From band insulator to Mott insulator in one dimension

We derive the phase diagram for the one-dimensional model of a ferroelectric perovskite recently introduced by Egami, Ishihara and Tachiki [Science, {\bf 261}, 1307 (1993)]. We show that the interplay between covalency, ionicity and strong correlations results in a spontaneously dimerized phase which separates the weak-coupling band insulator from the strong-coupling Mott insulator. The transition from the band insulator to the dimerized phase is identified as an Ising critical point. The charge gap vanishes at this single point with the optical conductivity diverging as $\sigma(\omega)\sim \omega^{-3/4}$. The spin excitations are gapless above the second transition to the Mott insulator phase.Comment: 4 pages LaTex (RevTex) and 1 postscript figure included by eps

Superconductor-to-Spin-Density-Wave Transition in Quasi-One-Dimensional Metals with Ising Anisotropy

We study a mechanism for superconductivity in quasi-one-dimensional materials with Ising anisotropy. In an isolated chain Ising anisotropy opens a spin gap; if inter-chain coupling is sufficiently weak, single particle hopping is suppressed and the physics of coupled chains is controlled by a competition between pair hopping and exchange interaction. Spin density wave and triplet superconductivity phases are found separated by a first order phase transition. For particular parameter values a second order transition described by SO(4) symmetry is found.Comment: 18 pages, 1 figur

Análise da estrutura fatorial dos Testes de Torrance em estudantes portugueses

In order to verify the factorial structures of the Torrance verbal and figural tests, two activities of each instrument were applied with 193 students from the 10th and 12th years of education in Portugal. We tried to demonstrate that the collinearity of the fluency and flexibility variables could create methodological artifacts that hinder the understanding of the internal structure underlying the test. The principal component analysis without control of collinearity indicated a solution composed of four basic factors that separeted activities. Controlling for collinearity, we found a new solution, which also contained four factors that, unlike the previous result, grouped variables with similar processes but of different activities. The verbal and figural content is also an important element in the factor structure. This new arrangement makes more sense with the theory that underlies the instruments separating the different processes and content which are being measured by the activities.Com a finalidade de verificar a estrutura fatorial dos testes de Torrance, duas atividades verbais e duas figurais foram aplicadas em 193 estudantes do 10º e 12º ano do ensino secundário de Portugal. Tentou-se demonstrar que a colinearidade das variáveis fluência e flexibilidade podem criar artefatos metodológicos que dificultam o entendimento da estrutura interna subjacente ao teste. A análise fatorial dos componentes principais, sem controle da colinearidade, indicou uma solução composta por quatro fatores que separam basicamente as atividades. Controlando-se a colinearidade, encontrou-se uma nova solução, também composta por quatro fatores, que, diferentemente da anterior, organizou variáveis com processos semelhantes, mas de diferentes atividades. O tipo de conteúdo, verbal e figural, mostrou-se ainda um importante elemento na organização dos fatores. Esse novo arranjo fez mais sentido diante da teoria que embasa os instrumentos, ao separar os diferentes processos e conteúdos por eles avaliados

Formality and informality in the summative assessment of motor vehicle apprentices: a case study

This article explores the interaction of formal and informal attributes of competence‐based assessment. Specifically, it presents evidence from a small qualitative case study of summative assessment practices for competence‐based qualifications within apprenticeships in the motor industry in England. The data are analysed through applying an adaptation of a framework for exploring the interplay of formality and informality in learning. This analysis reveals informal mentoring as a significant element which influences not only the process of assessment, but also its outcomes. We offer different possible interpretations of the data and their analysis, and conclude that, whichever interpretation is adopted, there appears to be a need for greater capacity‐building for assessors at a local level. This could acknowledge a more holistic role for assessors; recognise the importance of assessors’ informal practices in the formal retention and achievement of apprentices; and enhance awareness of inequalities that may be reinforced by both informal and formal attributes of assessment practices

