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

Spectral properties and infrared absorption in manganites

Within a recently proposed variational approach it has been shown that, in $La_{1-x}A_xMnO_3$ perovskites with $0<x<0.5$, 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 $x=0.3$ 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 $T$ decreases. In the metallic ferromagnetic phase, at low $T$ 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

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

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

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

On the Momentum Distribution and Condensate Fraction in the Bose Liquid

The model recently proposed by A.A. Shanenko [Phys. Lett. A 227 (1997) 367] is used to derive linear integro-differential equations whose solutions provide reasonable estimates for the momentum distribution and condensate fraction in interacting many-boson system at zero temperature. An advantage of these equations is that they can be employed in the weak coupling regime and beyond. As an example, analytical treatment of the weak coupling case is given.Comment: 12 pages, REVTEX, no figures, submitted to Phys. Lett.

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

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 $\omega_0/t$ is found and a polaron phase diagram is proposed.Comment: 12 pages, 2 figure

Comments on "Translation-invariant bipolarons and the problem of high-temperature superconductivity"

We comment on the recent results of Refs. [1, 2] on the bipolaron problem derived using an approximation of Gross - Tulub. It is proved that, contrary to the claim made in Refs. [1, 2], the bipolaron ground state energy calculated there in the strong-coupling approximation has not been shown to constitute a variational upper bound.Comment: 3 pages, 2 figures, submitted to Solid State Communication

Polaron and bipolaron formation in the Hubbard-Holstein model: role of next-nearest neighbor electron hopping

The influence of next-nearest neighbor electron hopping, $t^{\prime}$, 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 $t^{\prime}$ induces a strong anisotropy in the lattice distortions favoring the formation of nearest neighbor intersite bipolaron. The role of $t^{\prime}$, electron-phonon and electron-electron interactions is briefly discussed in view of the formation of charged striped domains.Comment: 4 figure