1,696 research outputs found
Origin of the metamagnetic transitions in Y1-xErxFe2(H,D)4.2 compounds
The structural and magnetic properties of Y1-xErxFe2 intermetallic compounds
and their hydrides and deuterides Y1-xErxFe2H(D)4.2 have been investigated
using X-ray diffraction and magnetic measurements under static and pulsed
magnetic field up to 60 T. The intermetallics crystallize in the C15 cubic
structure , whereas corresponding hydrides and deuterides crystallize in a
monoclinic structure. All compounds display a linear decrease of the unit cell
volume versus Er concentration; the hydrides have a 0.8% larger cell volume
compared to the deuterides with same Er content. They are ferrimagnetic at low
field and temperature with a compensation point at x = 0.33 for the
intermetallics and x = 0.57 for the hydrides and deuterides. A sharp first
order ferromagnetic-antiferromagnetic (FM-AFM) transition is observed upon
heating at TFM-AFM for both hydrides and deuterides. These compounds show two
different types of field induced transitions, which have different physical
origin. At low temperature (T < 50 K), a forced ferri-ferromagnetic
metamagnetic transition with Btrans1 = 8 T, related to the change of the Er
moments orientation from antiparallel to parallel Fe moment, is observed.
Btrans1 is not sensitive to Er concentration, temperature and isotope effect. A
second metamagnetic transition resulting from antiferromagnetic to
ferrimagnetic state is also observed. The transition field Btrans2 increases
linearly versus temperature and relates to the itinerant electron metamagnetic
behavior of the Fe sublattice. An onset temperature TM0 is obtained by
extrapolating TFM-AFM (B) at zero field. TM0 decreases linearly versus the Er
content and is 45(5) K higher for the hydrides compared to the corresponding
deuteride. The evolution of TM0 versus cell volume shows that it cannot be
attributed exclusively to a pure volume effect and that electronic effects
should also be considered.Comment: 22 pages, 10 figure
A basal ganglia inspired model of action selection evaluated in a robotic survival task.
The basal ganglia system has been proposed as a possible neural substrate for action selection in the vertebrate brain. We describe a robotic implementation of a model of the basal ganglia and demonstrate the capacity of this system to generate adaptive switching between several acts when embedded in a robot that has to "survive" in a laboratory environment. A comparison between this brain-inspired selection mechanism and classical "winner-takes-all" selection highlights some adaptive properties specific to the model, such as avoidance of dithering and energy-saving. These properties derive, in part, from the capacity of simulated basal ganglia-thalamo-cortical loops to generate appropriate "behavioral persistence"
Equivariant pretheories and invariants of torsors
In the present paper we introduce and study the notion of an equivariant
pretheory: basic examples include equivariant Chow groups, equivariant K-theory
and equivariant algebraic cobordism. To extend this set of examples we define
an equivariant (co)homology theory with coefficients in a Rost cycle module and
provide a version of Merkurjev's (equivariant K-theory) spectral sequence for
such a theory. As an application we generalize the theorem of
Karpenko-Merkurjev on G-torsors and rational cycles; to every G-torsor E and a
G-equivariant pretheory we associate a graded ring which serves as an invariant
of E. In the case of Chow groups this ring encodes the information concerning
the motivic J-invariant of E and in the case of Grothendieck's K_0 -- indexes
of the respective Tits algebras.Comment: 23 pages; this is an essentially extended version of the previous
preprint: the construction of an equivariant cycle (co)homology and the
spectral sequence (generalizing the long exact localization sequence) are
adde
Dessins, their delta-matroids and partial duals
Given a map on a connected and closed orientable surface, the
delta-matroid of is a combinatorial object associated to which captures some topological information of the embedding. We explore how
delta-matroids associated to dessins d'enfants behave under the action of the
absolute Galois group. Twists of delta-matroids are considered as well; they
correspond to the recently introduced operation of partial duality of maps.
