1,131 research outputs found
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 , is the
zone boundary phonon energy and 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 .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
over the ensemble of relevant particle paths which mainly contribute to the
total partition function. In the low 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
Path integrals approach to resisitivity anomalies in anharmonic systems
Different classes of physical systems with sizeable electron-phonon coupling
and lattice distortions present anomalous resistivity behaviors versus
temperature. We study a molecular lattice Hamiltonian in which polaronic charge
carriers interact with non linear potentials provided by local atomic
fluctuations between two equilibrium sites. We study a molecular lattice
Hamiltonian in which polaronic charge carriers interact with non linear
potentials provided by local atomic fluctuations between two equilibrium sites.
A path integral model is developed to select the class of atomic oscillations
which mainly contributes to the partition function and the electrical
resistivity is computed in a number of representative cases. We argue that the
common origin of the observed resistivity anomalies lies in the time retarded
nature of the polaronic interactions in the local structural instabilities.Comment: 4 figures, to appear in Phys.Rev.B, May 1st (2001
Clinical Applications of Scanning Electron Microscopy in Gastrointestinal Diseases
We considered the role of scanning electron microscopy (SEM) in clinical investigation of different gastrointestinal diseases. The following clinical applications of SEM may be suggested on the basis of our original data and those reported in literature:
in peptic ulcer: assessment of the completeness of healing, by observing the mucosal surface architecture of the scars; identification of mucosal changes, namely enterocytic surface membrane alterations, predictive of recurrence;
in coeliac disease: early assessment of the response to gluten-free diet and follow-up of the patients by staging the process of mucosal repair in cerebriform, intermediate and villous patterns;
in ulcerative and Crohn\u27s colitis: enhancement of the diagnostic sensitivity of perendoscopic biopsy, by detecting differences in surface structure of mucosa surrounding ulcers in both diseases. This is subverted in ulcerative colitis and preserved in Crohn\u27s colitis.
Finally the complementary role of SEM in relation to endoscopy and light microscopy is emphasized
Anharmonic stacking in supercoiled DNA
Multistep denaturation in a short circular DNA molecule is analyzed by a
mesoscopic Hamiltonian model which accounts for the helicoidal geometry.
Computation of melting profiles by the path integral method suggests that
stacking anharmonicity stabilizes the double helix against thermal disruption
of the hydrogen bonds. Twisting is essential in the model to capture the
importance of nonlinear effects on the thermodynamical properties. In a ladder
model with zero twist, anharmonic stacking scarcely affects the thermodynamics.
Moderately untwisted helices, with respect to the equilibrium conformation,
show an energetic advantage against the overtwisted ones. Accordingly
moderately untwisted helices better sustain local fluctuational openings and
make more unlikely the thermally driven complete strand separation.Comment: In pres
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 -X, X-W, and -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
Standardization of Cu2O nanocubes synthesis: Role of precipitation process parameters on physico-chemical and photo-electrocatalytic properties
A facile, reproducible, and scalable wet precipitation method was optimized to synthetise Cu2O nanocubes with tuneable morphology and photocatalytic properties. The synthesis process was standardized by controlling the flow rate of addition of the reducing agent. This allowed to control the Cu2O crystallites size, which decreased from 60 nm to 30 nm by increasing the L-ascorbic acid flow rate, while maintaining a high yield (ranging from 87% to 97%) and reproducibility, as confirmed by X-Ray diffraction, scanning electron microscopy, and X-Ray photoelectron spectroscopy analyses. Moreover, the role of the synthesis conditions on the Cu2O nanocubes specific surface area and electrochemical surface area (ECSA) were investigated and correlated to their photo-electrocatalytic activity for the reduction of water and CO2 under ambient conditions, on electrodes made by air brushing. Decreasing of the Cu2O crystallites size enhanced the photo-electrocatalytic activity most probably due to a superior surface area, ECSA and an optimum valence and conduction band positions, which improves the charge transfer properties of the photocatalyst. The here proposed methodology and outcomes are very promising for the scale-up of the precipitation synthesis, not only of Cu2O but also of other nanostructured metal oxides to be exploited as photo-catalysts for environmental and energy applications
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
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