14,919 research outputs found
The Nature of Quantum Hall States near the Charge Neutral Dirac Point in Graphene
We investigate the quantum Hall (QH) states near the charge neutral Dirac
point of a high mobility graphene sample in high magnetic fields. We find that
the QH states at filling factors depend only on the perpendicular
component of the field with respect to the graphene plane, indicating them to
be not spin-related. A non-linear magnetic field dependence of the activation
energy gap at filling factor suggests a many-body origin. We therefore
propose that the and states arise from the lifting of the spin
and sub-lattice degeneracy of the LL, respectively.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Scaling and exact solutions for the flux creep problem in a slab superconductor
The flux creep problem for a superconductor slab placed in a constant or
time-dependent magnetic field is considered. Logarithmic dependence of the
activation energy on the current density is assumed, U=U0 ln(J/Jc), with a
field dependent Jc. The density B of the magnetic flux penetrating into the
superconductor, is shown to obey a scaling law, i.e., the profiles B(x) at
different times can be scaled to a function of a single variable. We found
exact solution for the scaling function in some specific cases, and an
approximate solution for a general case. The scaling also holds for a slab
carrying transport current I resulting in a power-law V(I) with exponent p~1.
When the flux fronts moving from two sides of the slab collapse at the center,
the scaling is broken and p crosses over to U0/kT.Comment: RevTex, 10 pages including 6 figures, submitted to Phys.Rev.
The structure of the graviton self-energy at finite temperature
We study the graviton self-energy function in a general gauge, using a hard
thermal loop expansion which includes terms proportional to T^4, T^2 and
log(T). We verify explicitly the gauge independence of the leading T^4 term and
obtain a compact expression for the sub-leading T^2 contribution. It is shown
that the logarithmic term has the same structure as the ultraviolet pole part
of the T=0 self-energy function. We argue that the gauge-dependent part of the
T^2 contribution is effectively canceled in the dispersion relations of the
graviton plasma, and present the solutions of these equations.Comment: 27 pages, 6 figure
La représentation de Dieu : Comment les enfants japonais dessinent Dieu
L'étude de la représentation de Dieu chez l'enfant à l'aide de la technique du dessin n'est pas nouvelle. Dans une large enquête conduite aux Etats-Unis, Harms (1944) montrait des modifications du contenu des représentations en fonction de l'âge : du conte de fée aux représentations plus réalistes, de l'anthropomorphisme à des représentations plus symboliques ou abstraites. Depuis, d'autres travaux ont repris cette technique avec des enfants européens, montrant des différences suivant que l'enfant a reçu ou non une éducation religieuse (Hanisch, 1996) ou qu'il est garçon ou fille (Klein, 2000). Dans le prolongement de ces travaux, l'enquête présentée cherche à mettre en évidence l'effet de la culture en sortant d'un contexte inspiré par la conception judéo-chrétienne de Dieu. Près de 150 dessins ont été récoltés au Japon dans des écoles bouddhistes et publiques, auprès d'enfants entre sept ans et 14 ans. Trois groupes d'âges ont été constitués : 7-8 ans, 10-11 ans, 13-14 ans. Chaque dessin a été décrit à l'aide d'une quarantaine de traits qui ont permis de définir 17 types. Ces types, ainsi que quelques variables saillantes ont été corrélés avec l'âge, le genre du dessinateur, et l'école suivie. Contrairement aux dessins récoltés en Occident, où presque tous les dessins anthropomorphes présentent des figures masculines, la moitié des filles japonaises ont représenté un dieu féminin. Parallèlement, on constate aussi que l'éducation religieuse (ici le bouddhisme) favorise la production des représentations non anthropomorphiques chez les enfants plus âgés (30% des dessins chez les enfants fréquentant des écoles bouddhistes contre 8% chez ceux fréquentant des écoles publiques). Indépendamment des types qui ont pu être décrits opérationnellement, on constate que certains moyens utilisés pour différencier la représentation de la figure de Dieu d'autres figures sont largement partagés. Les enfants puisent dans un répertoire graphique et symbolique en combinant des motifs, certains typiques du Japon, d'autres propres à l'imagerie occidentale largement popularisée par les médias. Il en ressort que la représentation (picturale) de Dieu n'est pas tant le résultat de la reproduction plus ou moins habile d'un stéréotype traditionnel plus ou moins bien assimilé, mais bien plutôt la tentative de signifier une différence ontologique à l'aide d'une grammaire de signes empruntés à divers systèmes de référence
Dyadic pulsations as a signature of sustainability in correspondence networks
In this paper, we introduce the concept of dyadic pulsations as a measure of sustainability in online discussion groups. Dyadic pulsations correspond to new communication exchanges occurring between two participants in a discussion group. A group that continuously integrates new participants in the on-going conversation is characterized by a steady dyadic pulsation rhythm. On the contrary, groups that either pursue close conversation or unilateral communication have no or very little dyadic pulsations. We show on two examples taken from Usenet discussion groups, that dyadic pulsations permit to anticipate future bursts in response delay time which are signs of group discussion collapses. We discuss ways of making this measure resilient to spam and other common algorithmic production that pollutes real discussion
Self-organization of vortices in type-II superconductors during magnetic relaxation
We revise the applicability of the theory of self-organized criticality (SOC)
to the process of magnetic relaxation in type-II superconductors. The driving
parameter of self-organization of vortices is the energy barrier for flux creep
and not the current density. The power spectrum of the magnetic noise due to
vortex avalanches is calculated and is predicted to vary with time during
relaxation.Comment: RevTex, 5 pages, 2 PS figures. Accepted in Phys. Rev.
