1,055 research outputs found
Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors
The magnetic structure of antiferromagnetically ordered phases of
quasi-one-dimensional organic conductors is studied theoretically at absolute
zero based on the mean field approximation to the quarter-filled band with
on-site and nearest-neighbor Coulomb interaction. The differences in magnetic
properties between the antiferromagnetic phase of (TMTTF)X and the spin
density wave phase in (TMTSF)X are seen to be due to a varying degrees of
roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb
interaction introduces charge disproportionation, which has the same spatial
periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic
phase. This is in accordance with the results of experiments on (TMTTF)Br
and (TMTTF)SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)Ag
is predicted to have a similar antiferromagnetic spin structure.Comment: 8 pages, LaTeX, 4 figures, uses jpsj.sty, to be published in J. Phys.
Soc. Jpn. 66 No. 5 (1997
Isotope effects in underdoped cuprate superconductors: a quantum phenomenon
We show that the unusual doping dependence of the isotope effects on
transition temperature and zero temperature in - plane penetration depth
naturally follows from the doping driven 3D-2D crossover, the 2D quantum
superconductor to insulator transition (QSI) in the underdoped limit and the
change of the relative doping concentration upon isotope substitution. Close to
the QSI transition both, the isotope coefficient of transition temperature and
penetration depth approach the coefficient of the relative dopant
concentration, and its divergence sets the scale. These predictions are fully
consistent with the experimental data and imply that close to the underdoped
limit the unusual isotope effect on transition temperature and penetration
depth uncovers critical phenomena associated with the quantum superconductor to
insulator transition in two dimensions.Comment: 6 pages, 3 figure
A skirmish in the early reception of Karl Barth in Scotland: The exchange between Thomas F. Torrance and Brand Blanshard
With an introduction by Iain Torrance, this paper reproduces a series of letters in The Scotsman newspaper between T. F. Torrance and the distinguished American philosopher Brand Blanshard. This (at times highly contentious) exchange was occasioned by views expressed by Blanshard in his 1952 Gifford Lectures on Barth and Brunner and what he called their ‘theology of crisis’. The letters give a fascinating insight into the way this new theology was perceived in the English-speaking world at the time
Excitation Spectrum of One-dimensional Extended Ionic Hubbard Model
We use Perturbative Continuous Unitary Transformations (PCUT) to study the
one dimensional Extended Ionic Hubbard Model (EIHM) at half-filling in the band
insulator region. The extended ionic Hubbard model, in addition to the usual
ionic Hubbard model, includes an inter-site nearest-neighbor (n.n.) repulsion,
. We consider the ionic potential as unperturbed part of the Hamiltonian,
while the hopping and interaction (quartic) terms are treated as perturbation.
We calculate total energy and ionicity in the ground state. Above the ground
state, (i) we calculate the single particle excitation spectrum by adding an
electron or a hole to the system. (ii) the coherence-length and spectrum of
electron-hole excitation are obtained. Our calculations reveal that for V=0,
there are two triplet bound state modes and three singlet modes, two anti-bound
states and one bound state, while for finite values of there are four
excitonic bound states corresponding to two singlet and two triplet modes. The
major role of on-site Coulomb repulsion is to split singlet and triplet
collective excitation branches, while tends to pull the singlet branches
below the continuum to make them bound states.Comment: 10 eps figure
Ordering phenomena in quasi one-dimensional organic conductors
Low-dimensional organic conductors could establish themselves as model
systems for the investigation of the physics in reduced dimensions. In the
metallic state of a one-dimensional solid, Fermi-liquid theory breaks down and
spin and charge degrees of freedom become separated. But the metallic phase is
not stable in one dimension: as the temperature is reduced, the electronic
charge and spin tend to arrange themselves in an ordered fashion due to strong
correlations. The competition of the different interactions is responsible for
which broken-symmetry ground state is eventually realized in a specific
compound and which drives the system towards an insulating state.
