235 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
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
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
Role of local fields in the optical properties of silicon nanocrystals using the tight binding approach
The role of local fields in the optical response of silicon nanocrystals is
analyzed using a tight binding approach. Our calculations show that, at
variance with bulk silicon, local field effects dramatically modify the silicon
nanocrystal optical response. An explanation is given in terms of surface
electronic polarization and confirmed by the fair agreement between the tight
binding results and that of a classical dielectric model. From such a
comparison, it emerges that the classical model works not only for large but
also for very small nanocrystals. Moreover, the dependence on size of the
optical response is discussed, in particular treating the limit of large size
nanocrystals.Comment: 4 pages, 4 figure
Tight binding formulation of the dielectric response in semiconductor nanocrystals
We report on a theoretical derivation of the electronic dielectric response
of semiconductor nanocrystals using a tight-binding framework. Extending to the
nanoscale the Hanke and Sham approach [Phys. Rev. B 12, 4501 (1975)] developed
for bulk semiconductors, we show how local field effects can be included in the
study of confined systems. A great advantage of this scheme is that of being
formulated in terms of localized orbitals and thus it requires very few
computational resources and times. Applications to the optical and screening
properties of semiconductor nanocrystals are presented here and discussed.
Results concerning the absorption cross section, the static polarizability and
the screening function of InAs (direct gap) and Si (indirect gap) nanocrystals
compare well to both first principles results and experimental data. We also
show that the present scheme allows us to easily go beyond the continuum
dielectric model, based on the Clausius-Mossotti equation, which is frequently
used to include the nanocrystal surface polarization. Our calculations indicate
that the continuum dielectric model, used in conjunction with a size dependent
dielectric constant, underestimates the nanocrystal polarizability, leading to
exceedingly strong surface polarization fields.Comment: 9 pages, 5 figures; corrected typos, added reference
Resonating bipolarons
Electrons coupled to local lattice deformations end up in selftrapped
localized molecular states involving their binding into bipolarons when the
coupling is stronger than a certain critical value. Below that value they exist
as essentially itinerant electrons. We propose that the abrupt crossover
between the two regimes can be described by resonant pairing similar to the
Feshbach resonance in binary atomic collision processes. Given the
intrinsically local nature of the exchange of pairs of itinerant electrons and
localized bipolarons, we demonstrate the occurrence of such a resonance on a
finite-size cluster made out of metallic atoms surrounding a polaronic ligand
center.Comment: 7 pages, 4 figures, to be published in Europhysics Letter
Polaron features of the one-dimensional Holstein Molecular Crystal Model
The polaron features of the one-dimensional Holstein Molecular Crystal Model
are investigated by improving a variational method introduced recently and
based on a linear superposition of Bloch states that describe large and small
polaron wave functions. The mean number of phonons, the polaron kinetic energy,
the electron-phonon local correlation function, and the ground state spectral
weight are calculated and discussed. A crossover regime between large and small
polaron for any value of the adiabatic parameter is found and a
polaron phase diagram is proposed.Comment: 12 pages, 2 figure
Wigner crystallization in a polarizable medium
We present a variational study of the 2D and 3D Wigner crystal phase of large
polarons. The method generalizes that introduced by S. Fratini,P.\
Qu{\'{e}}merais [Mod. Phys. Lett. B {\bf 12} 1003 (1998)]. We take into account
the Wigner crystal normal modes rather than a single mean frequency in the
minimization procedure of the variational free energy. We calculate the
renormalized modes of the crystal as well as the charge polarization
correlation function and polaron radius. The solid phase boundaries are
determined via a Lindemann criterion, suitably generalized to take into account
the classical-to-quantum cross-over.
In the weak electron-phonon coupling limit, the Wigner crystal parameters are
renormalized by the electron-phonon interaction leading to a stabilization of
the solid phase for low polarizability of the medium. Conversely, at
intermediate and strong coupling, the behavior of the system depends strongly
on the polarizability of the medium.
For weakly polarizable media, a density crossover occurs inside the solid
phase when the renormalized plasma frequency approaches the phonon frequency.
At low density, we have a renormalized polaron Wigner crystal, while at higher
densities the electron-phonon interaction is weakened irrespective of the {\it
bare} electron-phonon coupling.
For strongly polarizable media, the system behaves as a Lorentz lattice of
dipoles. The abrupt softening of the internal polaronic frequency predicted by
Fratini and Quemerais is observed near the actual melting point only at very
strong coupling, leading to a possible liquid polaronic phase for a wider range
of parameters.Comment: 24 pages, 13 figures v1.
Screening in semiconductor nanocrystals: \textit{Ab initio} results and Thomas-Fermi theory
A first-principles calculation of the impurity screening in Si and Ge
nanocrystals is presented. We show that isocoric screening gives results in
agreement with both the linear response and the point-charge approximations.
Based on the present ab initio results, and by comparison with previous
calculations, we propose a physical real-space interpretation of the several
contributions to the screening. Combining the Thomas-Fermi theory and simple
electrostatics, we show that it is possible to construct a model screening
function that has the merit of being of simple physical interpretation. The
main point upon which the model is based is that, up to distances of the order
of a bond length from the perturbation, the charge response does not depend on
the nanocrystal size. We show in a very clear way that the link between the
screening at the nanoscale and in the bulk is given by the surface
polarization. A detailed discussion is devoted to the importance of local field
effects in the screening. Our first-principles calculations and the
Thomas-Fermi theory clearly show that in Si and Ge nanocrystals, local field
effects are dominated by surface polarization, which causes a reduction of the
screening in going from the bulk down to the nanoscale. Finally, the model
screening function is compared with recent state-of-the-art ab initio
calculations and tested with impurity activation energies
Polaron and bipolaron formation in the Hubbard-Holstein model: role of next-nearest neighbor electron hopping
The influence of next-nearest neighbor electron hopping, , on the
polaron and bipolaron formation in a square Hubbard-Holstein model is
investigated within a variational approach. The results for electron-phonon and
electron-electron correlation functions show that a negative value of
induces a strong anisotropy in the lattice distortions favoring
the formation of nearest neighbor intersite bipolaron. The role of
, electron-phonon and electron-electron interactions is briefly
discussed in view of the formation of charged striped domains.Comment: 4 figure
- …