756 research outputs found
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
Path Integral Method for DNA Denaturation
The statistical physics of homogeneous DNA is investigated by the imaginary
time path integral formalism. The base pair stretchings are described by an
ensemble of paths selected through a macroscopic constraint, the fulfillement
of the second law of thermodynamics. The number of paths contributing to the
partition function strongly increases around and above a specific temperature
whereas the fraction of unbound base pairs grows continuosly around and
above . The latter is identified with the denaturation temperature.
Thus, the separation of the two complementary strands appears as a highly
cooperative phenomenon displaying a smooth crossover versus . The
thermodynamical properties have been computed in a large temperature range by
varying the size of the path ensemble at the lower bound of the range. No
significant physical dependence on the system size has been envisaged. The
entropy grows continuosly versus while the specific heat displays a
remarkable peak at . The location of the peak versus varies with the
stiffness of the anharmonic stacking interaction along the strand. The
presented results suggest that denaturation in homogeneous DNA has the features
of a second order phase transition. The method accounts for the cooperative
behavior of a very large number of degrees of freedom while the computation
time is kept within a reasonable limit.Comment: Physical Review E 2009 in pres
Helix untwisting and bubble formation in circular DNA
The base pair fluctuations and helix untwisting are examined for a circular
molecule. A realistic mesoscopic model including twisting degrees of freedom
and bending of the molecular axis is proposed. The computational method, based
on path integral techniques, simulates a distribution of topoisomers with
various twist numbers and finds the energetically most favorable molecular
conformation as a function of temperature. The method can predict helical
repeat, openings loci and bubble sizes for specific sequences in a broad
temperature range. Some results are presented for a short DNA circle recently
identified in mammalian cells.Comment: The Journal of Chemical Physics, vol. 138 (2013), in pres
J-factors of short DNA molecules
The propensity of short DNA sequences to convert to the circular form is
studied by a mesoscopic Hamiltonian method which incorporates both the bending
of the molecule axis and the intrinsic twist of the DNA strands. The base pair
fluctuations with respect to the helix diameter are treated as path
trajectories in the imaginary time path integral formalism. The partition
function for the sub-ensemble of closed molecules is computed by imposing chain
ends boundary conditions both on the radial fluctuations and on the angular
degrees of freedom. The cyclization probability, the J-factor, proves to be
highly sensitive to the stacking potential, mostly to its nonlinear parameters.
We find that the J-factor generally decreases by reducing the sequence length (
N ) and, more significantly, below N = 100 base pairs. However, even for very
small molecules, the J-factors remain sizeable in line with recent experimental
indications. Large bending angles between adjacent base pairs and anharmonic
stacking appear as the causes of the helix flexibility at short length scales.Comment: The Journal of Chemical Physics - May 2016 ; 9 page
Parasitological prevalence of bovine trypanosomosis in the Faro and Deo division valley of the Adamaoua plateau, Cameroon
A cross sectional survey to determine the distribution and prevalence of trypanosomosis was conducted in the Faro and Deo division valley, on the Adamaoua Plateau in Cameroon. A total of 334 adult cattle from 5 sedentary herds were examined in 5 villages. Dark field buffy coat method, as well as stained thin blood film examination and packed cell volume (PCV) evaluation were the diagnostic techniques used. The overall prevalence of bovine trypanosomosis in the area was 23%. Among the positive animals, 44 (57.1%), 26 (33.8%), 5 (6.5%) and 2 (2.6%) were due to Trypanosoma congolense, Trypanosoma brucei, Trypanosoma vivax and mixed infection (T. congolense and T. brucei) respectively. The mean PCV of the positive and negative animals ranged between 21.1-27% and 28.2-30.1% respectively. The mean PCV of negative animals (29.1±0.7%) was significantly higher than the mean PCV of positive animals (24.2±2.5%) (P< 0.005). In view of the high risk of trypanosomosis, in the area, an integrated intervention approach to which combines the strategic application of appropriate tsetse fly control methods to reduce host fly contact and chemotherapy and chemoprophylaxis against trypanosomosis is recommended.© 2009 International Formulae Group. All rights reserved.Key words: Adamaoua, Cameroon, Cattle, Prevalence, Trypanosomosis, Tsetse
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 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
Influence of Y2O3 addition on the mechanical and oxidation behaviour of carbon fibre reinforced ZrB2/SiC composites
The influence of Y2O3 addition on the microstructure, thermo-mechanical properties and oxidation resistance of carbon fibre reinforced ZrB2/SiC composites was investigated. Y2O3 reacted with oxide impurities present on the surface of ZrB2 and SiC grains and formed a liquid phase, effectively lowering the sintering temperature and allowing to reach full density at 1900 °C. The presence of a carbon source (fibres) led to additional reactions which resulted in the formation of new secondary phases such as yttrium boro-carbides. Mechanical properties were significantly enhanced compared to the un-doped composite. Further tests at high temperatures resulted in strength increase up to 700 MPa at 1500 °C which was attributed to stress relaxation. Oxidation tests carried out at 1500 °C and 1650 °C in air showed that the presence of the Y-based secondary phases enhanced the growth of ZrO2 grains, but offered limited protection to oxygen due to the lower availability of surficial SiO2 formed from SiC
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The multi-country transmission of sovereign and banking risk: a spatial vector autoregressive approach
This paper develops a spatial vector autoregressive (SpVAR) model to investigate the transmission of sovereign, banking, and corporate default risks among eleven Eurozone countries for the January 2008–December 2013 period. The results show that a significant proportion of default risk variation is explained by foreign shocks. However, the cross-border sovereign–bank nexus is statistically significant, but economically moderate. Among the three sectors, shocks to the banking sector play the most critical role. On average, for the eleven countries, a foreign banking shock can explain 7%, 23%, and 18% of the forecast error variance of changes in sovereign, banking, and corporate credit default swap (CDS) spreads, respectively
Toughening effect of non-periodic fiber distribution on crack propagation energy of UHTC composites
Different configurations of continuous carbon fiber-reinforced ultrahigh temperature ceramics (UHTCs), by combining coatings and matrix, were produced via electrophoretic deposition (EPD) and slurry infiltration. The toughening of non-periodic fiber distribution induced by the EPD process was investigated through work of fracture analysis. The results show that a non-periodic fiber distribution results in toughness increase from 8 MPa√m to 11 MPa√m with respect to a periodic fiber distribution. This toughness improvement does not strongly affect the flexural strength, which is mainly related to the fiber volumetric amount. It is shown that the assembling of carbon fibers into bundles (i.e. by dispersing the fibers with a non-periodic distribution) increases the crack propagation energy dissipated on the crack-wake from 0.5 kJ/m2 to 1 kJ/m2, which can be mainly ascribed to the fiber/bundle pull-out. On the other hand, the energy dissipated on the crack-tip (as fiber/matrix debonding) is fiber distribution-independent and increases from 0.3 kJ/m2 to 0.4 kJ/m2 with increasing the fiber amount from 33 vol% to 40 vol%. Finally, WoF analysis is proposed as test to evaluate pull-out toughening instead of push-in and push-out tests
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