Here we review the various ordering phenomena and how they can be identified
by optic and magnetic measurements. While the final results might look very
similar in the case of a charge density wave and a charge-ordered metal, for
instance, the physical cause is completely different. When density waves form,
a gap opens in the density of states at the Fermi energy due to nesting of the
one-dimension Fermi surface sheets. When a one-dimensional metal becomes a
charge-ordered Mott insulator, on the other hand, the short-range Coulomb
repulsion localizes the charge on the lattice sites and even causes certain
charge patterns.
We try to point out the similarities and conceptional differences of these
phenomena and give an example for each of them. Particular emphasis will be put
on collective phenomena which are inherently present as soon as ordering breaks
the symmetry of the system.Comment: Review article Naturwissenschaften 200
Mild place illusion: a virtual reality factor to spark creativity in writing
Developments in Virtual Reality (VR) technology have modified
the creative potential of each individual. We introduce a new con cept, called "mild place illusion", as a new paradigm for designing
VR-based user interfaces targeted at stimulating creativity. We
show that for creative tasks - such as creative writing, new product
ideation, and brainstorming - a "just-enough" amount of place illu sion leads to a greater self-perception of creativity, as opposed to a
"full-level" place illusion. This is a somewhat unexpected result
since one would suppose, a priori, to have the full-level place illu sion as the optimal setup for stimulating creativity. We considered
that the methodology in this work was fairly complex, but our re sults show – through a data triangulation approach – that we were
able to identify more consistent and personal creative experiences.
Therefore, the main contribution of this paper is a new paradigm
for designing VR user interfaces targeted at stimulating creativity
by showing that a “one-illusion interspace” leads to a greater self perception of creativity.info:eu-repo/semantics/publishedVersio
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
Anomalous electronic susceptibility in Bi2Sr2CuO6+d and comparison with other overdoped cuprates
We report magnetic susceptibility performed on overdoped Bi2Sr2CuO6+d powders
as a function of oxygen doping d and temperature T. The decrease of the spin
susceptibility with increasing T is confirmed. At sufficient high temperature,
the spin susceptibility Chi_s presents an unusual linear temperature dependence
Chi_s ~ Chi_s0 -Chi_1 T. Moreover, a linear correlation between Chi_1 and
Chi_s0 for increasing hole concentration is displayed. A temperature Tchi,
independent of hole doping characterizes this scaling. Comparison with other
cuprates of the literature(LSCO, Tl-2201 and Bi-2212), over the same overdoped
range, shows similarities with above results. These non conventional metal
features will be discussed in terms of a singular narrow-band structure.Comment: 16 pages, 4 figure
Spin-orbit coupling in interacting quasi-one-dimensional electron systems
We present a new model for the study of spin-orbit coupling in interacting
quasi-one-dimensional systems and solve it exactly to find the spectral
properties of such systems. We show that the combination of spin-orbit coupling
and electron-electron interactions results in: the replacement of separate spin
and charge excitations with two new kinds of bosonic mixed-spin-charge
excitation, and a characteristic modification of the spectral function and
single-particle density of states. Our results show how manipulation of the
spin-orbit coupling, with external electric fields, can be used for the
experimental determination of microscopic interaction parameters in quantum
wires.Comment: 5 pages including 4 figures; RevTeX; to appear in Phys.Rev.Let
Nature of the insulating phases in the half-filled ionic Hubbard model
We investigate the ground-state phase diagram of the one-dimensional "ionic"
Hubbard model with an alternating periodic potential at half-filling by
numerical diagonalization of finite systems with the Lanczos and density matrix
renormalization group (DMRG) methods. We identify an insulator-insulator phase
transition from a band to a correlated insulator with simultaneous charge and
bond-charge order. The transition point is characterized by the vanishing of
the optical excitation gap while simultaneously the charge and spin gaps remain
finite and equal. Indications for a possible second transition into a
Mott-insulator phase are discussed.Comment: final for
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