370 research outputs found
Quantum Chaos and Regularity in Ultracold Fermi Gases
Quantum fluctuation of the energy is studied for an ultracold gas of
interacting fermions trapped in a three-dimensional potential. Periodic-orbit
theory is explored, and energy fluctuations are studied versus particle number
for generic regular and chaotic systems, as well for a system defined by a
harmonic confinement potential. Temperature effects on the energy fluctuations
are investigated.Comment: 4 pages, 5 figure
Can we always get the entanglement entropy from the Kadanoff-Baym equations? The case of the T-matrix approximation
We study the time-dependent transmission of entanglement entropy through an
out-of-equilibrium model interacting device in a quantum transport set-up. The
dynamics is performed via the Kadanoff-Baym equations within many-body
perturbation theory. The double occupancy , needed to determine the entanglement entropy, is obtained from
the equations of motion of the single-particle Green's function. A remarkable
result of our calculations is that can become negative, thus not permitting to evaluate the
entanglement entropy. This is a shortcoming of approximate, and yet conserving,
many-body self-energies. Among the tested perturbation schemes, the -matrix
approximation stands out for two reasons: it compares well to exact results in
the low density regime and it always provides a non-negative . For the second part of this statement, we
give an analytical proof. Finally, the transmission of entanglement across the
device is diminished by interactions but can be amplified by a current flowing
through the system.Comment: 6 pages, 6 figure
Kadanoff-Baym approach to time-dependent quantum transport in AC and DC fields
We have developed a method based on the embedded Kadanoff-Baym equations to
study the time evolution of open and inhomogeneous systems. The equation of
motion for the Green's function on the Keldysh contour is solved using
different conserving many-body approximations for the self-energy. Our
formulation incorporates basic conservation laws, such as particle
conservation, and includes both initial correlations and initial embedding
effects, without restrictions on the time-dependence of the external driving
field. We present results for the time-dependent density, current and dipole
moment for a correlated tight binding chain connected to one-dimensional
non-interacting leads exposed to DC and AC biases of various forms. We find
that the self-consistent 2B and GW approximations are in extremely good
agreement with each other at all times, for the long-range interactions that we
consider. In the DC case we show that the oscillations in the transients can be
understood from interchain and lead-chain transitions in the system and find
that the dominant frequency corresponds to the HOMO-LUMO transition of the
central wire. For AC biases with odd inversion symmetry odd harmonics to high
harmonic order in the driving frequency are observed in the dipole moment,
whereas for asymmetric applied bias also even harmonics have considerable
intensity. In both cases we find that the HOMO-LUMO transition strongly mixes
with the harmonics leading to harmonic peaks with enhanced intensity at the
HOMO-LUMO transition energy.Comment: 16 pages, 9 figures. Submitted at "Progress in Nonequilibrium Green's
Functions IV" conferenc
Some open questions in TDDFT: Clues from Lattice Models and Kadanoff-Baym Dynamics
Two aspects of TDDFT, the linear response approach and the adiabatic local
density approximation, are examined from the perspective of lattice models. To
this end, we review the DFT formulations on the lattice and give a concise
presentation of the time-dependent Kadanoff-Baym equations, used to asses the
limitations of the adiabatic approximation in TDDFT. We present results for the
density response function of the 3D homogeneous Hubbard model, and point out a
drawback of the linear response scheme based on the linearized Sham-Schl\"uter
equation. We then suggest a prescription on how to amend it. Finally, we
analyze the time evolution of the density in a small cubic cluster, and compare
exact, adiabatic-TDDFT and Kadanoff-Baym-Equations densities. Our results show
that non-perturbative (in the interaction) adiabatic potentials can perform
quite well for slow perturbations but that, for faster external fields, memory
effects, as already present in simple many-body approximations, are clearly
required.Comment: 15 pages, submitted to Chemical Physic
Successes and Failures of Kadanoff-Baym Dynamics in Hubbard Nanoclusters
We study the non-equilibrium dynamics of small, strongly correlated clusters,
described by a Hubbard Hamiltonian, by propagating in time the Kadanoff-Baym
equations within the Hartree-Fock, 2nd Born, GW and T-matrix approximations. We
compare the results to exact numerical solutions. We find that the T-matrix is
overall superior to the other approximations, and is in good agreement with the
exact results in the low-density regime. In the long time limit, the many-body
approximations attain an unphysical steady state which we attribute to the
implicit inclusion of infinite order diagrams in a few-body system.Comment: 4 pages, 4 figure
Finite elements and the discrete variable representation in nonequilibrium Green's function calculations. Atomic and molecular models
In this contribution, we discuss the finite-element discrete variable
representation (FE-DVR) of the nonequilibrium Green's function and its
implications on the description of strongly inhomogeneous quantum systems. In
detail, we show that the complementary features of FEs and the DVR allows for a
notably more efficient solution of the two-time Schwinger/Keldysh/Kadanoff-Baym
equations compared to a general basis approach. Particularly, the use of the
FE-DVR leads to an essential speedup in computing the self-energies.
As atomic and molecular examples we consider the He atom and the linear
version of H in one spatial dimension. For these closed-shell models we,
in Hartree-Fock and second Born approximation, compute the ground-state
properties and compare with the exact findings obtained from the solution of
the few-particle time-dependent Schr\"odinger equation.Comment: 12 pages, 3 figures, submitted as proceedings of conference "PNGF IV
Optical absorption spectra of finite systems from a conserving Bethe-Salpeter equation approach
We present a method for computing optical absorption spectra by means of a
Bethe-Salpeter equation approach, which is based on a conserving linear
response calculation for electron-hole coherences in the presence of an
external electromagnetic field. This procedure allows, in principle, for the
determination of the electron-hole correlation function self-consistently with
the corresponding single-particle Green function. We analyze the general
approach for a "one-shot" calculation of the photoabsorption cross section of
finite systems, and discuss the importance of scattering and dephasing
contributions in this approach. We apply the method to the closed-shell
clusters Na_4, Na^+_9 and Na^+_(21), treating one active electron per Na atom.Comment: 9 pages, 3 figure
An Update on EMU Sovereign Yield Spread Drivers in Times of Crisis: A Panel Data Analysis
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