809 research outputs found
Missing derivative discontinuity of the exchange-correlation energy for attractive interactions: the charge Kondo effect
We show that the energy functional of ensemble Density Functional Theory
(DFT) [Perdew et al., Phys. Rev. Lett. 49, 1691 (1982)] in systems with
attractive interactions is a convex function of the fractional particle number
N and is given by a series of straight lines joining a subset of ground-state
energies. As a consequence the exchange-correlation (XC) potential is not
discontinuous for all N. We highlight the importance of this exact result in
the ensemble-DFT description of the negative-U Anderson model. In the atomic
limit the discontinuity of the XC potential is missing for odd N while for
finite hybridizations the discontinuity at even N is broadened. We demonstrate
that the inclusion of these properties in any approximate XC potential is
crucial to reproduce the characteristic signatures of the charge-Kondo effect
in the conductance and charge susceptibility.Comment: 5 pages, 5 eps figure. Phys. Rev. B 86, 081409(R) (2012
Dynamical formation and manipulation of Majorana fermions in driven quantum wires
Controlling the dynamics of Majorana fermions (MF) subject to time-varying
driving fields is of fundamental importance for the practical realization of
topological quantum computing. In this work we study how it is possible to
dynamically generate and maintain the topological phase in one-dimensional
superconducting nanowires after the temporal variation of the Hamiltonian
parameters. Remarkably we show that for a sudden quench the system can never
relax towards a state exhibiting fully developed MF, independently of the
initial and final Hamiltonians. Only for sufficiently slow protocols the system
behaves adiabatically, and the topological phase can be reached. Finally we
address the crucial question of how "adiabatic" a protocol must be in order to
manipulate the MF inside the topological phase without deteriorating their
Majorana character.Comment: 5 pages, 4 eps figure
The dissection algorithm for the second-Born self-energy
We describe an algorithm to efficiently compute the second-Born self-energy
of many-body perurbation theory. The core idea consists in dissecting the set
of all four-index Coulomb integrals into properly chosen subsets, thus avoiding
to loop over those indices for which the Coulomb integrals are zero or
negligible. The scaling properties of the algorithm with the number of basis
functions is discussed. The computational gain is demonstrated in the case of
one-particle Kohn-Sham basis for organic molecules.Comment: 6 pages, contribution to the proceedings of the workshop "Progress in
Nonequilibrium Green's Function VII
A theory of superconductivity in multi-walled carbon nanotubes
We devise an approach to describe the electronic instabilities of doped
multi-walled nanotubes, where each shell has in general a manifold of Fermi
points. Our analysis relies on the scale dependence of the different scattering
processes, showing that a pairing instability arises for a large enough number
of Fermi points as a consequence of their particular geometric arrangement. The
instability is enhanced by the tunneling of Cooper pairs between nearest
shells, giving rise to a transition from the Luttinger liquid to a
superconducting state in a wide region of the phase diagram.Comment: 4 pages, 2 figures, replaced with revised versio
Transient dynamics in the Anderson-Holstein model with interfacial screening
We study the combined effects of electron-phonon coupling and dot-lead
repulsion in the transport properties of the Anderson-Holstein model. We employ
a recently proposed nonperturbative method to calculate the transient response
of the system. By varying the initial conditions for the time propagation the
current exhibits transient oscillations of different nature. We are able to
disentangle two dynamical processes, namely the local charge rearrangement due
to the dot-lead contacting and the establishment of the nonequilbrium many-body
state due to the application of the external bias. These processes involve
either Franck-Condon excitations or transitions between the resonant level and
the Fermi energy of the leads.Comment: 6 pages, 6 figure
CHEERS: A tool for Correlated Hole-Electron Evolution from Real-time Simulations
We put forward a practical nonequilibrium Green's function (NEGF) scheme to
perform real-time evolutions of many-body interacting systems driven out of
equilibrium by external fields. CHEERS is a computational tool to solve the
NEGF equation of motion in the so called generalized Kadanoff-Baym ansatz and
it can be used for model systems as well as first-principles Hamiltonians.
Dynamical correlation (or memory) effects are added to the Hartree-Fock
dynamics through a many-body self-energy. Applications to time-dependent
quantum transport, time-resolved photoabsorption and other ultrafast phenomena
are discussed.Comment: 15 pages, 6 figures, to be published, J. Phys.: Condens. Matter
(2018
Time-dependent transport in graphene nanoribbons
We theoretically investigate the time-dependent ballistic transport in
metallic graphene nanoribbons after the sudden switch-on of a bias voltage .
The ribbon is divided in three different regions, namely two semi-infinite
graphenic leads and a central part of length , across which the bias drops
linearly and where the current is calculated. We show that during the early
transient time the system behaves like a graphene bulk under the influence of a
uniform electric field . In the undoped system the current does not grow
linearly in time but remarkably reaches a temporary plateau with dc
conductivity , which coincides with the minimal
conductivity of two-dimensional graphene. After a time of order
( being the Fermi velocity) the current departs from the first plateau
and saturates at its final steady state value with conductivity
typical of metallic nanoribbons of finite width.Comment: 5 pages, 5 figure
Unconventional quasiparticle lifetime in undoped graphene
We address the question of how small can the quasiparticle decay rate be at
low energies in undoped graphene, where kinematical constraints are known to
prevent the decay into particle-hole excitations. For this purpose, we study
the renormalization of the phonon dispersion by many-body effects, which turns
out to be very strong in the case of the out-of-plane phonons at the K point of
the spectrum. We show that these evolve into a branch of very soft modes that
provide the relevant channel for quasiparticle decay, at energies below the
scale of the optical phonon modes. In this regime, we find that the decay rate
is proportional to the cube of the quasiparticle energy. This implies that a
crossover should be observed in transport properties from the linear dependence
characteristic of the high-energy regime to the much slower decay rate due to
the soft phonon modes.Comment: 5 pages, 1 figur
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