2,218 research outputs found
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 LaCuO
The recently-discovered ``ladder'' compound LaCuO 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. The introduction of an on-site repulsion term, , to the band
scheme causes a transition to an antiferromagnetic insulator for rather small
but finite values of , 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 -dimensional subspace 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 equal
to the subspace of polynomials of degree less than . 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
Superconducting Spiral Phase in the two-dimensional t-J model
We analyse the t-t'-t''-J model, relevant to the superconducting cuprates. By
using chiral perturbation theory we have determined the ground state to be a
spiral for small doping \delta << 1 near half filling. In this limit the
solution does not contain any uncontrolled approximations. We evaluate the
spin-wave Green's functions and address the issue of stability of the spiral
state, leading to the phase diagram of the model. At t'=t''=0 the spiral state
is unstable towards a local enhancement of the spiral pitch, and the nature of
the true ground state remains unclear. However, for values of t' and t''
corresponding to real cuprates the (1,0) spiral state is stabilized by quantum
fluctuations (``order from disorder'' effect). We show that at \delta = 0.119
the spiral is commensurate with the lattice with a period of 8 lattice
spacings. It is also demonstrated that spin-wave mediated superconductivity
develops in the spiral state and a lower limit for the superconducting gap is
derived. Even though one cannot classify the gap symmetry according to the
lattice representations (s,p,d,...) since the symmetry of the lattice is
spontaneously broken by the spiral, the gap always has lines of nodes along the
(1,\pm 1) directions.Comment: 17 pages, 11 figure
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