168 research outputs found
Nonequilibrium Green's functions and atom-surface dynamics: Simple views from a simple model system
We employ Non-equilibrium Green's functions (NEGF) to describe the real-time
dynamics of an adsorbate-surface model system exposed to ultrafast laser
pulses. For a finite number of electronic orbitals, the system is solved
exactly and within different levels of approximation. Specifically i) the full
exact quantum mechanical solution for electron and nuclear degrees of freedom
is used to benchmark ii) the Ehrenfest approximation (EA) for the nuclei, with
the electron dynamics still treated exactly. Then, using the EA, electronic
correlations are treated with NEGF within iii) 2nd Born and with iv) a recently
introduced hybrid scheme, which mixes 2nd Born self-energies with
non-perturbative, local exchange-correlation potentials of Density Functional
Theory (DFT). Finally, the effect of a semi-infinite substrate is considered:
we observe that a macroscopic number of de-excitation channels can hinder
desorption. While very preliminary in character and based on a simple and
rather specific model system, our results clearly illustrate the large
potential of NEGF to investigate atomic desorption, and more generally, the non
equilibrium dynamics of material surfaces subject to ultrafast laser fields.Comment: 10 pages, 5 figure
Long-range excitations in time-dependent density functional theory
Adiabatic time-dependent density functional theory fails for excitations of a
heteroatomic molecule composed of two open-shell fragments at large separation.
Strong frequency-dependence of the exchange-correlation kernel is necessary for
both local and charge-transfer excitations. The root of this is static
correlation created by the step in the exact Kohn-Sham ground-state potential
between the two fragments. An approximate non-empirical kernel is derived for
excited molecular dissociation curves at large separation. Our result is also
relevant for the usual local and semi-local approximations for the ground-state
potential, as static correlation there arises from the coalescence of the
highest occupied and lowest unoccupied orbital energies as the molecule
dissociates.Comment: 7 pages, 2 figure
Multiple Core-Hole Coherence in X-Ray Four-Wave-Mixing Spectroscopies
Correlation-function expressions are derived for the coherent nonlinear
response of molecules to three resonant ultrafast pulses in the x-ray regime.
The ability to create two-core-hole states with controlled attosecond timing in
four-wave-mixing and pump probe techniques should open up new windows into the
response of valence electrons, which are not available from incoherent x-ray
Raman and fluorescence techniques. Closed expressions for the necessary
four-point correlation functions are derived for the electron-boson model by
using the second order cumulant expansion to describe the fluctuating
potentials. The information obtained from multidimensional nonlinear techniques
could be used to test and refine this model, and establish an anharmonic
oscillator picture for electronic excitations
Photoemission Beyond the Sudden Approximation
The many-body theory of photoemission in solids is reviewed with emphasis on
methods based on response theory. The classification of diagrams into loss and
no-loss diagrams is discussed and related to Keldysh path-ordering
book-keeping. Some new results on energy losses in valence-electron
photoemission from free-electron-like metal surfaces are presented. A way to
group diagrams is presented in which spectral intensities acquire a
Golden-Rule-like form which guarantees positiveness. This way of regrouping
should be useful also in other problems involving spectral intensities, such as
the problem of improving the one-electron spectral function away from the
quasiparticle peak.Comment: 18 pages, 11 figure
Time-dependent quantum transport: an exact formulation based on TDDFT
An exact theoretical framework based on Time Dependent Density Functional
Theory (TDDFT) is proposed in order to deal with the time-dependent quantum
transport in fully interacting systems. We use a \textit{partition-free}
approach by Cini in which the whole system is in equilibrium before an external
electric field is switched on. Our theory includes the interactions between the
leads and between the leads and the device. It is well suited for calculating
measurable transient phenomena as well as a.c. and other time-dependent
responses. We show that the steady-state current results from a
\textit{dephasing mechanism} provided the leads are macroscopic and the device
is finite. In the d.c. case, we obtain a Landauer-like formula when the
effective potential of TDDFT is uniform deep inside the electrodes.Comment: final version, 7 pages, 1 figur
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
Origin of static and dynamic steps in exact Kohn-Sham potentials
Knowledge of exact properties of the exchange-correlation (xc) functional is important for improving the approximations made within density functional theory. Features such as steps in the exact xc potential are known to be necessary for yielding accurate densities, yet little is understood regarding their shape, magnitude, and location. We use systems of a few electrons, where the exact electron density is known, to demonstrate general properties of steps. We find that steps occur at points in the electron density where there is a change in the 'local effective ionization energy' of the electrons. We provide practical arguments, based on the electron density, for determining the position, shape, and height of steps for ground-state systems, and extend the concepts to time-dependent systems. These arguments are intended to inform the development of approximate functionals, such as the mixed localization potential (MLP), which already demonstrate their capability to produce steps in the Kohn-Sham potential
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