141 research outputs found
Multimode vibrational effects in single molecule conductance: A nonequilibrium Green's function approach
The role of multimode vibrational dynamics in electron transport through
single molecule junctions is investigated. The study is based on a generic
model, which describes charge transport through a single molecule that is
attached to metal leads. To address vibrationally-coupled electron transport,
we employ a nonequilibrium Green's function approach that extends a method
recently proposed by Galperin et al. [Phys. Rev. B 73, 045314 (2006)] to
multiple vibrational modes. The methodology is applied to two systems: a
generic model with two vibrational degrees of freedom and
benzenedibutanethiolate covalently bound to gold electrodes. The results show
that the coupling to multiple vibrational modes can have a significant effect
on the conductance of a molecular junction. In particular, we demonstrate the
effect of electronically induced coupling between different vibrational modes
and study nonequilibrium vibrational effects by calculating the current-induced
excitation of vibrational modes.Comment: 31 pages, 10 figure
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Binding energy referencing for XPS in Alkali metal-based battery materials research (II): Application to complex composite electrodes
X-ray photoelectron spectroscopy (XPS) is a key method for studying (electro-)chemical changes in metal-ion battery electrode materials. In a recent publication, we pointed out a conflict in binding energy (BE) scale referencing at alkali metal samples, which is manifested in systematic deviations of the BEs up to several eV due to a specific interaction between the highly reactive alkali metal in contact with non-conducting surrounding species. The consequences of this phenomenon for XPS data interpretation are discussed in the present manuscript. Investigations of phenomena at surface-electrolyte interphase regions for a wide range of materials for both lithium and sodium-based applications are explained, ranging from oxide-based cathode materials via alloys and carbon-based anodes including appropriate reference chemicals. Depending on material class and alkaline content, specific solutions are proposed for choosing the correct reference BE to accurately define the BE scale. In conclusion, the different approaches for the use of reference elements, such as aliphatic carbon, implanted noble gas or surface metals, partially lack practicability and can lead to misinterpretation for application in battery materials. Thus, this manuscript provides exemplary alternative solutions
Theory of Vibrationally Inelastic Electron Transport through Molecular Bridges
Vibrationally inelastic electron transport through a molecular bridge that is
connected to two leads is investigated. The study is based on a generic model
of vibrational excitation in resonant transmission of electrons through a
molecular junction. Employing methods from electron-molecule scattering theory,
the transmittance through the molecular bridge can be evaluated numerically
exactly. The current through the junction is obtained approximately using a
Landauer-type formula. Considering different parameter regimes, which include
both the case of a molecular bridge that is weakly coupled to the leads,
resulting in narrow resonance structures, and the opposite case of a broad
resonance caused by strong interaction with the leads, we investigate the
characteristic effects of coherent and dissipative vibrational motion on the
electron transport. Furthermore, the validity of widely used approximations
such as the wide-band approximation and the restriction to elastic transport
mechanisms is investigated in some detail.Comment: Submited to PRB, revised version according to comments of referees
(minor text changes and new citations
Semiclassical Approximations in Phase Space with Coherent States
We present a complete derivation of the semiclassical limit of the coherent
state propagator in one dimension, starting from path integrals in phase space.
We show that the arbitrariness in the path integral representation, which
follows from the overcompleteness of the coherent states, results in many
different semiclassical limits. We explicitly derive two possible semiclassical
formulae for the propagator, we suggest a third one, and we discuss their
relationships. We also derive an initial value representation for the
semiclassical propagator, based on an initial gaussian wavepacket. It turns out
to be related to, but different from, Heller's thawed gaussian approximation.
It is very different from the Herman--Kluk formula, which is not a correct
semiclassical limit. We point out errors in two derivations of the latter.
Finally we show how the semiclassical coherent state propagators lead to
WKB-type quantization rules and to approximations for the Husimi distributions
of stationary states.Comment: 80 pages, 4 figure
Light-Induced Responses of Slow Oscillatory Neurons of the Rat Olivary Pretectal Nucleus
Background: The olivary pretectal nucleus (OPN) is a small midbrain structure responsible for pupil constriction in response to eye illumination. Previous electrophysiological studies have shown that OPN neurons code light intensity levels and therefore are called luminance detectors. Recently, we described an additional population of OPN neurons, characterized by a slow rhythmic pattern of action potentials in light-on conditions. Rhythmic patterns generated by these cells last for a period of approximately 2 minutes. Methodology: To answer whether oscillatory OPN cells are light responsive and whether oscillatory activity depends on retinal afferents, we performed in vivo electrophysiology experiments on urethane anaesthetized Wistar rats. Extracellular recordings were combined with changes in light conditions (light-dark-light transitions), brief light stimulations of the contralateral eye (diverse illuminances) or intraocular injections of tetrodotoxin (TTX). Conclusions: We found that oscillatory neurons were able to fire rhythmically in darkness and were responsive to eye illumination in a manner resembling that of luminance detectors. Their firing rate increased together with the strength of the light stimulation. In addition, during the train of light pulses, we observed two profiles of responses: oscillationpreserving and oscillation-disrupting, which occurred during low- and high-illuminance stimuli presentation respectively. Moreover, we have shown that contralateral retina inactivation eliminated oscillation and significantly reduced the firin
Basis set generation for quantum dynamics simulations using simple trajectory-based methods
Methods for solving the time-dependent Schrödinger equation generally employ either a global static basis set, which is fixed at the outset, or a dynamic basis set, which evolves according to classical-like or variational equations of motion; the former approach results in the well-known exponential scaling with system size, while the latter can suffer from challenging numerical problems, such as singular matrices, as well as violation of energy conservation. Here, we suggest a middle road: building a basis set using trajectories to place time-independent basis functions in the regions of phase space relevant to wave function propagation. This simple approach, which potentially circumvents many of the problems traditionally associated with global or dynamic basis sets, is successfully demonstrated for two challenging benchmark problems in quantum dynamics, namely, relaxation dynamics following photoexcitation in pyrazine, and the spin Boson model
Computational Investigation of Acene-Modified Zinc-Porphyrin Based Sensitizers for Dye-Sensitized Solar Cells
Simulation of Electron Transfer and Electron Transport in Molecular Systems at Surfaces
We have investigated electron transfer and transport processes in several molecular systems adsorbed at metal surfaces using a methodology that combines first-principles electronic structure methods with quantum dynamics and transport approaches. Specifically, we have analysed the molecular factors that control electron transfer in a series of nitrile-substituted alkanethiolate self-assembled monolayer models adsorbed at the Au(111) surface that differ in the size of the aliphatic spacer chain. In addition, we have analysed the possibility of using a proton transfer reaction triggered by an external electrostatic field as a novel mechanism for switching a molecular junction. To demonstrate the feasibility of the process, we have investigated electron transport in a junction containing a molecular bridge that can exist in two tautomeric forms, [2,5-(4-hydroxypyridine)] and 2,5-[4(1H)-pyridone], that exhibit very different conductance properties
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