investigate the exact solution of the many-body Schrödinger equation for a lattice
model. Using exact diagonalization, we explicitely construct the exact Hohenberg-Kohn functional and the mapping from densities to wavefunctions. Besides the normal inter-system derivative discontinuity
widely discussed in the density-functional theory community, we observe a new feature of the exact functional in the low-density
limit. This "intra-system derivative discontinuity" resembles the inter-system derivative discontinuity, but is within the system.
The description of many physical phenomena linked to charge-transfer processes (both in the static and dynamical regimes) require a proper account of this "intra-system derivative discontinuity".Peer reviewe
Identifying communities by influence dynamics in social networks
Communities are not static; they evolve, split and merge, appear and
disappear, i.e. they are product of dynamical processes that govern the
evolution of the network. A good algorithm for community detection should not
only quantify the topology of the network, but incorporate the dynamical
processes that take place on the network. We present a novel algorithm for
community detection that combines network structure with processes that support
creation and/or evolution of communities. The algorithm does not embrace the
universal approach but instead tries to focus on social networks and model
dynamic social interactions that occur on those networks. It identifies
leaders, and communities that form around those leaders. It naturally supports
overlapping communities by associating each node with a membership vector that
describes node's involvement in each community. This way, in addition to
overlapping communities, we can identify nodes that are good followers to their
leader, and also nodes with no clear community involvement that serve as a
proxy between several communities and are equally as important. We run the
algorithm for several real social networks which we believe represent a good
fraction of the wide body of social networks and discuss the results including
other possible applications.Comment: 10 pages, 6 figure
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