93 research outputs found
Ferromagnetic (Ga,Mn)N epilayers versus antiferromagnetic GaMnN clusters
Mn-doped wurtzite GaN epilayers have been grown by nitrogen plasma-assisted
molecular beam epitaxy. Correlated SIMS, structural and magnetic measurements
show that the incorporation of Mn strongly depends on the conditions of the
growth. Hysteresis loops which persist at high temperature do not appear to be
correlated to the presence of Mn. Samples with up to 2% Mn are purely
substitutional GaMnN epilayers, and exhibit paramagnetic
properties. At higher Mn contents, precipitates are formed which are identified
as GaMnN clusters by x-ray diffraction and absorption: this induces a
decrease of the paramagnetic magnetisation. Samples co-doped with enough Mg
exhibit a new feature: a ferromagnetic component is observed up to
K, which cannot be related to superparamagnetism of unresolved magnetic
precipitates.Comment: Revised versio
Remarks on evolution of space-times in 3+1 and 4+1 dimensions
A large class of vacuum space-times is constructed in dimension 4+1 from
hyperboloidal initial data sets which are not small perturbations of empty
space data. These space-times are future geodesically complete, smooth up to
their future null infinity, and extend as vacuum space-times through their
Cauchy horizon. Dimensional reduction gives non-vacuum space-times with the
same properties in 3+1 dimensions.Comment: 10pp, exposition improved; final versio
The Cauchy problems for Einstein metrics and parallel spinors
We show that in the analytic category, given a Riemannian metric on a
hypersurface and a symmetric tensor on , the metric
can be locally extended to a Riemannian Einstein metric on with second
fundamental form , provided that and satisfy the constraints on
imposed by the contracted Codazzi equations. We use this fact to study the
Cauchy problem for metrics with parallel spinors in the real analytic category
and give an affirmative answer to a question raised in B\"ar, Gauduchon,
Moroianu (2005). We also answer negatively the corresponding questions in the
smooth category.Comment: 28 pages; final versio
Incorporation of europium into gan nanowires by ion implantation
Rare earth (RE)-doped GaN nanowires (NWs), combining the well-defined and controllable optical emission lines of trivalent RE ions with the high crystalline quality, versatility, and small dimension of the NW host, are promising building blocks for future nanoscale devices in optoelectronics and quantum technologies. Europium doping of GaN NWs was performed by ion implantation, and structural and optical properties were assessed in comparison to thin film reference samples. Despite some surface degradation for high implantation fluences, the NW core remains of high crystalline quality with lower concentrations of extended defects than observed in ion-implanted thin films. Strain introduced by implantation defects is efficiently relaxed in NWs and the measured deformation stays much below that in thin films implanted in the same conditions. Optical activation is achieved for all samples after annealing, and while optical centers are similar in all samples, Eu^3+ emission from NW samples is shown to be less affected by residual implantation damage than for the case of thin films. The incorporation of Eu in GaN NWs was further investigated by nano-cathodoluminescence and X-ray absorption spectroscopy (XAS). Maps of the Eu-emission intensity within a single NW agree well with the Eu-distribution predicted by Monte Carlo simulations, suggesting that no pronounced Eu-diffusion takes place. XAS shows that 70-80% of Eu is found in the 3+ charge state while 20-30% is 2+ attributed to residual implantation defects. A similar local environment was found for Eu in NWs and thin films: for low fluences, Eu is mainly incorporated on substitutional Ga-sites, while for high fluences XAS points at the formation of a local EuN-like next neighbor structure. The results reveal the high potential of ion implantation as a processing tool at the nanoscale
General Argyres-Douglas Theory
We construct a large class of Argyres-Douglas type theories by compactifying
six dimensional (2,0) A_N theory on a Riemann surface with irregular
singularities. We give a complete classification for the choices of Riemann
surface and the singularities. The Seiberg-Witten curve and scaling dimensions
of the operator spectrum are worked out. Three dimensional mirror theory and
the central charges a and c are also calculated for some subsets, etc. Our
results greatly enlarge the landscape of N=2 superconformal field theory and in
fact also include previous theories constructed using regular singularity on
the sphere.Comment: 55 pages, 20 figures, minor revision and typos correcte
Prolongation on regular infinitesimal flag manifolds
Many interesting geometric structures can be described as regular
infinitesimal flag structures, which occur as the underlying structures of
parabolic geometries. Among these structures we have for instance conformal
structures, contact structures, certain types of generic distributions and
partially integrable almost CR-structures of hypersurface type. The aim of this
article is to develop for a large class of (semi-)linear overdetermined systems
of partial differential equations on regular infinitesimal flag manifolds a
conceptual method to rewrite these systems as systems of the form
, where is a linear connection
on some vector bundle over and is a
(vector) bundle map. In particular, if the overdetermined system is linear,
will be a linear connection on and hence the dimension of
its solution space is bounded by the rank of . We will see that the rank of
can be easily computed using representation theory.Comment: 35 pages; typos corrected and minor changes, final version to appear
in International Journal of Mathematic
FAME : A new beamline for X-ray absorption investigations of very-diluted systems of environmental, material and biological interests
International audienceFAME is the French Absorption spectroscopy beamline in Material and Environmental sciences at the ESRF (France), operational since September 2002. Technically speaking, the source is a 0.85 T bending magnet and the main optical element is a two-crystals monochromator using either Si(111) or Si(220) monocrystals so that the available energy ranges from 4 to 40 keV. The first crystal is liquid nitrogen cooled in order to avoid a thermal bump and thus preserve the energy resolution. The second crystal is dynamically bent during the energy scan in order to focus the beam in the horizontal plane. Two bendable mirrors are located before and after the monochromator, for beam-collimation (to optimize the energy resolution) and vertical focalization, respectively. During scans, the beam on the sample is kept constant in position and size (around 150 × 200 μm2, V × H). The high flux on the sample combined with the sensitivity of our 30-elements fluorescence detector allow to decrease the detection limit down to 10 ppm or around less than a monolayer. Moreover, quick-EXAFS acquisition is operational: the acquisition time may be reduced down to 30s
Twisted local systems solve the (holographic) loop equation of large-N QCD_4
We construct a holographic map from the loop equation of large-N QCD in d=2
and d=4, for planar self-avoiding loops, to the critical equation of an
equivalent effective action. The holographic map is based on two ingredients:
an already proposed holographic form of the loop equation, such that the
quantum contribution is reduced to a regularized residue; a new conformal map
from the region encircled by the based loop to a cuspidal fundamental domain in
the upper half-plane, such that the regularized residue vanishes at the cusp.
As a check, we study the first coefficient of the beta function and that part
of the second coefficient which arises from the rescaling anomaly, in passing
from the Wilsonian to the canonically normalised (holographic) effective
action.Comment: 42 pages, latex; abstract shortened and a reference added as
suggested by the referee; typos in Eq.(72,76,77,82,83,84) correcte
A classification of the torsion tensors on almost contact manifolds with B-metric
The space of the torsion (0,3)-tensors of the linear connections on almost
contact manifolds with B-metric is decomposed in 15 orthogonal and invariant
subspaces with respect to the action of the structure group. Three known
connections, preserving the structure, are characterized regarding this
classification.Comment: 17 pages, exposition clarified, references adde
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