254 research outputs found
Non-Linear Transport through a Molecular Nanojunction
We present a simple model of electrical transport through a
metal-molecule-metal nanojunction that includes charging effects as well as
aspects of the electronic structure of the molecule. The interplay of a large
charging energy and an asymmetry of the metal-molecule coupling can lead to
various effects in non-linear electrical transport. In particular, strong
negative differential conductance is observed under certain conditions.Comment: 7 pages, 5 figures, accepted by Europhys. Let
Super-poissonian noise, negative differential conductance, and relaxation effects in transport through molecules, quantum dots and nanotubes
We consider charge transport through a nanoscopic object, e.g. single
molecules, short nanotubes, or quantum dots, that is weakly coupled to metallic
electrodes. We account for several levels of the molecule/quantum dot with
level-dependent coupling strengths, and allow for relaxation of the excited
states. The current-voltage characteristics as well as the current noise are
calculated within first-order perturbation expansion in the coupling strengths.
For the case of asymmetric coupling to the leads we predict
negative-differential-conductance accompanied with super-poissonian noise. Both
effects are destroyed by fast relaxation processes. The non-monotonic behavior
of the shot noise as a function of bias and relaxation rate reflects the
details of the electronic structure and level-dependent coupling strengths.Comment: 8 pages, 7 figures, submitted to Phys. Rev. B, added reference
Pair-tunneling resonance in the single-electron transport regime
We predict a new electron pair-tunneling (PT) resonance in non-linear
transport through quantum dots with positive charging energies exceeding the
broadening due to thermal and quantum fluctuations. The PT resonance shows up
in the single-electron transport (SET) regime as a peak in the derivative of
the non-linear conductance when the electrochemical potential of one electrode
matches the average of two subsequent charge addition energies. For a single
level quantum dot (Anderson model) we find the analytic peak shape and the
dependence on temperature, magnetic field and junction asymmetry and compare
with the inelastic cotunneling peak which is of the same order of magnitude. In
experimental transport data the PT resonance may be mistaken for a weak SET
resonance judging only by the voltage dependence of its position. Our results
provide essential clues to avoid such erroneous interpretation of transport
spectroscopy data.Comment: 5 pages, 2 figures, published versio
Strongly enhanced shot noise in chains of quantum dots
We study charge transport through a chain of quantum dots. The dots are fully
coherent among each other and weakly coupled to metallic electrodes via the
dots at the interface, thus modelling a molecular wire. If the non-local
Coulomb interactions dominate over the inter-dot hopping we find strongly
enhanced shot noise above the sequential tunneling threshold. The current is
not enhanced in the region of enhanced noise, thus rendering the noise
super-Poissonian. In contrast to earlier work this is achieved even in a fully
symmetric system. The origin of this novel behavior lies in a competition of
"slow" and "fast" transport channels that are formed due to the differing
non-local wave functions and total spin of the states participating in
transport. This strong enhancement may allow direct experimental detection of
shot noise in a chain of lateral quantum dots.Comment: 4 pages, 2 figures, submitted to PR
Phase Diagram of the Hubbard Model: Beyond the Dynamical Mean Field
The Dynamical Cluster Approximation (DCA) is used to study non-local
corrections to the dynamical mean field phase diagram of the two-dimensional
Hubbard model. Regions of antiferromagnetic, d-wave superconducting,
pseudo-gapped non-Fermi liquid, and Fermi liquid behaviors are found, in rough
agreement with the generic phase diagram of the cuprates. The non-local
fluctuations beyond the mean field both suppress the antiferromagnetism and
mediate the superconductivity.Comment: 4 pages, 5 eps figures, submitted to PR
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