167 research outputs found
Electron transport through a metal-molecule-metal junction
Molecules of bisthiolterthiophene have been adsorbed on the two facing gold
electrodes of a mechanically controllable break junction in order to form
metal-molecule(s)-metal junctions. Current-voltage (I-V) characteristics have
been recorded at room temperature. Zero bias conductances were measured in the
10-100 nS range and different kinds of non-linear I-V curves with step-like
features were reproducibly obtained. Switching between different kinds of I-V
curves could be induced by varying the distance between the two metallic
electrodes. The experimental results are discussed within the framework of
tunneling transport models explicitly taking into account the discrete nature
of the electronic spectrum of the molecule.Comment: 12 pages, 12 figures to appear in Phys. Rev. B 59(19) 199
Incoherent charge transport through molecular wires: interplay of Coulomb interaction and wire population
The influence of Coulomb interaction on the electron transport through
molecular wires is studied in the regime of incoherent tunneling. In the limit
of strong Coulomb repulsion, the current for spinless electrons is determined.
It is shown that the voltage profile along the wire crucially influences the
dependence of the current on the wire length. Upon inclusion of the spin degree
of freedom one finds a blocking effect which depends both on the interaction
strength and on the population of the wire. For finite Coulomb interaction, the
temperature dependence of the blocking is studied and it is shown that several
regimes with different blocking strength may exist.Comment: 18 pages, 8 figures, elsart.cls v2.18 include
Charging induced asymmetry in molecular conductors
We investigate the origin of asymmetry in various measured current-voltage
(I-V) characteristics of molecules with no inherent spatial asymmetry, with
particular focus on a recent break junction measurement. We argue that such
asymmetry arises due to unequal coupling with the contacts and a consequent
difference in charging effects, which can only be captured in a self-consistent
model for molecular conduction. The direction of the asymmetry depends on the
sign of the majority carriers in the molecule. For conduction through highest
occupied molecular orbitals (i.e. HOMO or p-type conduction), the current is
smaller for positive voltage on the stronger contact, while for conduction
through lowest unoccupied molecular orbitals (i.e. LUMO or n-type conduction),
the sense of the asymmetry is reversed. Within an extended Huckel description
of the molecular chemistry and the contact microstructure (with two adjustable
parameters, the position of the Fermi energy and the sulphur-gold bond length),
an appropriate description of Poisson's equation, and a self-consistently
coupled non-equilibrium Green's function (NEGF) description of transport, we
achieve good agreement between theoretical and experimental I-V
characteristics, both in shape as well as overall magnitude.Comment: length of the paper has been extended (4 pages to 6 pages), two new
figures have been added (3 figures to 5 figures), has been accepted for PR
Stretching dependence of the vibration modes of a single-molecule Pt-H2-Pt bridge
A conducting bridge of a single hydrogen molecule between Pt electrodes is
formed in a break junction experiment. It has a conductance near the quantum
unit, G_0 = 2e^2/h, carried by a single channel. Using point contact
spectroscopy three vibration modes are observed and their variation upon
stretching and isotope substitution is obtained. The interpretation of the
experiment in terms of a Pt-H_2-Pt bridge is verified by Density Functional
Theory calculations for the stability, vibrational modes, and conductance of
the structure.Comment: 5 pages, 4 figure
From a local Green function to molecular charge transport
A local-orbital based ab initio approach to obtain the Green function for
large heterogeneous systems is developed. First a Green function formalism is
introduced based on exact diagonalization. Then the self energy is constructed
from an incremental scheme, rendering the procedure feasible, while at the same
time physical insight into different local correlation contributions is
obtained. Subsequently the Green function is used in the frame of the Landauer
theory and the wide band approximation to calculate the electronic transmission
coefficient across molecular junctions. The theory is applied to meta- and
para-ditholbenzene linked to gold electrodes and various correlation
contributions are analyzed
Path-integral Monte-Carlo simulations for electronic dynamics on molecular chains: I. Sequential hopping and super exchange
An improved real-time quantum Monte Carlo procedure is presented and applied
to describe the electronic transfer dynamics along molecular chains. The model
consists of discrete electronic sites coupled to a thermal environment which is
integrated out exactly within the path integral formulation. The approach is
numerically exact and its results reduce to known analytical findings (Marcus
theory, golden rule) in proper limits. Special attention is paid to the role of
superexchange and sequential hopping at lower temperatures in symmetric
donor-bridge-acceptor systems. In contrast to previous approximate studies,
superexchange turns out to play a significant role only for extremely high
lying bridges where the transfer is basically frozen or for extremely low
temperatures where for weaker dissipation a description in terms of rate
constants is no longer feasible. For bridges with increasing length an
algebraic decrease of the yield is found for short as well as for longer
bridges. The approach can be extended to electronic systems with more
complicated topologies including impurities and in presence of external time
dependent forces.Comment: 14 pages, 9 figures submitted to the Journal of Chemical Physic
Generalized Numerical Renormalization Group for Dynamical Quantities
In this paper we introduce a new approach for calculating dynamical
properties within the numerical renormalization group. It is demonstrated that
the method previously used fails for the Anderson impurity in a magnetic field
due to the absence of energy scale separation. The problem is solved by
evaluating the Green function with respect to the reduced density matrix of the
full system, leading to accurate spectra in agreement with the static
magnetization. The new procedure (denoted as DM-NRG) provides a unifying
framework for calculating dynamics at any temperature and represents the
correct extension of Wilson's original thermodynamic calculation.Comment: 4 pages RevTeX, 6 eps figures include
Coherent charge transport through molecular wires: influence of strong Coulomb repulsion
We derive a master equation for the electron transport through molecular
wires in the limit of strong Coulomb repulsion. This approach is applied to two
typical situations: First, we study transport through an open conduction
channel for which we find that the current exhibits an ohmic-like behaviour.
Second, we explore the transport properties of a bridged molecular wire, where
the current decays exponentially as a function of the wire length. For both
situations, we discuss the differences to the case of non-interacting
electrons.Comment: 15 pages, 4 figures, elsart style, accepted at Chem Phy
Landau-Zener transitions in a linear chain
We present an exact asymptotic solution for electron transition amplitudes in
an infinite linear chain driven by an external homogeneous time-dependent
electric field. This solution extends the Landau-Zener theory for the case of
infinite number of states in discrete spectrum. In addition to transition
amplitudes we calculate an effective diffusion constant.Comment: 3 figure
Shot noise in tunneling transport through molecules and quantum dots
We consider electrical transport through single molecules coupled to metal
electrodes via tunneling barriers. Approximating the molecule by the Anderson
impurity model as the simplest model which includes Coulomb charging effects,
we extend the ``orthodox'' theory to expand current and shot noise
systematically order by order in the tunnel couplings. In particular, we show
that a combined measurement of current and shot noise reveals detailed
information of the system even in the weak-coupling limit, such as the ratio of
the tunnel-coupling strengths of the molecule to the left and right electrode,
and the presence of the Coulomb charging energy. Our analysis holds for
single-level quantum dots as well.Comment: 8 page
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