130 research outputs found
Nonequilibrium Green's function theory for nonadiabatic effects in quantum electron transport
We develop nonequilibribrium Green's function based transport theory, which
includes effects of nonadiabatic nuclear motion in the calculation of the
electric current in molecular junctions. Our approach is based on the
separation of slow and fast timescales in the equations of motion for the
Green's functions by means of the Wigner representation. Time derivatives with
respect to central time serves as a small parameter in the perturbative
expansion enabling the computation of nonadiabatic corrections to molecular
Green's functions. Consequently, we produce series of analytic expressions for
non-adiabatic electronic Green's functions (up to the second order in the
central time derivatives); which depend not solely on instantaneous molecular
geometry but likewise on nuclear velocities and accelerations. Extended formula
for electric current is derived which accounts for the non-adiabatic
corrections. This theory is concisely illustrated by the calculations on a
model molecular junction
Expectation values of single-particle operators in the random phase approximation ground state
We developed a method for computing matrix elements of single-particle
operators in the correlated random phase approximation ground state. Working
with the explicit random phase approximation ground state wavefunction, we
derived practically useful and simple expression for a molecular property in
terms of random phase approximation amplitudes. The theory is illustrated by
the calculation of molecular dipole moments for a set of representative
molecules.Comment: Accepted to J.Chem.Phy
Waiting time distribution for electron transport in a molecular junction with electron-vibration interaction
On the elementary level, electronic current consists of individual electron
tunnelling events that are separated by random time intervals. The waiting time
distribution is a probability to observe the electron transfer in the detector
electrode at time given that an electron was detected in the same
electrode at earlier time . We study waiting time distribution for quantum
transport in a vibrating molecular junction. By treating the electron-vibration
interaction exactly and molecule-electrode coupling perturbatively, we obtain
master equation and compute the distribution of waiting times for electron
transport. The details of waiting time distributions are used to elucidate
microscopic mechanism of electron transport and the role of electron-vibration
interactions. We find that as nonequilibrium develops in molecular junction,
the skewness and dispersion of the waiting time distribution experience
stepwise drops with the increase of the electric current. These steps are
associated with the excitations of vibrational states by tunnelling electrons.
In the strong electron-vibration coupling regime, the dispersion decrease
dominates over all other changes in the waiting time distribution as the
molecular junction departs far away from the equilibrium
Chiral selectivity of amino acid adsorption on chiral surfaces - the case of alanine on Pt
We study the binding pattern of the amino acid alanine on the naturally
chiral Pt surfaces Pt(531), Pt(321) and Pt(643). These surfaces are all vicinal
to the {111} direction but have different local environments of their kink
sites and are thus a model for realistic roughened Pt surfaces. Alanine has
only a single methyl group attached to its chiral center, which makes the
number of possible binding conformations computationally tractable.
Additionally, only the amine and carboxyl group are expected to interact
strongly with the Pt substrate. On Pt(531) we study the molecule in its
pristine as well as its deprotonated form and find that the deprotonated one is
more stable by 0.39 eV. Therefore, we study the molecule in its deprotonated
form on Pt(321) and Pt(643). As expected, the oxygen and nitrogen atoms of the
deprotonated molecule provide a local binding "tripod" and the most stable
adsorption configurations optimize the interaction of this "tripod" with
undercoordinated surface atoms. However, the interaction of the methyl group
plays an important role: it induces significant chiral selectivity of about 60
meV on all surfaces. Hereby, the L-enantiomer adsorbs preferentially to the
Pt(321) and Pt(643) surfaces while the D-enantiomer is more stable on
Pt(531). The binding energies increase with increasing surface density of
kink sites, i.e. they are largest for Pt(531) and smallest for Pt(643)
Emergence of negative viscosities and colored noise under current-driven Ehrenfest molecular dynamics
Molecules in molecular junctions are subject to current-induced forces that
can break chemical bonds, induce reactions, destabilize molecular geometry, and
halt the operation of the junction. Theories behind current-driven molecular
dynamics simulations rely on a perturbative time-scale separation within the
system with subsequent use of nonequilibrium Green's functions (NEGF) to
compute conservative, non-conservative, and stochastic forces exerted by
electrons on nuclear degrees of freedom. We analyze the effectiveness of this
approximation, paying particular attention to the phenomenon of negative
viscosities. The perturbative approximation is directly compared to the
nonequilibrium Ehrenfest approach. We introduce a novel time-stepping approach
to calculate the forces present in the Ehrenfest method via exact integration
of the equations of motion for the nonequilibrium Green's functions, which does
not necessitate a time-scale separation within the system and provides an exact
description for the corresponding classical dynamics. We observe that negative
viscosities are not artifacts of a perturbative treatment but also emerge in
Ehrenfest dynamics. However, the effects of negative viscosity have the
possibility of being overwhelmed by the predominantly positive dissipation due
to the higher-order forces unaccounted for by the perturbative approach.
