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

Numerical studies of the interaction of an atomic sample with the electromagnetic field in two dimensions

We consider the interaction of electromagnetic radiation of arbitrary polarization with multi-level atoms in a self-consistent manner, taking into account both spatial and temporal dependencies of local fields. This is done by numerically solving the corresponding system of coupled Maxwell-Liouville equations for various geometries. In particular, we scrutinize linear optical properties of nanoscale atomic clusters, demonstrating the significant role played by collective effects and dephasing. It is shown that subwavelength atomic clusters exhibit two resonant modes, one of which is localized slightly below the atomic transition frequency of an individual atom, while the other is positioned considerably above it. As an initial exploration of future applications of this approach, the optical response of core-shell nanostructures, with a core consisting of silver and shell composed of resonant atoms, is examined.Comment: re-submitted to PR

Electron transfer in confined electromagnetic fields

The interaction between molecular (atomic) electron(s) and the vacuum field of a reflective cavity generates a significant interest thanks to the rapid developments in nanophotonics. Such interaction which lies within the realm of cavity quantum electrodynamic can substantially affect transport properties of molecular systems. In this work we consider non-adiabatic electron transfer process in the presence of a cavity mode. We present a generalized framework for the interaction between a charged molecular system and a quantized electromagnetic field of a cavity and apply it to the problem of electron transfer between a donor and an acceptor placed in a confined vacuum electromagnetic field. The effective system Hamiltonian corresponds to a unified Rabi and spin-boson model which includes a self-dipole energy term. Two limiting cases are considered: one where the electron is assumed much faster than the cavity mode and another in which the electron tunneling time is significantly larger than the mode period. In both cases a significant rate enhancement can be produced by coupling to the cavity mode in the Marcus inverted region. The results of this work offer new possibilities for controlling electron transfer processes using visible and infrared plasmonicsComment: 33 pages, 4 figure

Magnetic Fields Effects on the Electronic Conduction Properties of Molecular Ring Structures

While mesoscopic conducting loops are sensitive to external magnetic fields, as seen by observations of the Aharonov-Bohm (AB) effect in such structures, the field needed to observe the AB periodicity in small molecular rings is unrealistically large. The present study aims to identify conditions under which magnetic field dependence can be observed in electronic conduction through such molecules. We consider molecular ring structures modeled both within the tight-binding (H\"uckel) model and as continuous rings. In fact, much of the observed qualitative behavior can be rationalized in terms of a much simpler two-state model. Dephasing in these models is affected by two common tools: the B\"uttiker probe method and coherence damping within a density matrix formulation. We show that current through a benzene ring can be controlled by moderate fields provided that (a) conduction must be dominated by degenerate (in the free molecule) molecular electronic resonances, associated with multiple pathways as is often the case with ring molecules; (b) molecular-leads electronic coupling must is weak so as to affect relatively distinct conduction resonances; (c) molecular binding to the leads must be asymmetric (e.g., for benzene, connection in the meta or ortho, but not para, configurations) and, (d) dephasing has to be small. Under these conditions, considerable sensitivity to an imposed magnetic field normal to the molecular ring plane is found in benzene and other aromatic molecules. Interestingly, in symmetric junctions (e.g. para connected benzene) a large sensitivity of the transmission coefficient to magnetic field is not reflected in the current-voltage characteristic. Although sensitivity to magnetic field is suppressed by dephasing, quantitative estimates indicate that magnetic field control can be observed under realistic condition.Comment: 46 pages, 20 figure

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