362 research outputs found
Kinetic Schemes in Open Interacting Systems
We discuss utilization of kinetic schemes for description of open interacting
systems, focusing on vibrational energy relaxation for an oscillator coupled to
a nonequilibirum electronic bath. Standard kinetic equations with constant rate
coefficients are obtained under the assumption of timescale separation between
system and bath, with the bath dynamics much faster than that of the system of
interest. This assumption may break down in certain limits and we show that
ignoring this may lead to qualitatively wrong predictions. Connection with more
general, nonequilibrium Green's function (NEGF) analysis, is demonstrated. Our
considerations are illustrated within generic molecular junction models with
electron-vibration coupling.Comment: 22 pages, 4 figure
Nuclear dynamics at molecule-metal interfaces: A pseudoparticle perspective
We discuss nuclear dynamics at molecule-metal interfaces including
non-equilibrium molecular junctions. Starting from the many-body states
(pseudoparticle) formulation of the molecule-metal system in the molecular
vibronic basis, we introduce gradient expansion in order to reduce the
adiabatic nuclear dynamics (that is, nuclear dynamics on a single molecular
potential surface) into its semi-classical form while maintaining the effect of
the non-adiabatic electronic transitions between different molecular charge
states. This yields a set of equations for the nuclear dynamics in the presence
of these non-adiabatic transitions, which reproduce surface hopping formulation
in the limit of small metal-molecule coupling (where broadening of the
molecular energy levels can be disregarded) and Ehrenfest dynamics (motion on
the potential of mean force) when information on the different charging states
is traced out, which is relevant when this coupling is strong.Comment: 9 page
Optical spectroscopy of molecular junctions: Nonequilibrium Green's functions perspective
We consider optical spectroscopy of molecular junctions from the quantum
transport perspective when radiation field is quantized and optical response of
the system is simulated as photon flux. Using exact expressions for photon and
electronic fluxes derived within the nonequilibrium Green function (NEGF)
methodology and utilizing fourth order diagrammatic perturbation theory in
molecular coupling to radiation field we perform simulations employing
realistic parameters. Results of the simulations are compared to the bare
perturbation theory (PT) usually employed in studies on nonlinear optical
spectroscopy to classify optical processes. We show that the bare PT violates
conservation laws, while flux conserving NEGF formulation mixes optical
processes.Comment: 10 pages, 6 figure
Simulation of optical response functions in molecular junctions
We discuss theoretical approaches to nonlinear optical spectroscopy of
molecular junctions. Optical response functions are derived in the form
convenient for implementation of Green function techniques, and their
expressions in terms of pseudoparticle nonequilibrium Green functions are
proposed. The formulation allows to account for both intra-molecular
interactions and hybridization of molecular states due to coupling to contacts.
Two-dimensional optical spectroscopy in junctions is considered as an example.Comment: 11 pages, 7 figure
Electronic friction in interacting systems
We consider effects of strong light-matter interaction on electronic friction
in molecular junctions within generic model of single molecule nano cavity
junction. Results of the Hubbard NEGF simulations are compared with mean-field
NEGF and generalized Head-Gordon and Tully approaches. Mean-field NEGF is shown
to fail qualitatively at strong intra-system interactions, while accuracy of
the generalized Head-Gordon and Tully results is restricted to situations of
well separated intra-molecular excitations, when bath induced coherences are
negligible. Numerical results show effects of bias and cavity mode pumping on
electronic friction. We demonstrate non-monotonic behavior of the friction on
the bias and intensity of the pumping field and indicate possibility of
engineering friction control in single molecule junctions.Comment: 19 pages, 4 figure
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Numerically exact full counting statistics of the energy current in the Kondo regime
We use the inchworm Quantum Monte Carlo method to investigate the full
counting statistics of particle and energy currents in a strongly correlated
quantum dot. Our method is used to extract the heat fluctuations and entropy
production of a quantum thermoelectric device, as well as cumulants of the
particle and energy currents. The energy--particle current cross correlations
reveal information on the preparation of the system and the interplay of
thermal and electric currents. We furthermore demonstrate the signature of a
crossover from Coulomb blockade to Kondo physics in the energy current
fluctuations, and show how the conventional master equation approach to full
counting statistics systematically fails to capture this crossover
On the widths of Stokes lines in Raman scattering from molecules adsorbed at metal surfaces and in molecular conduction junctions
Within a generic model we analyze the Stokes linewidth in surface enhanced
Raman scattering (SERS) from molecules embedded as bridges in molecular
junctions. We identify four main contributions to the off-resonant Stokes
signal and show that under zero voltage bias (a situation pertaining also to
standard SERS experiments) and at low bias junctions only one of these
contributions is pronounced. The linewidth of this component is determined by
the molecular vibrational relaxation rate, which is dominated by interactions
with the essentially bosonic thermal environment when the relevant molecular
electronic energy is far from the metal(s) Fermi energy(ies). It increases when
the molecular electronic level is close to the metal Fermi level so that an
additional vibrational relaxation channel due to electron-hole (eh) excition in
the molecule opens. Other contributions to the Raman signal, of considerably
broader linewidths, can become important at larger junction bias.Comment: 17 pages, 5 figure
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