6,385 research outputs found
Normal state properties of an interacting large polaron gas
A simple approach to the many-polaron problem for both weak and intermediate
electron-phonon coupling and valid for densities much smaller than those
typical of metals is presented. Within the model the total energy, the
collective modes and the single-particle properties are studied and compared
with the available theories. It is shown the occurrence of a charge density
wave instability in the intermediate coupling regime.Comment: 26 pages, 12 figures. To appear on European Physical Journal
Modelling clusters of galaxies by f(R)-gravity
We consider the possibility that masses and gravitational potentials of
galaxy cluster, estimated at X-ray wavelengths, could be explained without
assuming huge amounts of dark matter, but in the context of -gravity.
Specifically, we take into account the weak field limit of such theories and
show that the corrected gravitational potential allows to estimate the total
mass of a sample of 12 clusters of galaxies. Results show that such a
gravitational potential provides a fair fit to the mass of visible matter (i.e.
gas + stars) estimated by X-ray observations, without the need of additional
dark matter while the size of the clusters, as already observed at different
scale for galaxies, strictly depends on the interaction lengths of the
corrections to the Newtonian potential.Comment: 18 pages, 34 figure
Optical properties of an interacting large polaron gas
The normal state conductivity of a system of interacting large polarons is
calculated within the Random Phase approximation and some numerical results are
presented. The behaviour of the optical absorption as a function of the charge
carrier density and of the temperature is analyzed for different values of the
electron-phonon coupling constant. It is shown that the conductivity exihibits
features similar to thos observed in the infrared spectra of the cuprates.Comment: 13 pages, 1 table, 3 figures (to be published ob Eur. Jour. Phys. B
On the interface polaron formation in organic field-effect transistors
A model describing the low density carrier state in an organic single crystal
FET with high- gate dielectrics is studied. The interplay between
charge carrier coupling with inter-molecular vibrations in the bulk of the
organic material and the long-range interaction induced at the interface with a
polar dielectric is investigated. This interplay is responsible for the
stabilization of a polaronic state with an internal structure extending on few
lattice sites, at much lower coupling strengths than expected from the polar
interaction alone. This effect could give rise to polaron self-trapping in
high- organic FET's without invoking unphysically large values of the
carrier interface interaction.Comment: 9 pages, 9 figure
Ballistic transport in one-dimensional loops with Rashba and Dresselhaus spin-orbit coupling
We discuss the combined effect of Rashba and Dresselhaus spin-orbit
interactions in polygonal loops formed by quantum wires, when the electron are
injected in a node and collected at the opposite one. The conditions that allow
perfect localization are found. Furthermore, we investigate the suppression of
the Al'tshuler--Aronov--Spivak oscillations that appear, in presence of a
magnetic flux, when the electrons are injected and collected at the same node.
Finally, we point out that a recent realization of a ballistic spin
interferometer can be used to obtain a reliable estimate of the magnitude ratio
of the two spin-orbit interactions.\bigskipComment: 6 figure
Elucidation of the disulfide folding pathway of hirudin by a topology-based approach
A theoretical model for the folding of proteins containing disulfide bonds is
introduced. The model exploits the knowledge of the native state to favour the
progressive establishment of native interactions. At variance with traditional
approaches based on native topology, not all native bonds are treated in the
same way; in particular, a suitable energy term is introduced to account for
the special strength of disulfide bonds (irrespective of whether they are
native or not) as well as their ability to undergo intra-molecular reshuffling.
The model thus possesses the minimal ingredients necessary to investigated the
much debated issue of whether the re-folding process occurs through partially
structured intermediates with native or non-native disulfide bonds. This
strategy is applied to a context of particular interest, the re-folding process
of Hirudin, a thrombin-specific protease inhibitor, for which conflicting
folding pathways have been proposed. We show that the only two parameters in
the model (temperature and disulfide strength) can be tuned to reproduce well a
set of experimental transitions between species with different number of formed
disulfide. This model is then used to provide a characterisation of the folding
process and a detailed description of the species involved in the rate-limiting
step of Hirudin refolding.Comment: 14 pages, 9 figure
Spectral properties and infrared absorption in manganites
Within a recently proposed variational approach it has been shown that, in
perovskites with , near the metal-insulator
transition, the combined effect of the magnetic and electron-phonon
interactions pushes the system toward a regime of two coexisting phases: a low
electron density one made by itinerant large polarons forming ferromagnetic
domains and a high electron density one made by localized small polarons giving
rise to paramagnetic or antiferromagnetic domains depending on temperature.
Employing the above-mentioned variational scheme, in this paper spectral and
optical properties of manganites are derived for at different
temperatures. It is found that the phase separation regime induces a robust
pseudogap in the excitation spectrum of the system. Then the conductivity
spectra are characterized by a transfer of spectral weight from high to low
energies, as the temperature decreases. In the metallic ferromagnetic
phase, at low two types of infrared absorption come out: a Drude term and a
broad absorption band due respectively to the coherent and incoherent motion of
large polarons. The obtained results turn out in good agreement with
experiments.Comment: 9 figure
Non-local composite spin-lattice polarons in high temperature superconductors
The non-local nature of the polaron formation in t-t'-t"-J model is studied
in large lattices up to 64 sites by developing a new numerical method. We show
that the effect of longer-range hoppings t' and t" is a large anisotropy of the
electron-phonon interaction (EPI) leading to a completely different influence
of EPI on the nodal and antinodal points in agreement with the experiments.
Furthermore, nonlocal EPI preserves polaron's quantum motion, which destroys
the antiferromagnetic order effectively, even at strong coupling regime,
although the quasi-particle weight in angle-resolved-photoemission spectroscopy
is strongly suppressed.Comment: 5 pages, 4 figure
Ground state features of the Frohlich model
Following the ideas behind the Feynman approach, a variational wave function
is proposed for the Fr\"ohlich model. It is shown that it provides, for any
value of the electron-phonon coupling constant, an estimate of the polaron
ground state energy better than the Feynman method based on path integrals. The
mean number of phonons, the average electronic kinetic and interaction
energies, the ground state spectral weight and the electron-lattice correlation
function are calculated and successfully compared with the best available
results.Comment: 6 figure
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