Alexander J. White

Raman scattering in molecular junctions: a pseudoparticle formulation.

We present a formulation of Raman spectroscopy in molecular junctions based on a many-body state representation of the molecule. The approach goes beyond the previous effective single orbital formalism and provides a convenient way to incorporate computational methods and tools proven for equilibrium molecular spectroscopy into the realm of current carrying junctions. The presented framework is illustrated by first principle simulations of Raman response in a three-ring oligophenylene vinylene terminating in amine functional groups (OPV3) junction. The calculated shift in Stokes lines and estimate of vibrational heating by electric current agree with available experimental data. In particular, our results suggest that participation of the OPV3 cation in Raman scattering under bias may be responsible for the observed shift, and that the direction of the shift depends on renormalization of normal modes. This work is a step toward atomistic quantum ab initio modeling of the optical response of nonequilibrium electronic dynamics in molecular junctions.

Molecular nanoplasmonics: Self-consistent electrodynamics in current-carrying junctions

We consider a biased molecular junction subjected to external time-dependent electromagnetic field. We discuss local field formation due to both surface plasmon-polariton excitations in the contacts and the molecular response. Employing realistic parameters we demonstrate that such self-consistent treatment is crucial for proper description of the junction transport characteristics.

Quantum transport with two interacting conduction channels

The transport properties of a conduction junction model characterized by two mutually coupled channels that strongly differ in their couplings to the leads are investigated. Models of this type describe molecular redox junctions (where a level that is weakly coupled to the leads controls the molecular charge, while a strongly coupled one dominates the molecular conduction), and electron counting devices in which the current in a point contact is sensitive to the charging state of a nearby quantum dot. Here we consider the case where transport in the strongly coupled channel has to be described quantum mechanically (covering the full range between sequential tunneling and co-tunneling), while conduction through the weakly coupled channel is a sequential process that could by itself be described by a simple master equation. We compare the result of a full quantum calculation based on the pseudoparticle non-equilibrium Green function method to that obtained from an approximate mixed quantum-classical calculation, where correlations between the channels are taken into account through either the averaged rates or the averaged energy. We find, for the steady state current, that the approximation based on the averaged rates works well in most of the voltage regime, with marked deviations from the full quantum results only at the threshold for charging the weekly coupled level. These deviations are important for accurate description of the negative differential conduction behavior that often characterizes redox molecular junctions in the neighborhood of this threshold.

Correlation and transport properties for mixtures at constant pressure and temperature

Transport properties of mixtures of elements in the dense plasma regime play an important role in natural astrophysical and experimental systems, e.g., inertial confinement fusion. We present a series of orbital-free molecular dynamics simulations on dense plasma mixtures with comparison to a global pseudo ion in jellium model. Hydrogen is mixed with elements of increasingly high atomic number (lithium, carbon, aluminum, copper, and silver) at a fixed temperature of 100 eV and constant pressure set by pure hydrogen at 2g/cm^{3}, namely, 370 Mbars. We compute ionic transport coefficients, such as self-diffusion, mutual diffusion, and viscosity for various concentrations. Small concentrations of the heavy atoms significantly change the density of the plasma and decrease the transport coefficients. The structure of the mixture evidences a strong Coulomb coupling between heavy ions and the appearance of a broad correlation peak at short distances between hydrogen atoms. The concept of an effective one component plasma is used to quantify the overcorrelation of the light element induced by the admixture of a heavy element.

Effects of Electromagnetic Coupling on Conductance Switching of a Gated Tunnel Junction

Using a combination of density functional theory and quantum master equations approach, we study the effect of electromagnetic (EM) coupling on the nonequilibrium steady-state behavior of a recently introduced gated molecular junction. This junction was demonstrated in a previous publication to exhibit sharp current switching near a certain critical DC field Ez*, which induces intramolecular charge transfer, and here, we analyze the steady-state population and current when an AC EM field (EMF) is present. The AC EMF at frequency ω0 produces pronounced population and current features at gate fields Ez = Ez* ± ℏω0/ez (where ez is the dipole of the charge-transfer state) and thus allows additional sharp switching capability at lower gate fields. We found that even when EMF is absent, the EM coupling itself changes the overall steady-state population and current distributions because it allows for relaxation via spontaneous emission.

Non-Markovian theory of collective plasmon-molecule excitations in nanojunctions combined with classicalelectrodynamic simulations

We present a pseudoparticle nonequilibrium Green function formalism as a tool to study the coupling between plasmons and excitons in nonequilibrium molecular junctions. The formalism treats plasmon-exciton couplings and intra-molecular interactions exactly, and is shown to be especially convenient for exploration of plasmonic absorption spectrum of plexitonic systems, where combined electron and energy transfers play an important role. We demonstrate the sensitivity of the molecule-plasmon Fano resonance to junction bias and intra-molecular interactions (Coulomb repulsion and intra-molecular exciton coupling). The electromagnetic theory is used in order to derive self-consistent ¯eld-induced coupling terms between the molecular and the plasmon excitations. Our study opens a way to deal with strongly interacting plasmon-exciton systems in nonequilibrium molecular devices.

