76 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
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
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
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
Driving current through single organic molecules
We investigate electronic transport through two types of conjugated
molecules. Mechanically controlled break-junctions are used to couple thiol
endgroups of single molecules to two gold electrodes. Current-voltage
characteristics (IVs) of the metal-molecule-metal system are observed. These
IVs reproduce the spatial symmetry of the molecules with respect to the
direction of current flow. We hereby unambigously detect an intrinsic property
of the molecule, and are able to distinguish the influence of both the molecule
and the contact to the metal electrodes on the transport properties of the
compound system.Comment: 4 pages, 5 figure
Fullerene-based molecular nanobridges: A first-principles study
Building upon traditional quantum chemistry calculations, we have implemented
an {\em ab-initio} method to study the electrical transport in nanocontacts. We
illustrate our technique calculating the conductance of C molecules
connected in various ways to Al electrodes characterized at the atomic level.
Central to a correct estimate of the electrical current is a precise knowledge
of the local charge transfer between molecule and metal which, in turn,
guarantees the correct positioning of the Fermi level with respect to the
molecular orbitals. Contrary to our expectations, ballistic transport seems to
occur in this system.Comment: 4 pages in two-column forma
Electrical transport through single-molecule junctions: from molecular orbitals to conduction channels
We present an atomistic theory of electronic transport through single organic
molecules that reproduces the important features of the current-voltage
characteristics observed in recent experiments. We trace these features to
their origin in the electronic structure of the molecules and their local
atomic environment. We demonstrate how conduction channels arise from the
molecular orbitals and elucidate which specific properties of the individual
orbitals determine their contribution to the current.Comment: Revtex4, 4 pages, 4 figures. Version with color figures in
http://www-tfp.physik.uni-karlsruhe.de/~cuevas/Publications.htm
Modeling transport through single-molecule junctions
Non-equilibrium Green's functions (NEGF) formalism combined with extended
Huckel (EHT) and charging model are used to study electrical conduction through
single-molecule junctions. Analyzed molecular complex is composed of asymmetric
1,4-Bis((2'-para-mercaptophenyl)-ethinyl)-2-acetyl-amino-5-nitro-benzene
molecule symmetrically coupled to two gold electrodes [Reichert et al., Phys.
Rev. Lett. Vol.88 (2002), pp. 176804]. Owing to this model, the accurate values
of the current flowing through such junction can be obtained by utilizing basic
fundamentals and coherently deriving model parameters. Furthermore, the
influence of the charging effect on the transport characteristics is
emphasized. In particular, charging-induced reduction of conductance gap,
charging-induced rectification effect and charging-generated negative value of
the second derivative of the current with respect to voltage are observed and
examined for molecular complex.Comment: 8 pages, 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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