8,083 research outputs found
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
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
A Thermal Gradient Approach for the Quasi-Harmonic Approximation and its Application to Improved Treatment of Anisotropic Expansion
We present a novel approach to efficiently implement thermal expansion in the
quasi-harmonic approximation (QHA) for both isotropic and more importantly,
anisotropic expansion. In this approach, we rapidly determine a crystal's
equilibrium volume and shape at a given temperature by integrating along the
gradient of expansion from zero Kelvin up to the desired temperature. We
compare our approach to previous isotropic methods that rely on a brute-force
grid search to determine the free energy minimum, which is infeasible to carry
out for anisotropic expansion, as well as quasi-anisotropic approaches that
take into account the contributions to anisotropic expansion from the lattice
energy. We compare these methods for experimentally known polymorphs of
piracetam and resorcinol and show that both isotropic methods agree to within
error up to 300 K. Using the Gr\"{u}neisen parameter causes up to 0.04 kcal/mol
deviation in the Gibbs free energy, but for polymorph free energy differences
there is a cancellation in error with all isotropic methods within 0.025
kcal/mol at 300 K.
Anisotropic expansion allows the crystals to relax into lattice geometries
0.01-0.23 kcal/mol lower in energy at 300 K relative to isotropic expansion.
For polymorph free energy differences all QHA methods produced results within
0.02 kcal/mol of each other for resorcinol and 0.12 kcal/mol for piracetam, the
two molecules tested here, demonstrating a cancellation of error for isotropic
methods.
We also find that when expanding in more than a single volume variable, there
is a non-negligible rate of failure of the basic approximations of QHA.
Specifically, while expanding into new harmonic modes as the box vectors are
increased, the system often falls into alternate, structurally distinct
harmonic modes unrelated by continuous deformation from the original harmonic
mode.Comment: 38 pages, including 9 pages supporting informatio
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
Born Oppenheimer Dynamics Near Metal Surfaces
We discuss the usefulness of Born-Oppenheimer potential surfaces for nuclear
dynamics for molecules strongly coupled to metal surfaces. A simple model
demonstrating the construction of such surface for a molecular junction is
discussed.Comment: 5 pages, 2 figure
Reply to Comment by Alexandrov and Bratkovsky [cond-mat/0603467, cond-mat/0606366]
The submitted Comment is based on using an isolated quantum dot approach to
discuss the situation where the coupling to the leads is considerable (not
negligible). This finite lead coupling is the situation in most molecular
transport junctions. In such situations the population on the molecule is not
static and dynamical effects (fluctuations) are crucial. While the discussion
in [Phys. Rev. B 67, 235312 (2003)], and in the Comment may well apply for an
isolated molecule, it is simply irrelevant for the very different transport
situation analyzed in our original contribution.Comment: 3 pages, no figures; Original Comment [cond-mat/0603467] was
submitted to Nano Letters and following our reply the editor has decided
against publication; Later submission [cond-mat/0606366] is essentially a
reproduction (with new title and abstract and equations rearranged) of the
original Comment [cond-mat/0603467
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