223 research outputs found
Critical Behavior of the Antiferromagnetic Heisenberg Model on a Stacked Triangular Lattice
We estimate, using a large-scale Monte Carlo simulation, the critical
exponents of the antiferromagnetic Heisenberg model on a stacked triangular
lattice. We obtain the following estimates: ,
. These results contradict a perturbative
Renormalization Group calculation that points to Wilson-Fisher O(4) behaviour.
While these results may be coherent with results from
Landau-Ginzburg analysis, they show the existence of an unexpectedly rich
structure of the Renormalization Group flow as a function of the dimensionality
and the number of components of the order parameter.Comment: Latex file, 10 pages, 1 PostScript figure. Was posted with a wrong
Title !
Fermions out of Dipolar Bosons in the lowest Landau level
In the limit of very fast rotation atomic Bose-Einstein condensates may
reside entirely in the lowest two-dimensional Landau level (LLL). For small
enough filling factor of the LLL, one may have formation of fractional quantum
Hall states. We investigate the case of bosons with dipolar interactions as may
be realized with Chromium-52 atoms. We show that at filling factor equal to
unity the ground state is a Moore-Read (a.k.a Pfaffian) paired state as is the
case of bosons with purely s-wave scattering interactions. This Pfaffian state
is destabilized when the interaction in the s-wave channel is small enough and
the ground state is a stripe phase with unidimensional density modulation. For
filling factor 1/3, we show that there is formation of a Fermi sea of
``composite fermions''. These composites are made of one boson bound with three
vortices. This phase has a wide range of stability and the effective mass of
the fermions depends essentially only of the scattering amplitude in momentum
channels larger or equal to 2. The formation of such a Fermi sea opens up a new
possible route to detection of the quantum Hall correlations.Comment: 12 pages, 5 figures, published versio
Convex hull test of the linear separability hypothesis in visual search
AbstractVisual search for a colour target in distractors of two other colours is dramatically affected by the configuration of the colours in CIE (x, y) space. To a first approximation, search is difficult when a target’s chromaticity falls directly between (i.e. is not linearly separable from) two distractor chromaticities, otherwise search is easy (D’Zmura [1991, Vision Research, 31, 951–966]; Bauer, Jolicoeur, & Cowan [1996a, Vision Research, 36, 1439–1466]; Bauer, Jolicoeur, & Cowan [1996b, Perception, 25, 1282–1294]). In this paper, we demonstrate that the linear separability effect transcends the two distractor case. Placing a target colour inside the convex hull defined by a set of distractors hindered search performance compared with a target placed outside the convex hull. This is true whether the target was linearly separable in chromaticity only (Experiments 1 and 2), or in a combination of luminance and chromaticity (Experiments 3 and 4)
Belleza y atracción en traducción literaria
The approach to literary translation proposed here relies on three concepts: beauty, generated from ambiguity; attraction, produced on the reader-translator by the source text; and the effect caused by the source text and the translated text. Attraction is linked to what we may deem beautiful, or to what we could call the beauty effect; and it is what triggers the movement towards translation. The reader-translator must thus be seduced by the text while at the same time trying to make up for the absence generated by it. As for the effect, that the translator must identify in order to eventually reproduce it, it is linked to the internal cohesion of the text, to the marks left by the author and to the ambiguity inherent in the aesthetics of the text. In other words, the effect of the text is what the translator should yearn to reproduce, after finding his or her way.La noción de traducción literaria que les proponemos en estas páginas se cimenta en tres conceptos: la belleza, definida por su relación con el vacÃo y lo ambiguo, la atracción que ejerce el texto origen sobre el lector-traductor y el efecto provocado tanto por el texto origen como por el traducido. La atracción, estrechamente relacionada con lo que nos gusta, con lo que podrÃamos denominar el efecto de lo bello, es considerada en estas páginas como el motor de la traducción, en cuanto que el lector-traductor debe ser seducido por el texto que va a traducir, al tiempo que intentar colmar la ausencia que lo interpela. En cuanto al efecto, que hay que saber aprehender si queremos reproducirlo en la traducción, está unido a la coherencia interna del texto, a las huellas del autor, a la ambigüedad inherente a la estética del texto. Resumiendo, el traductor debe consagrase al efecto que produce el texto, una vez haya encontrado su vÃa y su voz