Additionally, we assess the validity of the white-noise approximation for the
stochastic forces, finding that it is justifiable in the presence of a clear
time-scale separation and is more applicable when the current-carrying
molecular orbital is moved outside of the voltage window. Finally, we
demonstrate the method for molecular junction models consisting of one and two
classical degrees of freedom
First-passage time theory of activated rate chemical processes in electronic molecular junctions
Confined nanoscale spaces, electric fields and tunneling currents make the
molecular electronic junction an experimental device for the discovery of new,
out-of-equilibrium chemical reactions. Reaction-rate theory for
current-activated chemical reactions is developed by combining a Keldysh
nonequilibrium Green's functions treatment of electrons, Fokker-Planck
description of the reaction coordinate, and Kramers' first-passage time
calculations. The NEGF provide an adiabatic potential as well as a diffusion
coefficient and temperature with local dependence on the reaction coordinate.
Van Kampen's Fokker-Planck equation, which describes a Brownian particle moving
in an external potential in an inhomogeneous medium with a position-dependent
friction and diffusion coefficient, is used to obtain an analytic expression
for the first-passage time. The theory is applied to several transport
scenarios: a molecular junction with a single, reaction coordinate dependent
molecular orbital, and a model diatomic molecular junction. We demonstrate the
natural emergence of Landauer's blowtorch effect as a result of the interplay
between the configuration dependent viscosity and diffusion coefficients. The
resultant localized heating in conjunction with the bond-deformation due to
current-induced forces are shown to be the determining factors when considering
chemical reaction rates; each of which result from highly tunable parameters
within the system
Manifestation of nonequilibrium initial conditions in molecular rotation: the generalized J-diffusion model
In order to adequately describe molecular rotation far from equilibrium, we
have generalized the J-diffusion model by allowing the rotational relaxation
rate to be angular momentum dependent. The calculated nonequilibrium rotational
correlation functions (CFs) are shown to decay much slower than their
equilibrium counterparts, and orientational CFs of hot molecules exhibit
coherent behavior, which persists for several rotational periods. As distinct
from the results of standard theories, rotational and orientational CFs are
found to dependent strongly on the nonequilibrium preparation of the molecular
ensemble. We predict the Arrhenius energy dependence of rotational relaxation
times and violation of the Hubbard relations for orientational relaxation
times. The standard and generalized J-diffusion models are shown to be almost
indistinguishable under equilibrium conditions. Far from equilibrium, their
predictions may differ dramatically
Current Profiles of Molecular Nanowires; DFT Green Function Representation
The Liouville-space Green function formalism is used to compute the current
density profile across a single molecule attached to electrodes. Time ordering
is maintained in real, physical, time, avoiding the use of artificial time
loops and backward propagations. Closed expressions for molecular currents,
which only require DFT calculations for the isolated molecule, are derived to
fourth order in the molecule/electrode coupling.Comment: 21 page
Hadrons in Dense Resonance-Matter: A Chiral SU(3) Approach
A nonlinear chiral SU(3) approach including the spin 3/2 decuplet is
developed to describe dense matter. The coupling constants of the baryon
resonances to the scalar mesons are determined from the decuplet vacuum masses
and SU(3) symmetry relations. Different methods of mass generation show
significant differences in the properties of the spin-3/2 particles and in the
nuclear equation of state.Comment: 28 pages, 9 figure
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