Mass Renormalization in the Su-Schrieffer-Heeger Model

This study of the one dimensional Su-Schrieffer-Heeger model in a weak coupling perturbative regime points out the effective mass behavior as a function of the adiabatic parameter $\omega_{\pi}/J$, $\omega_{\pi}$ is the zone boundary phonon energy and $J$ is the electron band hopping integral. Computation of low order diagrams shows that two phonons scattering processes become appreciable in the intermediate regime in which zone boundary phonons energetically compete with band electrons. Consistently, in the intermediate (and also moderately antiadiabatic) range the relevant mass renormalization signals the onset of a polaronic crossover whereas the electrons are essentially undressed in the fully adiabatic and antiadiabatic systems. The effective mass is roughly twice as much the bare band value in the intermediate regime while an abrupt increase (mainly related to the peculiar 1D dispersion relations) is obtained at $\omega_{\pi}\sim \sqrt{2}J$.Comment: To be published in Phys.Rev.B - 3 figure

Thermodynamic properties of Holstein polarons and the effects of disorder

The ground state and finite temperature properties of polarons are studied considering a two-site and a four-site Holstein model by exact diagonalization of the Hamiltonian. The kinetic energy, Drude weight, correlation functions involving charge and lattice deformations, and the specific heat have been evaluated as a function of electron-phonon (e-ph) coupling strength and temperature. The effects of site diagonal disorder on the above properties have been investigated. The disorder is found to suppress the kinetic energy and the Drude weight, reduces the spatial extension of the polaron, and makes the large-to-small polaron crossover smoother. Increasing temperature also plays similar role. For strong coupling the kinetic energy arises mainly from the incoherent hopping processes owing to the motion of electrons within the polaron and is almost independent of the disorder strength. From the coherent and incoherent contributions to the kinetic energy, the temperature above which the incoherent part dominates is determined as a function of e-ph coupling strength.Comment: 17 pages. 17 figure

Path Integral of the Two Dimensional Su-Schrieffer-Heeger Model

The equilibrium thermodynamics of the two dimensional Su-Schrieffer-Heeger Model is derived by means of a path integral method which accounts for the variable range of the electronic hopping processes. While the lattice degrees of freedom are classical functions of time and are integrated out exactly, the electron particle paths are treated quantum mechanically. The free energy of the system and its temperature derivatives are computed by summing at any $T$ over the ensemble of relevant particle paths which mainly contribute to the total partition function. In the low $T$ regime, the {\it heat capacity over T} ratio shows un upturn peculiar of a glassy like behavior. This feature is more sizeable in the square lattice than in the linear chain as the overall hopping potential contribution to the total action is larger in higher dimensionality.Comment: Phys.Rev.B vol.71 (2005

Characteristic features of anharmonic effects in the lattice dynamics of fcc metals

The dispersion in the entire Brillouin zone and the temperature dependence (right up to the melting temperature) of the anharmonic frequency shift and phonon damping in a number of fcc metals is investigated on the basis of microscopic calculations. It is found that the anharmonic effects depend sharply on the wave vector in the directions $\Gamma$-X, X-W, and $\Gamma$-L and, in contrast to bcc metals, the magnitude of the effects is not due to the softness of the initial phonon spectrum. It is shown that the relative frequency shifts and the phonon damping near melting do not exceed 10-20%. The relative role of various anharmonic processes is examined, and the relation between the results obtained and existing experimental data is discussed.Comment: 4 pages, 5 figures, LaTe

IL MODELLO DI REGRESSIONE QUANTILE NELL’ANALISI DELLE DETERMINANTI DELLA QUALITA’ DELLA VITA IN UNA POPOLAZIONE ANZIANA

Obiettivi. L\u2019obiettivo di questo studio \ue8 spiegare l\u2019associazione tra alcune importanti covariate e la qualit\ue0 della vita percepita (HRQoL) in soggetti anziani. Metodi. I dati sono stati raccolti nell\u2019ambito di uno studio longitudinale che coinvolge 5256 soggetti ultrasesantacinquenni. La scala-termometro del questionario EQ-5D, la EQ-VAS, \ue8 stata utilizzata per ottenere una misura sintetica di qualit\ue0 della vita. L\u2019EQ-VAS Score \ue8 stato modellato utilizzando la regressione quantile. Questo approccio metodologico \ue8 stato preferito alla regressione lineare a causa della particolare distribuzione del punteggio EQ-VAS. Le principali covariate inserite nel modello sono: quantit\ue0 settimanale di attivit\ue0 fisica, capaocit\ue0 di svolgimento delle attivit\ue0 quotidiane ( questionari ADL e IADL ), presenza di patologie cardiovascolari, diabete, ipertensione e colesterolo, dolori articolare e informazioni socio-demografiche. Principali Risultati. 1) Anche valori bassi di attivit\ue0 fisica sono associati significativamente ed in modo positivo alla qualit\ue0 della vita percepita. 2) Problemi nella attivit\ue0 quotidiane, almeno un evento cardiovascolare e dolori articolari contribuiscono fortemente a ridurre il punteggio EQ-VAS

Polarons and slow quantum phonons

We describe the formation and properties of Holstein polarons in the entire parameter regime. Our presentation focuses on the polaron mass and radius, which we obtain with an improved numerical technique. It is based on the combination of variational exact diagonalization with an improved construction of phonon states, providing results even for the strong coupling adiabatic regime. In particular we can describe the formation of large and heavy adiabatic polarons. A comparison of the polaron mass for the one and three dimensional situation explains how the different properties in the static oscillator limit determine the behavior in the adiabatic regime. The transport properties of large and small polarons are characterized by the f-sum rule and the optical conductivity. Our calculations are approximation-free and have negligible numerical error. This allows us to give a conclusive and impartial description of polaron formation. We finally discuss the implications of our results for situations beyond the Holstein model.Comment: Final version, 10 pages, 10 figure

Particle Path Correlations in a Phonon Bath

The path integral formalism is applied to derive the full partition function of a generalized Su-Schrieffer-Heeger Hamiltonian describing a particle motion in a bath of oscillators. The electronic correlations are computed versus temperature for some choices of oscillators energies. We study the perturbing effect of a time averaged particle path on the phonon subsystem deriving the relevant temperature dependent cumulant corrections to the harmonic partition function and free energy. The method has been applied to compute the total heat capacity up to room temeperature: a low temperature upturn in the heat capacity over temperature ratio points to a glassy like behavior ascribable to a time dependent electronic hopping with variable range in the linear chain.Comment: To be published in J.Phys.:Condensed Matte