20 research outputs found
A perturbation theory for the Anderson model of magnetic impurities in simple metals
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Temperature dependence of the charge carrier mobility in gated quasi-one-dimensional systems
The many-body Monte Carlo method is used to evaluate the frequency dependent
conductivity and the average mobility of a system of hopping charges,
electronic or ionic on a one-dimensional chain or channel of finite length. Two
cases are considered: the chain is connected to electrodes and in the other
case the chain is confined giving zero dc conduction. The concentration of
charge is varied using a gate electrode. At low temperatures and with the
presence of an injection barrier, the mobility is an oscillatory function of
density. This is due to the phenomenon of charge density pinning. Mobility
changes occur due to the co-operative pinning and unpinning of the
distribution. At high temperatures, we find that the electron-electron
interaction reduces the mobility monotonically with density, but perhaps not as
much as one might intuitively expect because the path summation favour the
in-phase contributions to the mobility, i.e. the sequential paths in which the
carriers have to wait for the one in front to exit and so on. The carrier
interactions produce a frequency dependent mobility which is of the same order
as the change in the dc mobility with density, i.e. it is a comparably weak
effect. However, when combined with an injection barrier or intrinsic disorder,
the interactions reduce the free volume and amplify disorder by making it
non-local and this can explain the too early onset of frequency dependence in
the conductivity of some high mobility quasi-one-dimensional organic materials.Comment: 9 pages, 8 figures, to be published in Physical Review
Charge localization in a layer induced by electron-phonon interaction: application to transient polaron formation
We describe electron transfer and localization in a finite two-dimensional transporting layer (15 × 15) using a tight binding Hamiltonian where each site is coupled to phonons. For a narrow electronic band, a polaron is formed with a population that peaks in the middle of the layer and exhibits a concomitant energy lowering. A “local defect” can be simulated by lowering or raising the corresponding site energy. As an example, if we put the defect in one corner, the consequence is that the electron population builds up a polaron which is repelled from this region. The model has been applied to describe the experimentally observed real time polaron formation process in organic layers and in particular in the surface bands of ice-covered metal. We simulate the polaron formation, population distribution and energy relaxation in time. We also investigate the effect of local fluctuations on polaron formation. The formalism can be generalized to excitonic trapping, and has many potential applications
Polaron assisted charge transfer in model biological systems
We use a tight binding Hamiltonian to simulate the electron transfer from an initial
charge-separating exciton to a final target state through a two-arm transfer model. The
structure is copied from the model frequently used to describe electron harvesting in
photosynthesis (photosystems I). We use this network to provide proof of principle for
dynamics, in quantum system/bath networks, especially those involving interference
pathways, and use these results to make predictions on artificially realizable systems.
Each site is coupled to the phonon bath via several electron-phonon couplings. The assumed
large energy gaps and weak tunneling integrals linking the last 3 sites give rise to“Stark
Wannier like” quantum localization; electron transfer to the target cluster becomes
impossible without bath coupling. As a result of the electron-phonon coupling, local
electronic energies relax when the site is occupied, and transient polaronic states are
formed as photo-generated electrons traverse the system. For a symmetric constructively
interfering two pathway network, the population is shared equally between two sets of
equivalent sites and therefore the polaron energy shift is smaller. The smaller energy
shift however makes the tunnel transfer to the last site slower or blocks it altogether.
Slight disorder (or thermal noise) can break the symmetry, permitting essentially a “one
path”, and correspondingly more efficient transfer
Magnetotransport in the Insulating Regime of Mn-Doped Gaas
We consider transport in the insulating regime of GaMnAs. We calculate the
resistance, magnetoresitance and Hall effect, assuming that the Fermi energy is
in the region of localized states above the valence band mobility edge. Both
hopping and activated band transport contributions are included. The anomalous
Hall current from band states is very different from the hopping Hall current
and has extrinsic (skew) and intrinsic (Luttinger) contributions. Comparison
with experiment allows us to assess the degree to which band and hopping
contribution determine each of the three transport coefficients in a particular
temperature range. There are strong indications that the insulating state
transport in GaMnAs is controlled primarily by extended state, band edge,
transport rather than by variable range hopping, as reported in the literature.Comment: submitted to Phys. Rev. B, we changed the title, we corrected typos
and we added few explanation