Thermal decomposition of a honeycomb-network sheet - A Molecular Dynamics simulation study

The thermal degradation of a graphene-like two-dimensional triangular membrane with bonds undergoing temperature-induced scission is studied by means of Molecular Dynamics simulation using Langevin thermostat. We demonstrate that the probability distribution of breaking bonds is highly peaked at the rim of the membrane sheet at lower temperature whereas at higher temperature bonds break at random anywhere in the hexagonal flake. The mean breakage time $\tau$ is found to decrease with the total number of network nodes $N$ by a power law $\tau \propto N^{-0.5}$ and reveals an Arrhenian dependence on temperature $T$. Scission times are themselves exponentially distributed. The fragmentation kinetics of the average number of clusters can be described by first-order chemical reactions between network nodes $n_i$ of different coordination. The distribution of fragments sizes evolves with time elapsed from a $\delta$-function through a bimodal one into a single-peaked again at late times. Our simulation results are complemented by a set of $1^{st}$-order kinetic differential equations for $n_i$ which can be solved exactly and compared to data derived from the computer experiment, providing deeper insight into the thermolysis mechanism.Comment: 21pages, 9 figures, LaTeX, revised versio

Optical properties, electron-phonon coupling, and Raman scattering of vanadium ladder compounds

The electronic structure of two V-based ladder compounds, the quarter-filled NaV$_2$O$_5$ in the symmetric phase and the iso-structural half-filled CaV$_2$O$_5$ is investigated by ab initio calculations. Based on the bandstructure we determine the dielectric tensor $\epsilon(\omega)$ of these systems in a wide energy range. The frequencies and eigenvectors of the fully symmetric A$_{g}$ phonon modes and the corresponding electron-phonon and spin-phonon coupling parameters are also calculated from first-principles. We determine the Raman scattering intensities of the A$_g$ phonon modes as a function of polarization and frequency of the exciting light. All results, i.e. shape and magnitude of the dielectric function, phonon frequencies and Raman intensities show very good agreement with available experimental data.Comment: 11 pages, 10 figure

Magnetization and specific heat of TbFe3(BO3)4: Experiment and crystal field calculations

We have studied the thermodynamic properties of single-crystalline TbFe3(BO3)4. Magnetization measurements have been carried out as a function of magnetic field (up to 50 T) and temperature up to 350K with the magnetic field both parallel and perpendicular to the trigonal c-axis of the crystal. The specific heat has been measured in the temperature range 2-300K with a magnetic field up to 9 T applied parallel to the c-axis. The data indicate a structural phase transition at 192 K and antiferromagnetic spin ordering at 40 K. A Schottky anomaly is present in the specific heat data around 20 K, arising due to two low-lying energy levels of the Tb3+ ions being split by f-d coupling. Below TN magnetic fields parallel to the c-axis drive a spin-flop phase transition, which is associated with a large magnetization jump. The highly anisotropic character of the magnetic susceptibility is ascribed mainly to the Ising-like behavior of the Tb3+ ions in the trigonal crystal field. We describe our results in the framework of an unified approach which is based on mean-field approximation and crystal-field calculations.Comment: 10 pages, 10 figures, 20 references, accepted by Phys. Rev.

Inversionless light amplification and optical switching controlled by state-dependent alignment of molecules

We propose a method to achieve amplification without population inversion by anisotropic molecules whose orientation by an external electric field is state-dependent. It is based on decoupling of the lower-state molecules from the resonant light while the excited ones remain emitting. The suitable class of molecules is discussed, the equation for the gain factor is derived, and the magnitude of the inversionless amplification is estimated for the typical experimental conditions. Such switching of the sample from absorbing to amplifying via transparent state is shown to be possible both with the aid of dc and ac control electric fields.Comment: AMS-LaTeX v1.2, 4 pages with 4 figure

Analysis of signalling pathways using continuous time Markov chains

We describe a quantitative modelling and analysis approach for signal transduction networks. We illustrate the approach with an example, the RKIP inhibited ERK pathway [CSK+03]. Our models are high level descriptions of continuous time Markov chains: proteins are modelled by synchronous processes and reactions by transitions. Concentrations are modelled by discrete, abstract quantities. The main advantage of our approach is that using a (continuous time) stochastic logic and the PRISM model checker, we can perform quantitative analysis such as what is the probability that if a concentration reaches a certain level, it will remain at that level thereafter? or how does varying a given reaction rate affect that probability? We also perform standard simulations and compare our results with a traditional ordinary differential equation model. An interesting result is that for the example pathway, only a small number of discrete data values is required to render the simulations practically indistinguishable

