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

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

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

Current-induced forces for nonadiabatic molecular dynamics

We present general first principles derivation of expression for current-induced forces. The expression is applicable in non-equilibrium molecular systems with arbitrary intra-molecular interactions and for any electron-nuclei coupling. It provides a controlled consistent way to account for quantum effects of nuclear motion, accounts for electronic non-Markov character of the friction tensor, and opens way to treatments beyond strictly adiabatic approximation. We show connection of the expression with previous studies, and discuss effective ways to evaluate the friction tensor.Comment: 6 pages, 3 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