21 research outputs found
The electrostatic potential profile along a biased molecular wire: A model quantum mechanical calculation
We study the electrostatic potential of a molecular wire bridging two
metallic electrodes in the limit of weak contacts. With the use of a
tight-binding model including a fully three-dimensional treatment of the
electrostatics of the molecular junction, the potential is shown to be poorly
screened, dropping mostly along the entire molecule. In addition, we observe
pronounced Friedel oscillations that can be related to the breaking of
electron-hole symmetry. Our results are in semi-quantitative agreement with
recent state-of-the-art ab initio calculations and point to the need of a
three-dimensional treatment to properly capture the behavior of the
electrostatic potential. Based on these results, current-voltage curves are
calculated within the Landauer formalism. It is shown that Coulomb interaction
partially compensates the localization of the charges induced by the electric
field and consequently tends to suppress zones of negative differential
resistance.Comment: 8 pages, 5 figures, RevTeX
Molecular crystal approach for pi-conjugated polymers: from PPP Hamiltonian to Holstein model for polaron states
Starting from the -electron Pariser-Parr-Pople (PPP) Hamiltonian which
includes both strong electron-phonon and electron-electron interactions, we
propose some strongly correlated wave functions of increasing quality for the
ground state of conjugated polymers. These wavefunctions are built by combining
different finite sets of local configurations extended at most over two
nearest-neighbour monomers. With this picture, the doped case with one
additional particle is expressed in terms of quasi-particle. Thus, the polaron
formation problem goes back to the study of a Holstein like model.Comment: 27 pages, 6 eps figs, Revtex; enlarged version. Submitted to Journal
of Physics: Condensed Matte
Functional Model of a Nanoparticle Organic Memory Transistor for Use as a Spiking Synapse
Excitonic Strings in one dimensional organic compounds
Important questions concern the existence of excitonic strings in organic
compounds and their signatures in the photophysics of these systems. A model in
terms of Hard Core Bosons is proposed to study this problem in one dimension.
Mainly the cases with two and three particles are studied for finite and
infinite lattices, where analytical results are accessible. It is shown that if
bi-excitonic states exist, three-excitonic and even, n-excitonic strings, at
least in a certain range of parameters, will exist. Moreover, the behaviour of
the transitions from one exciton to the biexciton is fully clarified. The
results are in agreement with exact finite cluster diagonalizations of several
model Hamiltonians.Comment: 36 pages, 4 eps figs. to appear in Phys. Rev.