Mass enhancement in narrow band systems

A perturbative study of the Holstein Molecular Crystal Model which accounts for lattice structure and dimensionality effects is presented. Antiadiabatic conditions peculiar of narrow band materials and an intermediate to strong electron-phonon coupling are assumed. The polaron effective mass depends crucially in all dimensions on the intermolecular coupling strengths which also affect the size of the lattice deformation associated with the small polaron formation.Comment: Istituto Nazionale di Fisica della Materia - Dipartimento di Matematica e Fisica, Istituto Nazionale di Fisica della Materia Universita' di Camerino, 62032 Camerino, Ital

Polaron self-trapping in a honeycomb net

Small polaron behavior in a two dimensional honeycomb net is studied by applying the strong coupling perturbative method to the Holstein molecular crystal model. We find that small optical polarons can be mobile also if the electrons are strongly coupled to the lattice. Before the polarons localize and become very heavy, there is infact a window of {\it e-ph} couplings in which the polarons are small and have masses of order $\simeq 5 - 50$ times the bare band mass according to the value of the adiabaticity parameter. The 2D honeycomb net favors the mobility of small optical polarons in comparison with the square lattice.Comment: 6 pages, 3 figures, to appear in J.Phys.:Condensed Matter {PACS: 63.10.+a, 63.20.Dj, 71.38.+i

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

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

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

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

Lattice dynamics effects on small polaron properties

This study details the conditions under which strong-coupling perturbation theory can be applied to the molecular crystal model, a fundamental theoretical tool for analysis of the polaron properties. I show that lattice dimensionality and intermolecular forces play a key role in imposing constraints on the applicability of the perturbative approach. The polaron effective mass has been computed in different regimes ranging from the fully antiadiabatic to the fully adiabatic. The polaron masses become essentially dimension independent for sufficiently strong intermolecular coupling strengths and converge to much lower values than those tradition-ally obtained in small-polaron theory. I find evidence for a self-trapping transition in a moderately adiabatic regime at an electron-phonon coupling value of .3. Our results point to a substantial independence of the self-trapping event on dimensionality.Comment: 8 pages, 5 figure

Purification and characterization of circulating Onchocerca volvulus antigens from epileptic and non-epileptic onchocerciasis patient sera

Studies conducted during the past 25 years to investigate the possible relationship between onchocerciasis and epilepsy have led to contradictory results. In the present study aimed at contributing to the investigation of a possible relationship between onchocerciasis and epilepsy, we proceeded to purify and characterize circulating O. volvulus antigens from sera of onchocerciasis patients with and without epilepsy. Out of 539 onchocerciasis patients included in the study, sera from 78 epileptics and 20 non epileptics with high antigen titres were separately pooled and subjected to affinity purification using immunosorbent columns prepared using human and rabbit anti-O. volvulus IgG antibodies. Eluates of purified circulating O. volvulus antigens were concentrated, and then the protein contents were determined using the Bradford method. The antigenicity of the purified antigens was evaluated in a direct ELISA using onchocerciasis patient sera. Finally, the molecular composition of the purified proteins was determined by SDS-PAGE. The purified antigens were highly antigenic and there was no significant difference in the reaction profiles of the two groups or categories of patients. SDS-PAGE analysis showed that the purified antigens ranged from 31.63 to 102.40 KDa and there was no difference in the molecular composition of antigens purified from sera of the two classes of patients. Based on this antigen profiling between epileptic and non-epileptic onchocerciasis patients, we cannot conclude with certainty whether onchocerciasis is really a cause of epilepsy in areas where it is hyperendemic as predicted by some epidemiological studies.Keywords: Antigen-detection ELISA, Immunoadsorbent columns, Affinity chromatography, Antigenicity, SDS-PAG

Properties of large scale ultra-high temperature ceramic matrix composites made by filament winding and spark plasma sintering

In this paper, for the first time, we report the manufacturing and characterization of large UHTCMCs discs, made of a ZrB2/SiC matrix reinforced with PyC-coated PAN-based carbon fibres. This work was the result of a long term collaboration between different institutions and shows how it is possible to scale-up the production process of UHTCMCs for the fabrication of large components. 150 mm large discs were produced by filament winding and consolidated by spark plasma sintering and specimens were machined to test a large set of material properties at room and elevated temperature (up to 1800 °C). The extensive characterization revealed a new material with mechanical behaviour similar to CMCs, but with intrinsic higher thermal stability. Furthermore, the scale-up demonstrated in this work increases the appeal of UHTCMCs in sectors such as aerospace, where severe operating conditions limit the application of conventional materials

Stacking Interactions in Denaturation of DNA Fragments

A mesoscopic model for heterogeneous DNA denaturation is developed in the framework of the path integral formalism. The base pair stretchings are treated as one-dimensional, time dependent paths contributing to the partition function. The size of the paths ensemble, which measures the degree of cooperativity of the system, is computed versus temperature consistently with the model potential physical requirements. It is shown that the ensemble size strongly varies with the molecule backbone stiffness providing a quantitative relation between stacking and features of the melting transition. The latter is an overall smooth crossover which begins from the \emph{adenine-thymine} rich portions of the fragment. The harmonic stacking coupling shifts, along the $T$-axis, the occurrence of the multistep denaturation but it does not change the character of the crossover. The methods to compute the fractions of open base pairs versus temperature are discussed: by averaging the base pair displacements over the path ensemble we find that such fractions signal the multisteps of the transition in good agreement with the indications provided by the specific heat plots.Comment: European Physical Journal E (2011) in pres