On the possible reason for superconductivity strengthening in multiwall carbon nanotubes

Basing on the structural peculiarity of two-layer graphene which consists of translational and energetical nonequivalency of carbon atoms from different sublattices, it is shown that the density of long-wave electronic states at the Fermi level is finite (in contrast to the monolayer graphene). It is suggested that the same may be the reason why the critical temperature of superconducting transition in multiwall nanotubes more than ten times higher than in single-wall nanotubes

Effects of spin-elastic interactions in frustrated Heisenberg antiferromagnets

The Heisenberg antiferromagnet on a compressible triangular lattice in the spin- wave approximation is considered. It is shown that the interaction between quantum fluctuations and elastic degrees of freedom stabilizes the low symmetric L-phase with a collinear Neel magnetic ordering. Multi-stability in the dependence of the on-site magnetization on an unaxial pressure is found.Comment: Revtex, 4 pages, 2 eps figure

Adatom interaction effects in surface diffusion

Motivated by recent research of Nikitin et al. [J. Phys. D: Appl. Phys., 2009, 49, 055301], we examine the effects of interatomic interactions on adatom surface diffusion. By using a mean-field approach in the random walk problem, we derive a nonlinear diffusion equation and analyze its solutions. The results of our analysis are in good agreement with direct numerical simulations of the corresponding discrete model. It is shown that by analyzing a time dependence of adatom concentration profiles one can estimate the type and strength of interatomic interactions.Мотивованi недавнiми дослiдженнями Нiкiтiна та iн. [J. Phys. D: Appl. Phys., 2009, 49, 055301], ми вивчаємо вплив мiжатомних взаємодiй на адатомну поверхневу дифузiю. Використовуючи середньо-польовий пiдхiд у проблемi випадкових блукань, ми виводимо нелiнiйне рiвняння дифузiї i аналiзуємо його розв’язки. Результати нашого аналiзу добре узгоджуються з прямими числовими симуляцiями вiдповiдної дискретної моделi. Показано, що, аналiзуючи часову залежнiсть профiлiв адатомної концентрацiї, можна оцiнити тип i силу мiжатомної взаємодiї

Feedback Loops Between Fields and Underlying Space Curvature: an Augmented Lagrangian Approach

We demonstrate a systematic implementation of coupling between a scalar field and the geometry of the space (curve, surface, etc.) which carries the field. This naturally gives rise to a feedback mechanism between the field and the geometry. We develop a systematic model for the feedback in a general form, inspired by a specific implementation in the context of molecular dynamics (the so-called Rahman-Parrinello molecular dynamics, or RP-MD). We use a generalized Lagrangian that allows for the coupling of the space's metric tensor (the first fundamental form) to the scalar field, and add terms motivated by RP-MD. We present two implementations of the scheme: one in which the metric is only time-dependent [which gives rise to ordinary differential equation (ODE) for its temporal evolution], and one with spatio-temporal dependence [wherein the metric's evolution is governed by a partial differential equation (PDE)]. Numerical results are reported for the (1+1)-dimensional model with a nonlinearity of the sine-Gordon type.Comment: 5 pages, 3 figures, Phys. Rev. E in pres

Breathers in a system with helicity and dipole interaction

Recent papers that have studied variants of the Peyrard-Bishop model for DNA, have taken into account the long range interaction due to the dipole moments of the hydrogen bonds between base pairs. In these models the helicity of the double strand is not considered. In this particular paper we have performed an analysis of the influence of the helicity on the properties of static and moving breathers in a Klein--Gordon chain with dipole-dipole interaction. It has been found that the helicity enlarges the range of existence and stability of static breathers, although this effect is small for a typical helical structure of DNA. However the effect of the orientation of the dipole moments is considerably higher with transcendental consequences for the existence of mobile breathers.Comment: 4pages, 5 eps figure

Energy funneling in a bent chain of Morse oscillators with long-range coupling

A bent chain of coupled Morse oscillators with long-range dispersive interaction is considered. Moving localized excitations may be trapped in the bending region. Thus chain geometry acts like an impurity. An energy funneling effect is observed in the case of random initial conditions.Comment: 6 pages, 12 figures. Submitted to Physical Review E, Oct. 13, 200

Stability of trapped Bose-Einstein condensates

In three-dimensional trapped Bose-Einstein condensate (BEC), described by the time-dependent Gross-Pitaevskii-Ginzburg equation, we study the effect of initial conditions on stability using a Gaussian variational approach and exact numerical simulations. We also discuss the validity of the criterion for stability suggested by Vakhitov and Kolokolov. The maximum initial chirp (initial focusing defocusing of cloud) that can lead a stable condensate to collapse even before the number of atoms reaches its critical limit is obtained for several specific cases. When we consider two- and three-body nonlinear terms, with negative cubic and positive quintic terms, we have the conditions for the existence of two phases in the condensate. In this case, the magnitude of the oscillations between the two phases are studied considering sufficient large initial chirps. The occurrence of collapse in a BEC with repulsive two-body interaction is also shown to be possible.Comment: 15 pages, 11 figure

Consistent low-energy reduction of the three-band model for copper oxides with O-O hopping to the effective t-J model

A full three-band model for the CuO$_{2}$ plane with inclusion of all essential interactions - Cu-O and O-O hopping, repulsion at the copper and oxygen and between them - is considered. A general procedure of the low-energy reduction of the primary Hamiltonian to the Hamiltonian of the generalized $t$-$t'$-$J$ model is developed. An important role of the direct O-O hopping is discussed. Parameters of the effective low-energy model (the hopping integral, the band position and the superexchange constant $J$ are calculated. An analysis of the obtained data shows that the experimental value of $J$ fixes the charge transfer energy $\Delta =(\epsilon_{p}-\epsilon_{d})$ in a narrow region of energies.Comment: 32 pp. (LATEX), two figures (PostScript) appende