Insulator-to-metal crossover induced by local spin fluctuations in strongly correlated systems
We study the simplified Hubbard (SH) model in the presence of a transverse
field in the infinite dimension limit. The relevant one-particle Green's
functions of the model are obtained by means a perturbative treatment of the
hopping and of the transverse field around the atomic limit. We consider an
analytical solution for the impurity problem. It is shown that this solution is
very accurate in describing the spectral properties of the heavy-particles of
the SH for intermediate and strong values of the on-site Coulomb interaction
. We find that for large values of an insulator-metal transition takes
place as a function of the transverse field. We analyze the metallic phase
through the behavior of the density of states and of the optical conductivity
and static resistivity. Our results for the latter quantity agree with what is
observed in experiments on .Comment: 6 pages, 5 figures, to appear in Journal of Physics: Condensed Matte
Atlantic Meridional Overturning Circulation at 14.5° N in 1989 and 2013 and 24.5° N in 1992 and 2015: volume, heat, and freshwater transports
The Atlantic
Meridional Overturning Circulation (AMOC) is analyzed by applying a box
inverse model to hydrographic data from transatlantic sections along
14.5° N, occupied in 1989 and 2013, and along 24.5° N,
occupied in 1992 and 2015. Direct comparison of water mass properties among
the different realizations at the respective latitudes shows that the
Antarctic Intermediate Water (AAIW) became warmer and saltier at
14.5° N, and the densest Antarctic Bottom Water became lighter,
while the North Atlantic Deep Water freshened at both latitudes. The inverse
solution shows that the intermediate layer transport at 14.5° N was
also markedly weaker in 2013 than in 1989, indicating that the AAIW property
changes at this latitude may be related to changes in the circulation. The
inverse solution was validated using the RAPID and MOVE array data, and the
GECCO2 ocean state estimate. Comparison among these datasets indicates that
the AMOC has not significantly weakened over the past 2 decades at both
latitudes. Sensitivity tests of the inverse solution suggest that the
overturning structure and heat transport across the 14.5° N section
are sensitive to the Ekman transport, while freshwater transport is sensitive
to the transport-weighted salinity at the western boundary.</p
Evidence of local superconductivity in granular Bi nanowires fabricated by electrodeposition
An unusual enhancement of resistance (i.e., superresistivity) below a certain
characteristic temperature Tsr was observed in granular Bi nanowires. This
superresistive state was found to be dependent on the applied magnetic field
(H) as well as the excitation current (I). The suppression of Tsr by magnetic
field resembles that of a superconductor. The observed superresistivity appears
to be related to the nucleation of local superconductivity inside the granular
nanowire without long-range phase coherence. The phenomenon is reminiscent of
the Bose-insulator observed previously in ultra thin two-dimensional (2D)
superconducting films and 3D percolative superconducting films.Comment: 11 pages, 5 figures. submitted to PR
Detection of X-ray Emission from Gravitationally Lensed Submillimeter Sources in the Field of Abell 370
We report the detection by Chandra of SMM J02399-0134 and SMM J02399-0136,
two distant (z=1.06 and z=2.81, respectively) submillimeter sources
gravitationally magnified by the galaxy cluster Abell 370. These are
high-significance (> 7-sigma) X-ray detections of the high-redshift
submillimeter source population. The X-ray positions are coincident with the
optical positions to within one arcsecond. The X-ray spectra, while of low
signal-to-noise ratio, are quite hard. Absorbed power law models with fixed
photon indices of imply local absorbing columns cm and unabsorbed luminosities erg s in both
sources. These results imply that nuclear activity is responsible for the bulk
of the luminosity in SMM J02399-0134, and for at least 20% of the luminosity of
SMM J02399-0136, consistent with previous optical observations. We also place
an upper limit on the X-ray flux of a third submillimeter source, SMM
J02400-0134. Considered together with previously published Chandra upper limits
on X-ray flux from submillimeter sources, our results imply that
% of submillimeter sources exhibit X-ray emission from AGN
(90% confidence), consistent with expectations of their contribution to the
diffuse X-ray background.Comment: Corrected typos in Figure 1 labels; Accepted for publication in ApJ
Letters, 6 pages, 2 figures, latex requires emulateapj5.st
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