The optical absorption of the Fröhlich polaron model is obtained by an approximation-free Diagrammatic Monte Carlo method and compared with two new approximate approaches that treat lattice relaxation effects in different ways. We show that: i) a strong coupling expansion, based on the the Franck-Condon principle, well describes the optical conductivity for large coupling strengths (α > 10); ii) a Memory Function Formalism with phonon broadened levels reproduces the optical response for weak coupling strengths (α < 6) taking the dynamic lattice relaxation into account. In the coupling regime 6 < α < 10 the optical conductivity is a rapidly changing superposition of both Franck-Condon and dynamic contributions.

We propose a very accurate computational scheme for the dynamics of a classical oscillator coupled to a molecular junction driven by a finite bias, including the finite-mass effect. We focus on two minimal models for the molecular junction: the Anderson-Holstein and two-site Su-Schrieffer-Heeger (SSH) models. As concerns the oscillator dynamics, we are able to recover a Langevin equation confirming what has been found by other authors with different approaches and indicating that quantum effects come from the electronic subsystem only. Solving numerically the stochastic equation, we study the position and velocity distribution probabilities of the oscillator and the electronic transport properties at arbitrary values of electron-oscillator interaction and gate and bias voltages. The range of validity of the adiabatic approximation is established in a systematic way by analyzing the behavior of the kinetic energy of the oscillator. Due to the dynamical fluctuations, at intermediate bias voltages, the velocity distributions deviate from a Gaussian shape and the average kinetic energy shows a nonmonotonic behavior. In this same regime of parameters, the dynamical effects favor conduction far from electronic resonances where small currents are observed in the infinite-mass approximation. These effects are enhanced in the two-site SSH model due to the presence of the intermolecular hopping t. For sufficiently large hopping with respect to tunneling on the molecule, small interaction strengths, and at intermediate bias (non-Gaussian regime), we point out a correspondence between the minima of the kinetic energy and the maxima of the dynamical conductance

Dynamical mean field theory combined with finite-temperature exact diagonalization is shown to be a suitable method to study local Coulomb correlations in realistic multi-band materials. By making use of the sparseness of the impurity Hamiltonian, exact eigenstates can be evaluated for significantly larger clusters than in schemes based on full diagonalization. Since finite-size effects are greatly reduced this approach allows the study of three-band systems down to very low temperatures, for strong local Coulomb interactions and full Hund exchange. It is also shown that exact diagonalization yields smooth subband quasi-particle spectra and self-energies at real frequencies. As a first application the correlation induced charge transfer between t2g bands in Na0.3CoO2 is investigated. For both Hund and Ising exchange the small e ′ g Fermi surface hole pockets are found to be slightly enlarged compared to the non-interacting limit, in agreement with previous Quantum Monte Carlo dynamical mean field calculations for Ising exchange, but in conflict with photoemission data.

Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in the colossal magnetoresistance regime. The calculations are based on a generalization of a variational approach previously proposed for the study of manganite bulk. It is found that a reduction in the thickness of the film causes a decrease of critical temperature and magnetization, and an increase of resistivity at low temperatures. The strain is introduced through the modifications of in-plane and out-of-plane electron hopping amplitudes due to substrate-induced distortions of the film unit cell. The strain effects on the transition temperature and transport properties are in good agreement with experimental data only if the dependence of the hopping matrix elements on the M n − O − M n bond angle is properly taken into account. Finally variations of the electron-phonon coupling linked to the presence of strain turn out important in influencing the balance of coexisting phases in the film.

We study a general model describing a self-detecting single electron transistor realized by a suspended carbon nanotube actuated by a nearby antenna. The main features of the device, recently observed in a number of experiments, are accurately reproduced. When the device is in a low current-carrying state, a peak in the current signals a mechanical resonance. On the contrary, a dip in the current is found in high current-carrying states. In the nonlinear vibration regime of the resonator, we are able to reproduce quantitatively the characteristic asymmetric shape of the current-frequency curves. We show that the nonlinear effects coming out at high values of the antenna amplitude are related to the effective nonlinear force induced by the electronic flow. The interplay between electronic and mechanical degrees of freedom is understood in terms of a unifying model, including in an intrinsic way the nonlinear effects driven by the external probe.

Within a recently proposed variational approach it has been shown that, in La12xAxMnO3 perovskites with\ud 0,x,0.5, near the metal-insulator transition, the combined effect of the magnetic and electron-phonon interactions\ud pushes the system toward a regime of two coexisting phases: a low electron density one made by\ud itinerant large polarons forming ferromagnetic domains and a high electron density one made by localized\ud small polarons giving rise to paramagnetic or antiferromagnetic domains depending on temperature. Employing\ud the above-mentioned variational scheme, in this paper spectral and optical properties of manganites are\ud derived for x50.3 at different temperatures. It is found that the phase separation regime induces a robust\ud pseudogap in the excitation spectrum of the system. Then the conductivity spectra are characterized by a\ud transfer of spectral weight from high to low energies, as the temperature T decreases. In the metallic ferromagnetic\ud phase, at low T two types of infrared absorption come out: a Drude term and a broad absorption band\ud due respectively to the coherent and incoherent motion of large polarons. The obtained results turn out in good\ud agreement with experiments

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The thermoelectric properties of a molecular junction model, appropriate for large molecules such as fullerenes, are studied within a non-equilibrium adiabatic approach in the linear regime at room temperature. A self-consistent calculation is implemented for electron and phonon thermal conductance showing that both increase with the inclusion of the electron-vibration coupling. Moreover, we show that the deviations from the Wiedemann-Franz law are progressively reduced upon increasing the interaction between electronic and vibrational degrees of freedom. Consequently, the junction thermoelectric efficiency is substantially reduced by the electron-vibration coupling. Even so, for realistic parameters values, the thermoelectric figure of merit can still have peaks of the order of unity. Finally, in the off-resonant electronic regime, our results are compared with those of an approach which is exact for low molecular electron densities. We give evidence that in this case additional quantum effects, not included in the first part of this work, do not affect significantly the junction thermoelectric properties in any temperature regime.