Dimerization-induced enhancement of the spin gap in the quarter-filled two-leg rectangular ladder

We report density-matrix renormalization group calculations of spin gaps in the quarter-filled correlated two-leg rectangular ladder with bond-dimerization along the legs of the ladder. In the small rung-coupling region, dimerization along the leg bonds can lead to large enhancement of the spin gap. Electron-electron interactions further enhance the spin gap, which is nonzero for all values of the rung electron hopping and for arbitrarily small bond-dimerization. Very large spin gaps, as are found experimentally in quarter-filled band organic charge-transfer solids with coupled pairs of quasi-one-dimensional stacks, however, occur within the model only for large dimerization and rung electron hopping that are nearly equal to the hopping along the legs. Coexistence of charge order and spin gap is also possible within the model for not too large intersite Coulomb interaction

Magnetoelectric coupling in MnTiO3

We give general arguments that show that the linear magnetoelectric effect in antiferromagnetic materials gives rise to a magnetocapacitance anomaly—a divergence of the dielectric constant at the magnetic ordering temperature TN that appears in an applied magnetic field. The measurement of magnetodielectric response thus provides a definitive and experimentally accessiblemethod to recognize antiferromagnetic linear magnetoelectric materials, circumventing the experimental difficulties often involved in measuring electric polarization. We confirm this result experimentally using the example of MnTiO3, which we show to exhibit the linear magnetoelectric effect. No dielectric anomaly is observed at TN in the absence of an applied magnetic field. However, a sharp peak in the dielectric constant appears here when a magnetic field is applied along the c axis, reflecting a linear coupling of the polarization P with the antiferromagnetic order parameter L. In accordance with our theoretical analysis, the dielectric constant close to TN increases with the square of the magnetic field.

Introduction to Vassiliev Knot Invariants

This book is a detailed introduction to the theory of finite type (Vassiliev) knot invariants, with a stress on its combinatorial aspects. It is intended to serve both as a textbook for readers with no or little background in this area, and as a guide to some of the more advanced material. Our aim is to lead the reader to understanding by means of pictures and calculations, and for this reason we often prefer to convey the idea of the proof on an instructive example rather than give a complete argument. While we have made an effort to make the text reasonably self-contained, an advanced reader is sometimes referred to the original papers for the technical details of the proofs. Version 3: some typos and inaccuracies are corrected.Comment: 512 pages, thousands picture

Ordering in Two-Dimensional Ising Models with Competing Interactions

We study the 2D Ising model on a square lattice with additional non-equal diagonal next-nearest neighbor interactions. The cases of classical and quantum (transverse) models are considered. Possible phases and their locations in the space of three Ising couplings are analyzed. In particular, incommensurate phases occurring only at non-equal diagonal couplings, are predicted. We also analyze a spin-pseudospin model comprised of the quantum Ising model coupled to XY spin chains in a particular region of interactions, corresponding to the Ising sector's super-antiferromagnetic (SAF) ground state. The spin-SAF transition in the coupled Ising-XY model into a phase with co-existent SAF Ising (pseudospin) long-range order and a spin gap is considered. Along with destruction of the quantum critical point of the Ising sector, the phase digram of the Ising-XY model can also demonstrate a re-entrance of the spin-SAF phase. A detailed study of the latter is presented. The mechanism of the re-entrance, due to interplay of interactions in the coupled model, and the conditions of its appearance are established. Applications of the spin-SAF theory for the transition in the quarter-filled ladder compound NaV2O5 are discussed.Comment: Minor revisions and refs. added; published version of the invited paper in a special issue of "Low Temp. Physics

Comprehensive aerodynamic and dynamic study of independence of ukraine monument

Розглянуто результати комплексних досліджень вирішення інженерної проблеми створення висотного монумента колонного типу в Києві, динамічних досліджень моделі в масштабі 1:25 та аеродинамічних досліджень великомасштабної моделі (1:8) в аеродинамічній трубі ТАД-2 Національного авіаційного університету. Наведено процедуру визначення на натурному об’єкті реальних динамічних характеристик та забезпечення ефективності демпфіруванняРассмотрены результаты комплексных исследований решения инженерной задачи создания высотного монумента колонного типа в Киеве, динамических исследований модели в масштабе 1:25 и аэродинамических испытаний крупномасштабной (1:8) модели в аэродинамической трубе ТАД-2 Национального авиационного университета. Приведена процедура определения на натурном объекте реальных динамических характеристик и обеспечения эффективности демпфированияComprehensive approach for solution the engineering problems o f creation the high-rise extended pillartype Monument in Kiyv is described. The results o f dynamic tests o f 1:25 scale model and aerodynamictests o f 1:8 scale model o f the Monument in TAD-2 wind tunnel are given. The procedures fordetermination o f actual dynamic characteristics and assurance the efficiency o f damping on the siteare describe

Density of Neutral Solitons in Weakly Disordered Peierls Chains

We study the effects of weak off-diagonal disorder on Peierls systems with a doubly degenerate ground state. We show that for these systems disorder in the electron hopping amplitudes induces a finite density of solitons in the minimal-energy lattice configuration of a single chain. These disorder-induced dimerization kinks are neutral and have spin 1/2. Using a continuum model for the Peierls chain and treating the lattice classically, we analytically calculate the average free energy and density of kinks. We compare these results to numerical calculations for a discrete model and discuss the implications of the kinks for the optical and magnetic properties of the conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 5 Postscript figures, to appear in Phys. Rev.

Critical temperature and density of spin-flips in the anisotropic random field Ising model

We present analytical results for the strongly anisotropic random field Ising model, consisting of weakly interacting spin chains. We combine the mean-field treatment of interchain interactions with an analytical calculation of the average chain free energy (``chain mean-field'' approach). The free energy is found using a mapping on a Brownian motion model. We calculate the order parameter and give expressions for the critical random magnetic field strength below which the ground state exhibits long range order and for the critical temperature as a function of the random magnetic field strength. In the limit of vanishing interchain interactions, we obtain corrections to the zero-temperature estimate by Imry and Ma [Phys. Rev. Lett. 35, 1399 (1975)] of the ground state density of domain walls (spin-flips) in the one-dimensional random field Ising model. One of the problems to which our model has direct relevance is the lattice dimerization in disordered quasi-one-dimensional Peierls materials, such as the conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 4 postscript figures, to appear in Phys. Rev.

Magnetic excitations in the low-temperature ferroelectric phase of multiferroic YMn2O5 using inelastic neutron scattering

We studied magnetic excitations in a low-temperature ferroelectric phase of the multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify low-energy magnon modes and establish a correspondence between the magnon peaks observed by INS and electromagnon peaks observed in optical absorption [1]. Furthermore, we explain the microscopic mechanism, which results in the lowest-energy electromagnon peak, by comparing the inelastic neutron spectral weight with the polarization in the commensurate ferroelectric phase.Comment: 4 pages, 4 figure

Phase diagram of disordered spin-Peierls systems

We study the competition between the spin-Peierls and the antiferromagnetic ordering in disordered quasi-one-dimensional spin systems. We obtain the temperature vs disorder-strength phase diagram, which qualitatively agrees with recent experiments on doped CuGeO_3.Comment: 4 pages, revtex, epsf, 2 Postscript figure