Furthermore, we prove that every map has a partial dual defined over its field
of moduli. A relationship between dessins, partial duals and tropical curves
arising from the cartography groups of dessins is observed as well.Comment: 34 pages, 20 figures. Accepted for publication in the SIGMAP14
Conference Proceeding
Quantum electrodynamics of relativistic bound states with cutoffs
We consider an Hamiltonian with ultraviolet and infrared cutoffs, describing
the interaction of relativistic electrons and positrons in the Coulomb
potential with photons in Coulomb gauge. The interaction includes both
interaction of the current density with transversal photons and the Coulomb
interaction of charge density with itself. We prove that the Hamiltonian is
self-adjoint and has a ground state for sufficiently small coupling constants.Comment: To appear in "Journal of Hyperbolic Differential Equation
Extrasolar Giant Planets under Strong Stellar Irradiation
We investigate the effects on extrasolar giant planets [EGPs] of intense
irradiation by their parent stars, describing the issues involved in treating
the model atmosphere problem correctly. We treat the radiative transfer in
detail, allowing the flux from the parent star to interact with all relevant
depths of the planetary atmosphere, with no need for a pre-assumed albedo. We
present a low-resolution optical and near-IR spectrum of a close-in EGP,
focusing on the differences from an isolated planet.
In our dust-free planetary atmospheres we find that Rayleigh scattering
increases the EGP's flux by orders of magnitude shortward of the CaII H&K
doublet (393 nm), and the spectral features of the parent star are exactly
reflected. In the optical and near-IR the thermal absorption of the planet
takes over, but the absorption features are changed by the irradiation. The
inclusion of dust increases the reflected flux in the blue; the stellar
spectral lines can be seen blueward of H-beta (486 nm).Comment: 14 pages, 4 figures, LaTex, accepted in ApJ
Inverse Eigenvalue Problems for Perturbed Spherical Schroedinger Operators
We investigate the eigenvalues of perturbed spherical Schr\"odinger operators
under the assumption that the perturbation satisfies . We show that the square roots of eigenvalues are given by the square
roots of the unperturbed eigenvalues up to an decaying error depending on the
behavior of near . Furthermore, we provide sets of spectral data
which uniquely determine .Comment: 14 page
Human Amniocytes Are Receptive to Chemically Induced Reprogramming to Pluripotency
Restoring pluripotency using chemical compounds alone would be a major step forward in developing clinical-grade pluripotent stem cells, but this has not yet been reported in human cells. We previously demonstrated that VPA_ AFS cells, human amniocytes cultivated with valproic acid (VPA) acquired functional pluripotency while remaining distinct from human embryonic stem cells (hESCs), questioning the relationship between the modulation of cell fate and molecular regulation of the pluripotency network. Here, we used single-cell analysis and functional assays to reveal that VPA treatment resulted in a homogeneous population of self-renewing non-transformed cells that fulfill the hallmarks of pluripotency, i.e., a short G1 phase, a dependence on glycolytic metabolism, expression of epigenetic modifications on histones 3 and 4, and reactivation of endogenous OCT4 and downstream targets at a lower level than that observed in hESCs. Mechanistic insights into the process of VPA-induced reprogramming revealed that it was dependent on OCT4 promoter activation, which was achieved independently of the PI3K (phosphatidylinositol 3-kinase)/ AKT/ mTOR (mammalian target of rapamycin) pathway or GSK3 beta inhibition but was concomitant with the presence of acetylated histones H3K9 and H3K56, which promote pluripotency. Our data identify, for the first time, the pluripotent transcriptional and molecular signature and metabolic status of human chemically induced pluripotent stem cells
Spectral theory for a mathematical model of the weak interaction: The decay of the intermediate vector bosons W+/-, II
We do the spectral analysis of the Hamiltonian for the weak leptonic decay of
the gauge bosons W+/-. Using Mourre theory, it is shown that the spectrum
between the unique ground state and the first threshold is purely absolutely
continuous. Neither sharp neutrino high energy cutoff nor infrared
regularization are assumed.Comment: To appear in Ann. Henri Poincar\'
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