Photoexcited Nonadiabatic Dynamics of Solvated Push-Pull π-Conjugated Oligomers with the NEXMD Software

Solvation can be modeled implicitly by embedding the solute in a dielectric cavity. This approach models the induced surface charge density at the solute-solvent boundary, giving rise to extra Coulombic interactions. Herein, the Nonadiabatic EXcited-state Molecular Dynamics (NEXMD) software was used to model the photoexcited nonradiative relaxation dynamics in a set of substituted donor-acceptor oligo( p-phenylenevinylene) (PPVO) derivatives in the presence of implicit solvent. Several properties of interest including optical spectra, excited state lifetimes, exciton localization, excited state dipole moments, and structural relaxation are calculated to elucidate dependence of functionalization and solvent polarity on photoinduced nonadiabatic dynamics. Results show that solvation generally affects all these properties, where the magnitude of these effects vary from one system to another depending on donor-acceptor substituents and molecular polarizability. We conclude that implicit solvation can be directly incorporated into nonadiabatic simulations within the NEXMD framework with little computational overhead and that it qualitatively reproduces solvent-dependent effects observed in solution-based spectroscopic experiments.

Stretching Hollow Jets in Potential Flow

Why model hollow stretching jets? As well as offering some interesting new mathematics, the dynamics of such jets are of potential interest in the consideration of methods of creating circular penetration cuts into targets, in understanding better the coherency of solid shaped charge jets, and possibly as a means of engendering cracking in targets over a wider target area than can be accomplished with a solid shaped charge jet. This paper presents and solves numerically the boundary-value problem of a stretching hollow jet in potential flow. The behavior of the hollow cylindrical jet is analyzed as a function of the initial inner and outer radii, the rate of stretching, and the initial radial velocity component associated with the inner radius. It is shown that under different initial conditions the hollow can close up or expand. The evolution of the associated pressure field is determined. INTRODUCTION How do the characteristics of elongating hollow jets differ from solid jets? This question is of importance in the study of the stretching jet created by a shaped charge. Shaped charges are widely used for military applications and for oil recovery. They usually produce solid jets which stretch and become thinner. Much work on modelling such solid jets has been done [1] -– far less on hollow jets. This is because, in general, hollowness is not a desirable characteristic for a shaped charge jet. For one example, links between hollow jets and incoherency (radial dispersion of jet) have been postulated [2]. Hollowness is likely in general to degrade the penetrative capability [3]. But understanding hollowness effects may nonetheless offer other benefits, e.g. by making greater damaged zones in some targets or by making circular cuts in targets. To gain such understanding of the dynamics of a stretching hollow jet, it was logical to consider first the simplest constitutive model, namely potential flow as in earlier work by the authors on instability [4] and bi-material jets [5]. The adoption of potential flow implicitly implies incompressibility. Following again the philosophy of working initially with the simplest possible scenario, it was decided to model a stretching uniform tubular jet, making the assumptions that the uniformity is maintained as the jet stretches and that the axial velocity component increases linearly with distance along the jet. These assumptions are entirely in accord with previous work on the stability of shaped charge jets [4, 6-10]. In the next section the mathematical model for a stretching hollow jet in potential flow is derived, assuming that the pressure is so small as to be effectively zero on the curved inner and outer surfaces. The analysis results in three ordinary differential equations in time to be solved simultaneously. The numerical solution scheme used to solve these equations is described and applied for input parameters of values typical in the shaped charge regime. Predictions are made for a variety of initial conditions. A correspondingly varied range of responses of the jet is seen. The results are discussed, conclusions are drawn and finally a range of potential future research is outlined. Shock Compression of Condensed Matter 2017 AIP Conf. Proc. 1979, 120003-1–120003-6; https://doi.org/10.1063/1.5044941 Published by AIP Publishing. 978-0-7354-1693-2/

The formation and stretching of bi-material shaped charge jets

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Collective Plasmon-Molecule Excitations in Nanojunctions: Quantum Consideration

The equations for the formation of a bi-material jet from a laminated shaped charge liner are presented. A coupled pair of boundary-value problems is then established for an idealised stretching jet in cases where the outer material occupies a hollow uniform cylinder surrounding the inner material in the uniform hollow. This is done first where the materials are inviscid fluids. Making the assumption that the axial velocity in each part of the jet is the same and linearly decreasing from the front to the rear of the composite jet, solutions for the pressure field in each part are obtained. The problem is then reformulated where the two materials are both perfectly plastic solids but with differing densities and yield strengths. The equations of plastic flow (Levy-Mises with von-Mises yield criterion) are solved for each material to derive the stress field in both parts of the jet. These analytical solutions offer a basis for future stability and target penetration studies.