121 research outputs found
First order Mott transition at zero temperature in two dimensions: Variational plaquette study
The nature of the metal-insulator Mott transition at zero temperature has
been discussed for a number of years. Whether it occurs through a quantum
critical point or through a first order transition is expected to profoundly
influence the nature of the finite temperature phase diagram. In this paper, we
study the zero temperature Mott transition in the two-dimensional Hubbard model
on the square lattice with the variational cluster approximation. This takes
into account the influence of antiferromagnetic short-range correlations. By
contrast to single-site dynamical mean-field theory, the transition turns out
to be first order even at zero temperature.Comment: 6 pages, 5 figures, version 2 with additional results for 8 bath
site
Tuning the spin Hamiltonian of NENP by external pressure: a neutron scattering study
We report an inelastic neutron scattering study of antiferromagnetic spin
dynamics in the Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP) under external
hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations
in NENP are dominated by a long-lived triplet mode with a gap which is split by
orthorhombic crystalline anisotropy into a lower doublet centered at
1.2meV and a singlet at 2.5meV.
With pressure we observe appreciable shifts in these levels, which move to
1.45 meV and
2.2meV. The dispersion of these modes in the crystalline c-direction
perpendicular to the chain was measured here for the first time, and can be
accounted for by an interchain exchange J'_c approximately 3e-4*J which changes
only slightly with pressure. Since the average gap value 1.64
meV remains almost unchanged with P, we conclude that in NENP the application
of external pressure does not affect the intrachain coupling J appreciably, but
does produce a significant decrease of the single-ion anisotropy constant from
D/J = 0.16(2) at ambient pressure to D/J = 0.09(7) at P = 2.5 GPa.Comment: LaTeX file nenp_p.tex, 10 pages, 1 table, 5 figures. Submitted to
Phys. Rev.
Experiências de contacto dos bebés com a linguagem escrita
The purpose of this study is to characterize the literacy experiences ofbabies aged between 14 and 24 months. In this study, four issues were analysed: which type of written language experiences are offered to the children, how parents value these literacy experiences, the way parents perceive their own role in the literacy development of their children and what parents know about their means of contributing to the literacy development of their children. Six mothers were interviewed. These interviews were analysed with the use of a categorical content analysis. The results suggest that mothers value and include in their practices facilitative and mediation actions of literacy experiences with their children, noting some variability depending on the respective qualifications.Este estudo exploratório visa descrever as experiências de literacia de bebés entre os 14 e os 25 meses. Analisamse quatro questões, que experiências envolvendo a linguagem escrita no seio familiar são proporcionadas aos bebés, qual o valor que os pais lhes atribuem, como percepcionam o seu papel na promoção do desenvolvimento literácito dos filhos e como podem contribuir para o desenvolvimento da literacia dos mesmos. Adotou-se uma metodologia qualitativa, com recurso à análise de conteúdo categorial das respostas dadas por seis mães em entrevista individual e semiestruturada. Concluiu-se que estas mães valorizam e incluem nas suas práticas ações facilitadoras e de mediação de experiências de literacia por parte dos seus bebés, notando-se alguma variabilidade em função das respetivas habilitações académicas.(undefined)info:eu-repo/semantics/publishedVersio
Two-Dimensional Quantum Spin Systems with Ladder and Plaquette Structure
We investigate low-energy properties of two-dimensional quantum spin systems
with the ladder and plaquette structures, which are described by a generalized
antiferromagnetic Heisenberg model with both of the bond and spin alternations.
By exploiting a non-linear model technique and a modified spin wave
approach, we evaluate the spin gap and the spontaneous magnetization to discuss
the quantum phase transition between the ordered and disordered states. We
argue how the spin-gapped phase is driven to the antiferromagnetic phase in the
phase diagram.Comment: 8 pages (9 figures), accepted by JPS
Superconducting instability in 3 band metallic nanotubes
Motivated by recent experiments on small radius nanotubes, we study the
superconducting instabilities of cylindrical (5,0) nanotubes. According to band
structure calculations, thesenanotubes possess three bands at the Fermi energy.
Using a fermionic renormalization group approach and a careful bosonization
treatment,we consider the effect of different attractive interactions, mediated
by phonons, within the Luttinger Liquid framework. We particularly focus on a
superconducting instability specific to the three bands model we consider for
the description of these
(5,0) cylindrical nanotubes.Comment: RevTeX 4, 17 pages, 10 EPS figure
Strong-Coupling Expansion for the Hubbard Model
A strong-coupling expansion for models of correlated electrons in any
dimension is presented. The method is applied to the Hubbard model in
dimensions and compared with numerical results in . Third order expansion
of the Green function suffices to exhibit both the Mott metal-insulator
transition and a low-temperature regime where antiferromagnetic correlations
are strong. It is predicted that some of the weak photoemission signals
observed in one-dimensional systems such as should become stronger as
temperature increases away from the spin-charge separated state.Comment: 4 pages, RevTex, 3 epsf figures include
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