1,493 research outputs found
A combinatorial smoothness criterion for spherical varieties
We suggest a combinatorial criterion for the smoothness of an arbitrary
spherical variety using the classification of multiplicity-free spaces,
generalizing an earlier result of Camus for spherical varieties of type .Comment: 14 pages, 2 table
Charge, Orbital and Magnetic Order in Nd0.5Ca0.5MnO3
In the manganite Nd0.5Ca0.5MnO3, charge ordering occurs at much higher
temperature than the antiferromagnetic order (TCO=250K,TN=160K).
The magnetic behavior of the phase TN<T<TCO is puzzling: its magnetization
and susceptibility are typical of an antiferromagnet while no magnetic order is
detected by neutron diffraction.We have undertaken an extensive study of the
cristallographic, electric and magnetic properties of Nd0.5Ca0.5MnO3 and
established its phase diagram as a function of temperature and magnetic field.
The charge disordered, paramagnetic phase above TCO present ferromagnetic
correlations. An antiferromagnetic CE phase prevails below TN, with complete
charge and orbital ordering. In the intermediate temperature range, charge
ordering occurs while orbital ordering sets in progressively, with no magnetic
order. Strong magnetic fields destroy the charge ordered phases in a fisrt
order transition towards a ferromagnetic state.Comment: 17 pages, 11 Figures to appear in Phys. Rev.
Spherical orbit closures in simple projective spaces and their normalizations
Let G be a simply connected semisimple algebraic group over an algebraically
closed field k of characteristic 0 and let V be a rational simple G-module of
finite dimension. If G/H \subset P(V) is a spherical orbit and if X is its
closure, then we describe the orbits of X and those of its normalization. If
moreover the wonderful completion of G/H is strict, then we give necessary and
sufficient combinatorial conditions so that the normalization morphism is a
homeomorphism. Such conditions are trivially fulfilled if G is simply laced or
if H is a symmetric subgroup.Comment: 24 pages, LaTeX. v4: Final version, to appear in Transformation
Groups. Simplified some proofs and corrected minor mistakes, added
references. v3: major changes due to a mistake in previous version
The X-ray flaring activity of the galactic nucleus observed with XMM-Newton
We report the results of XMM-Newton observations of Sgr A*, the radiative
counterpart of the massive black hole at the nucleus of our Galaxy, performed
in the frame of the guaranteed time survey program of the Galactic Center
region. The discovery of bright X-ray flares from Sgr A* with Chandra in
October 2000 have opened new perspectives to understand the processes at work
in this object and in general in black holes accreting at low accretion rates.
We report here the important results obtained with XMM-Newton on the Sgr A*
high-energy flaring activity and we discuss the implications on the models and
the future observational perspectives.Comment: 4 pages, 3 figures, proc. of the SF2A conf. held in Bordeaux, France,
June 2003, eds.: F. Combes, D. Barret and T. Contini, EdP-Sciences Conf.
Serie
Effect of magnesium doping on the orbital and magnetic order in LiNiO2
In LiNiO2, the Ni3+ ions, with S=1/2 and twofold orbital degeneracy, are
arranged on a trian- gular lattice. Using muon spin relaxation (MuSR) and
electron spin resonance (ESR), we show that magnesium doping does not stabilize
any magnetic or orbital order, despite the absence of interplane Ni2+. A
disordered, slowly fluctuating state develops below 12 K. In addition, we find
that magnons are excited on the time scale of the ESR experiment. At the same
time, a g factor anisotropy is observed, in agreement with
orbital occupancy
Mechatronic design of a fast and long range 4 degrees of freedom humanoid neck
This paper describes the mechatronic design of a humanoid neck. To research human machine interaction, the head and neck combination should be able to approach the human behavior as much as possible. We present a novel humanoid neck concept that is both fast, and has a long range of motion in 4 degrees of freedom (DOFs). This enables the head to track fast objects, and the neck design is suitable for mimicking expressions. The humanoid neck features a differential drive design for the lower 2 DOFs resulting in a low moving mass and the ability to use strong actuators. The performance of the neck has been\ud
optimized by minimizing backlash in the mechanisms, and by using gravity compensation. Two cameras in the head are used for scanning and interaction with the environment
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