We use classical molecular dynamics simulations to study the dynamics of sodium atoms in amorphous Na$_2$O-4SiO$_2$. We find that the sodium trajectories form a well connected network of pockets and channels. Inside these channels the motion of the atoms is not cooperative but rather given by independent thermally activated hops of individual atoms between the pockets. By determining the probability that an atom returns to a given starting site, we show that such events are not important for the dynamics of this system.Comment: 10 pages of Latex, 5 figures, one figure added, text expande

We determine, through numerical energy optimization, the fundamental-state energy E associated with an extended model for ferrofluids (colloidal suspension of magnetic particles assumed to form rings of N dipoles). The extension consists in generalizing, into 1/${\mathit{r}}^{\mathrm{\alpha }}$ (0≤α), a standard 1/${\mathit{r}}^6$ contribution to the attrative potential. We show that E∝N, N lnN, or ${\mathit{N}}^{2\mathrm{-}\mathrm{\alpha }}$, respectively, for α>1, α=1, or 0≤α<1. In addition, a connection with generalized statistical mechanics is conjecturally proposed and the analogy with the d=2 XY ferromagnet is exhibited.

We use ab initio and parametrized calculations to investigate the morphology and structural stability at the growing edge of multiwall carbon nanotubes. We find that these open-ended structures are stabilized against dome closure by strong covalent bonds connecting the exposed edges of adjacent walls. Growth at the open edge involves rearrangement of these bonds, which are mediated by carbon atoms bridging the gap and change the tip morphology significantly. Presence of a strong “lip-lip” interaction can explain formation of carbon nanotubes under annealing conditions.

We present a study of the thermodynamic and physical properties of ${\text{Ta}}_5{\text{Si}}_3$ compounds by means of density-functional theory based calculations. Among the three different structures $\left(\text{D}{8}_{\text{m}},\text{D}{8}_{\text{l}},\text{D}{8}_8\right)$, the $\text{D}{8}_{\text{l}}$ structure (${\text{Cr}}_5{\text{B}}_3$ prototype) is the low-temperature phase with a high formation enthalpy of $-449.20\text{kJ}/\text{mol}$, the $\text{D}{8}_{\text{m}}$ structure (${\text{W}}_5{\text{Si}}_3$ prototype) is the high-temperature phase with a formation enthalpy of $-419.36\text{kJ}/\text{mol}$, and the $\text{D}{8}_8$ structure (${\text{Mn}}_5{\text{Si}}_3$ prototype) is a metastable phase. The optimized lattice constants of the different ${\text{Ta}}_5{\text{Si}}_3$ compounds are also in good agreement with the experimental data. The electronic density of states and the bonding charge density have also been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. The elastic constants of the $\text{D}{8}_{\text{m}}$ and $\text{D}{8}_{\text{l}}$ structures have been calculated together with the different moduli. Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion, and the Grüneisen parameter have also been obtained in the present work. The transformation temperature (2303.7 K) between the $\text{D}{8}_{\text{m}}$ and the $\text{D}{8}_{\text{l}}$ structures has been predicted by means of the Gibbs energy, and this predicted temperature (2303.7 K) is close to the experimental value (2433.5 K).