Remarks on the Collective Quantization of the SU(2) Skyrme Model

We point out the question of ordering momentum operator in the canonical \break quantization of the SU(2) Skyrme Model. Thus, we suggest a new definition for the momentum operator that may solve the infrared problem that appears when we try to minimize the Quantum Hamiltonian.Comment: 8 pages, plain tex, IF/UFRJ/9

Suppression of static stripe formation by next-neighbor hopping

We show from real-space Hartree-Fock calculations within the extended Hubbard model that next-nearest neighbor (t') hopping processes act to suppress the formation of static charge stripes. This result is confirmed by investigating the evolution of charge-inhomogeneous corral and stripe phases with increasing t' of both signs. We propose that large t' values in YBCO prevent static stripe formation, while anomalously small t' in LSCO provides an additional reason for the appearance of static stripes only in these systems.Comment: 4 pages, 5 figure

On possible superconductivity in the doped ladder compound La_(1-x)Sr_xCuO_2.5

LaCuO_2.5 is a system of coupled, two-chain, cuprate ladders which may be doped systematically by Sr substitution. Motivated by the recent synthesis of single crystals, we investigate theoretically the possibility of superconductivity in this compound. We use a model of spin fluctuation-mediated superconductivity, where the pairing potential is strongly peaked at \pi in the ladder direction. We solve the coupled gap equations on the bonding and antibonding ladder bands to find superconducting solutions across the range of doping, and discuss their relevance to the real material.Comment: RevTex, 4 pages, 7 figure

Detection of topological transitions by transport through molecules and nanodevices

We analyze the phase transitions of an interacting electronic system weakly coupled to free-electron leads by considering its zero-bias conductance. This is expressed in terms of two effective impurity models for the cases with and without spin degeneracy. We demonstrate using the half-filled ionic Hubbard ring that the weight of the first conductance peak as a function of external flux or of the difference in gate voltages between even and odd sites allows one to identify the topological charge transition between a correlated insulator and a band insulator.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let

The Effects of Negative Legacies on the Adjustment of Parentally Bereaved Children and Adolescents

This is a report of a qualitative analysis of a sample of bereaved families in which one parent died and in which children scored in the clinical range on the Child Behavior Check List. The purpose of this analysis was to learn more about the lives of these children. They were considered to be at risk of developing emotional and behavioral problems associated with the death. We discovered that many of these “high risk” children had a continuing bond with the deceased that was primarily negative and troubling for them in contrast to a comparison group of children not at risk from the same study. Five types of legacies, not mutually exclusive, were identified: health related, role related, personal qualities, legacy of blame, and an emotional legacy. Coping behavior on the part of the surviving parent seemed to make a difference in whether or not a legacy was experienced as negative

Electronic and Magnetic Structure of LaCuO2.5_{2.5}

The recently-discovered ``ladder'' compound LaCuO$_{2.5}$ has been found to admit hole doping without altering its structure of coupled copper oxide ladders. While susceptibility measurements on the parent compound suggest a spin gap and a spin-liquid state, NMR results indicate magnetic order at low temperatures. These seemingly contradictory results may be reconciled if in fact the magnetic state is near the crossover from spin liquid to antiferromagnet, and we investigate this possibility. From a tight-binding fit to the valence LDA bandstructure, we deduce that the strength of the interladder hopping term is approximately half that of intraladder hopping, showing that the material is three-dimensional in character. A mean-field treatment of the insulating magnetic state gives a spin-liquid phase whose spin gap decreases with increasing interladder coupling, vanishing (signalling a transition to the ordered phase) at a value somewhat below that obtained for LaCuO$_{2.5}$. The introduction of an on-site repulsion term, $U$, to the band scheme causes a transition to an antiferromagnetic insulator for rather small but finite values of $U$, reflecting the predominance of (one-dimensional) ladder behavior, and an absence of any special nesting features.Comment: 8 pages + 5 figure

Magnetic properties of (VO)_2P_2O_7: two-plane structure and spin-phonon interactions

Detailed experiments on single-crystal (VO)_2P_2O_7 continue to reveal new and unexpected features. We show that a model composed of two, independent planes of spin chains with frustrated magnetic coupling is consistent with nuclear magnetic resonance and inelastic neutron scattering measurements. The pivotal role of PO_4 groups in mediating intrachain exchange interactions explains both the presence of two chain types and their extreme sensitivity to certain lattice vibrations, which results in the strong magnetoelastic coupling observed by light scattering. We compute the respective modifications of the spin and phonon dynamics due to this coupling, and illustrate their observable consequences on the phonon frequencies, magnon dispersions, static susceptibility and specific heat.Comment: 10 pages, 9 figure

Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3

Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers. Interdimer superexchange interactions give a three-dimensional magnon dispersion and a spin gap significantly smaller than the dimer coupling. This gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a magnetic field of 5.6T, offering a unique opportunity to explore the both types of quantum phase transition and their associated critical phenomena. We use a bond-operator formulation to obtain a continuous description of all disordered and ordered phases, and thus of the transitions separating these. Both pressure- and field-induced transitions may be considered as the Bose-Einstein condensation of triplet magnon excitations, and the respective phases of staggered magnetic order as linear combinations of dimer singlet and triplet modes. We focus on the evolution with applied pressure and field of the magnetic excitations in each phase, and in particular on the gapless (Goldstone) modes in the ordered regimes which correspond to phase fluctuations of the ordered moment. The bond-operator description yields a good account of the magnetization curves and of magnon dispersion relations observed by inelastic neutron scattering under applied fields, and a variety of experimental predictions for pressure-dependent measurements.Comment: 20 pages, 17 figure

Block orthogonal polynomials: I. Definition and properties

Constrained orthogonal polynomials have been recently introduced in the study of the Hohenberg-Kohn functional to provide basis functions satisfying particle number conservation for an expansion of the particle density. More generally, we define block orthogonal (BO) polynomials which are orthogonal, with respect to a first Euclidean scalar product, to a given $i$-dimensional subspace ${\cal E}_i$ of polynomials associated with the constraints. In addition, they are mutually orthogonal with respect to a second Euclidean scalar product. We recast the determination of these polynomials into a general problem of finding particular orthogonal bases in an Euclidean vector space endowed with distinct scalar products. An explicit two step Gram-Schmidt orthogonalization (G-SO) procedure to determine these bases is given. By definition, the standard block orthogonal (SBO) polynomials are associated with a choice of ${\cal E}_i$ equal to the subspace of polynomials of degree less than $i$. We investigate their properties, emphasizing similarities to and differences from the standard orthogonal polynomials. Applications to classical orthogonal polynomials will be given in forthcoming papers.Comment: This is a reduced version of the initial manuscript, the number of pages being reduced from 34 to 2

Magnetic Properties of (VO)_2P_2O_7 from Frustrated Interchain Coupling

Neutron-scattering experiments on (VO)_2P_2O_7 reveal both a gapped magnon dispersion and an unexpected, low-lying second mode. The proximity and intensity of these modes suggest a frustrated coupling between the alternating spin chains. We deduce the minimal model containing such a frustration, and show that it gives an excellent account of the magnon dispersion, static susceptibility and electron spin resonance absorption. We consider two-magnon states which bind due to frustration, and demonstrate that these may provide a consistent explanation for the second mode.Comment: RevTeX, 5 pages, 6 figures, compressed from first versio