Does Adult Sex Ratio Predict Regional Variation in Facial Dominance Perceptions? Evidence From an Analysis of U.S. States

When the adult sex ratio of the local population is biased toward women, men face greater costs due to increased direct intrasexual competition. In order to mitigate these costs, men may be more attuned to cues of other men’s physical dominance under these conditions. Consequently, we investigated the relationships between the extent to which people (N = 3,586) ascribed high dominance to masculinized versus feminized faces and variation in adult sex ratio across U.S. states. Linear mixed models showed that masculinized faces were perceived as more dominant than feminized faces, particularly for judgments of men’s facial dominance. Dominance perceptions were weakly related to adult sex ratio, and this relationship was not moderated by face sex, participant sex, or their interaction. Thus, our results suggest that dominance perceptions are relatively unaffected by broad geographical differences in adult sex ratios

Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ

The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction band width.Comment: 4 pages, 4 figures; to appear in PR

Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition

We present exact diagonalization results on a modified Peierls-Hubbard model for the neutral-ionic phase transition. The ground state potential energy surface and the infrared intensity of the Peierls mode point to a strong, non-linear electron-phonon coupling, with effects that are dominated by the proximity to the electronic instability rather than by electronic correlations. The huge infrared intensity of the Peierls mode at the ferroelectric transition is related to the temperature dependence of the dielectric constant of mixed-stack organic crystals.Comment: 4 pages, 4 figure

Effects of Lattice and Molecular Phonons on Photoinduced Neutral-to-Ionic Transition Dynamics in Tetrathiafulvalene-pp-Chloranil

For electronic states and photoinduced charge dynamics near the neutral-ionic transition in the mixed-stack charge-transfer complex tetrathiafulvalene-$p$-chloranil (TTF-CA), we review the effects of Peierls coupling to lattice phonons modulating transfer integrals and Holstein couplings to molecular vibrations modulating site energies. The former stabilizes the ionic phase and reduces discontinuities in the phase transition, while the latter stabilizes the neutral phase and enhances the discontinuities. To reproduce the experimentally observed ionicity, optical conductivity and photoinduced charge dynamics, both couplings are quantitatively important. In particular, strong Holstein couplings to form the highly-stabilized neutral phase are necessary for the ionic phase to be a Mott insulator with large ionicity. A comparison with the observed photoinduced charge dynamics indicates the presence of strings of lattice dimerization in the neutral phase above the transition temperature.Comment: 9 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp

Evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in Eu1−x_{1-x}Gdx_{x}O

Raman scattering studies as functions of temperature, magnetic field, and Gd-substitution are used to investigate the evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in Eu$_{1-x}$Gd$_{x}$O. These studies reveal a greater richness of phase behavior than have been previously observed using transport measurements: a spin-fluctuation-dominated paramagnetic (PM) phase regime for T $>$ T$^{*}$ $>$ T$_{C}$, a two-phase regime for T $<$ T$^{*}$ in which magnetic polarons develop and coexist with a remnant of the PM phase, and an inhomogeneous ferromagnetic phase regime for T $<$ T$_{C}$

Valence changes associated with the metal-insulator transition in Bi1−x_{1-x}Lax_xNiO3_3

Perovskite-type BiNiO$_3$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray absorption (XAS) spectra of Bi$_{1-x}$La$_{x}$NiO$_{3}$ to investigate how the electronic structure changes with La doping. From Ni $2p$ XAS, we observed an increase of the valence of Ni from 2+ toward 3+. Combined with the core-level photoemission study, it was found that the average valence of Bi remains $\sim 4+$ and that the Ni valence behaves as $\sim (2+x)+$, that is, La substitution results in hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for $x>0$, consistent with the metallic behavior of the La-doped compounds. The Ni $2p$ XAS, Ni $2p$ core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shapes were found to be consistent with the gradual Ni$^{2+} \to$ Ni$^{3+}$ valence change.Comment: 6 pages, 7 figure