Folded modes in the infrared spectra of the spin-Peierls phase of CuGeO_3

Polarized far-infrared transmittance spectra of CuGeO_3 single crystals were measured at different temperatures (6K < T < 300K). Two spectral lines, at 284.2 cm-1 in E||c polarization and at 311.7 cm-1 in E||b polarization, appear at the temperature of the spin-Peierls transition and grow in intensity with decreasing temperature. Both of them are, most probably, folded modes of the dimerized lattice. We discuss a possible role of the spin-phonon interaction in the formation of the 311.7 cm-1 feature.Comment: 4 pages, 5 figures, 1 table; Submitted to Phys.Rev.B Second revision. Figures and text were slightly change

МІКРОБНО-ВІРУСНІ АСОЦІАЦІЇ ДЕЯКИХ ВНУТРІШНЬОКЛІТИННИХ ПАТОГЕНІВ (MYCOPLASMA PNEUMONIAE ТА HERPES SIMPLEX VIRUS) У ХВОРИХ НА НЕГОСПІТАЛЬНУ ПНЕВМОНІЮ

We analyzed contamination data on mixed infection (Mycoplasma pneumoniae and Herpes simplex virus) in patients with community-acquired pneumonia. It was found that patients with associated infection pneumonia duration had more severe character with negative consequences.Проаналізовано дані інфікованості на мікст-інфекцію (Mycoplasma рneumoniae та Herpes simplex virus) у хворих на негоспітальну пневмонію (НП). Встановлено, що за асоційованої інфікованості перебіг пневмонії був найтяжчим з негативними наслідками

Force-induced breakdown of flexible polymerized membrane

We consider the fracture of a free-standing two-dimensional (2D) elastic-brittle network to be used as protective coating subject to constant tensile stress applied on its rim. Using a Molecular Dynamics simulation with Langevin thermostat, we investigate the scission and recombination of bonds, and the formation of cracks in the 2D graphene-like hexagonal sheet for different pulling force $f$ and temperature $T$. We find that bond rupture occurs almost always at the sheet periphery and the First Mean Breakage Time of bonds decays with membrane size as $ \propto N^{-\beta}$ where $\beta \approx 0.50\pm 0.03$ and $N$ denotes the number of atoms in the membrane. The probability distribution of bond scission times $t$ is given by a Poisson function $W(t) \propto t^{1/3} \exp (-t / )$. The mean failure time that takes to rip-off the sheet declines with growing size $N$ as a power law $\propto N^{-\phi(f)}$. We also find $\propto \exp(\Delta U_0/k_BT)$ where the nucleation barrier for crack formation $\Delta U_0 \propto f^{-2}$, in agreement with Griffith's theory. $$displays an Arrhenian dependence of$$ on temperature $T$. Our results indicate a rapid increase in crack spreading velocity with growing external tension $f$.Comment: 12 pages, 10 figures, LaTeX, misprints correcte

Charge ordering in quarter-filled ladder systems coupled to the lattice

We investigate charge ordering in the presence of electron-phonon coupling for quarter-filled ladder systems by using Exact Diagonalization. As an example we consider NaV2O5 using model parameters obtained from first-principles band-structure calculations. The relevant Holstein coupling to the lattice considerably reduces the critical value of the nearest-neighbor Coulomb repulsion at which formation of the zig-zag charge-ordered state occurs, which is then accompanied by a static lattice distortion. Energy and length of a kink-like excitation on the background of the distorted lattice are calculated. Spin and charge spectra on ladders with and without static distortion are obtained, and the charge gap and the effective spin-spin exchange parameter J are extracted. J agrees well with experimental results. Analysis of the dynamical Holstein model, restricted to a small number of phonons, shows that low frequency lattice vibrations increase the charge order, accompanied by dynamically produced zig-zag lattice distortions.Comment: 11 pages, 17 figures, revised version as to appear in Phys. Rev.