The Three Dimensional Thirring Model for Small N_f
We formulate the three dimensional Thirring model on a spacetime lattice and
study it for various even numbers of fermion flavors N_f by Monte Carlo
simulation. We find clear evidence for spontaneous chiral symmetry breaking at
strong coupling, contradicting the predictions of the 1/N_f expansion. The
critical point appears to correspond to an ultra-violet fixed point of the
renormalisation group; a fit to a RG-inspired equation of state in the vicinity
of the fixed point yields distinct critical exponents for N_f=2 and N_f=4,
while no fit is found for N_f=6, suggesting there is a critical number N_fc<6
beyond which no chiral symmetry breaking occurs. The spectrum of the N_f=2
theory is studied; the states examined vary sharply but continuously across the
transition.Comment: 50 pages LaTeX, including 16 tables and 20 figures - uses style file
ldd_art.sty (included
Exchange couplings in the magnetic molecular cluster Mn12Ac
The magnetic properties of the molecular cluster Mn12Ac are due to the four
Mn3+ ions which have spins S=3/2 and the eight Mn4+ ions with spins S=2. These
spins are coupled by superexchange mechanism. We determine the four exchange
couplings assuming a Heisenberg-type interaction between the ions. We use exact
diagonalization of the spin Hamiltonian by a Lanczos algorithm and we adjust
the couplings to reproduce the magnetization curve of Mn12Ac. We also impose
the constraint of reproducing a gap of 35K between a S=10 ground state and a
first excited state with S=9. We predict that there is an excited level with
S=8 at 37K above the ground state, only slightly above the S=9 excited state
which lies at 35K and the next excited state is a S=9 multiplet at 67K above
the S=10 ground state.Comment: 15 pages, 6 figures, submitted to Phys Rev B, corrected a misTeX:
values of J1, J2 have changed, refs update
The Amusic Brain: Lost in Music, but Not in Space
Congenital amusia is a neurogenetic disorder of music processing that is currently ascribed to a deficit in pitch processing. A recent study challenges this view and claims the disorder might arise as a consequence of a general spatial-processing deficit. Here, we assessed spatial processing abilities in two independent samples of individuals with congenital amusia by using line bisection tasks (Experiment 1) and a mental rotation task (Experiment 2). Both amusics and controls showed the classical spatial effects on bisection performance and on mental rotation performance, and amusics and controls did not differ from each other. These results indicate that the neurocognitive impairment of congenital amusia does not affect the processing of space
Slave bosons in radial gauge: a bridge between path integral and hamiltonian language
We establish a correspondence between the resummation of world lines and the
diagonalization of the Hamiltonian for a strongly correlated electronic system.
For this purpose, we analyze the functional integrals for the partition
function and the correlation functions invoking a slave boson representation in
the radial gauge. We show in the spinless case that the Green's function of the
physical electron and the projected Green's function of the pseudofermion
coincide. Correlation and Green's functions in the spinful case involve a
complex entanglement of the world lines which, however, can be obtained through
a strikingly simple extension of the spinless scheme. As a toy model we
investigate the two-site cluster of the single impurity Anderson model which
yields analytical results. All expectation values and dynamical correlation
functions are obtained from the exact calculation of the relevant functional
integrals. The hole density, the hole auto-correlation function and the Green's
function are computed, and a comparison between spinless and spin 1/2 systems
provides insight into the role of the radial slave boson field. In particular,
the exact expectation value of the radial slave boson field is finite in both
cases, and it is not related to a Bose condensate.Comment: 28 pages, 1 figure. Summary extended, corrected typos, added
reference
Percolation Transition in the random antiferromagnetic spin-1 chain
We give a physical description in terms of percolation theory of the phase
transition that occurs when the disorder increases in the random
antiferromagnetic spin-1 chain between a gapless phase with topological order
and a random singlet phase. We study the statistical properties of the
percolation clusters by numerical simulations, and we compute exact exponents
characterizing the transition by a real-space renormalization group
calculation.Comment: 9 pages, 4 encapsulated Postscript figures, REVTeX 3.
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