acta physica slovaca

Abstract: This review concerns with numerical studies of density matrix renormalization group (DMRG) technique applied to various two- and three-dimensional classical spin lattice models. The main goal is to introduce DMRG as a powerful method suitable to treat models at the criticality, i. e., to determine critical points, all critical exponents as well as spatially modulated commensurate and incommensurate phases. In particular, a generalization of the DMRG algorithm is proposed and applied to a complete construction of phase diagrams of two-dimensional antiferromagnetic next-nearest-neighbor Ising (ANNNI) model as well as antiferromagnetic triangular nearest-neighbor Ising (ATNNI) model. Both models exhibit incommensurate modulated structures. The nonexistence of the Lifshitz point in the ATNNI model is conjectured. A proposal how to modify the DMRG method for two-dimensional classical models with periodic boundary conditions is given. It is shown that this approach is much more efficient to reveal critical properties of two-dimensional classical models in connection with a finite-size scaling than that starting from the standard DMRG when the open or fixed boundary conditions were imposed. A criterion for an optimum strip width is found. It enables to obtain the critical points as well as all critical exponents with high accuracy. The two-dimensional Ising and Potts models are studied in detail. A commensurate-disordered (C-D) phase transition in the ATNNI model has not been studied yet. Therefore, the C-D phase transition is studied in detail applying the modified DMRG algorithm with periodic boundary conditions and a finite-size scaling. Critical points (Tc, Hc) as well as all critical exponents are calculated on the C-D transition line. The Tensor Product Variational Approach (TPVA) algorithm is employed to treat the first order phase transition of the three-dimensional q=3 and q=4 Potts models by independent observations of the free energy, internal energy and magnetization for two different boundary conditions.

Abstract: We study new strong resonances associated with the physics responsible for the strong electroweak symmetry breaking. We write down the lowest order effective chiral Lagrangians describing the couplings of these new resonances to the Standard Model fields and calculate signals for I=J=0 S-resonances and for I=J=1 -resonances in the WWtt scattering in the process e^+ e^-  tt at the Next Linear Collider. We also find low-energy constraints on the -resonance couplings to the top quark.

Abstract: To increase the transmission capacity of future communication networks is becoming very critical. This task can only be accomplished by taking advantage of optical networks where multiplexing techniques such as Dense Wavelength Division Multiplexing (DWDM) and Optical Time Division Multiplexing (OTDM) are employed. To avoid electronic bottlenecks a whole new generation of ultrafast devices is needed. To fulfill these needs a new class of all optical devices has been proposed and developed. By taking advantage of the nonlinear dynamics in semiconductor optical amplifiers in combination with the fiber interferometers a new generation of ultrafast all-optical demultiplexers and wavelength converters has been demonstrated. Other switching technologies are also promising for the future. The latest technologies in the area of micro-machining have created very attractive low cost MEMS. Recently announced use of bubble technology for all-optical switching might also lead to the development of next generation large scale switching fabrics. This paper is an overview of the recent development in these areas.

Abstract: The paper reports results of the X-ray diffraction structural studies of liquid 1-phenylnaphthalene, C10H7-C6H5 at 338 K, with the use of MoK radiation of the wavelength =0.071069 nm. The curves of reduced radiation r^2_{j,k}^a[Kj k(r)-0] was calculated and some intra- and intermolecular distances in liquid 1-phenylnaphthalene were determined.