1,321 research outputs found
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
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
Zero kinetic energy-pulsed field ionization and resonance enhanced multiphoton ionization photoelectron spectroscopy: Ionization dynamics of Rydberg states in HBr
The results of rotationally resolved resonance enhanced multiphoton ionization photoelectron spectroscopy and zero kinetic energy‐pulsed field ionization studies on HBr via various rotational levels of the F^ 1Δ_2 and f^ 3Δ_2 Rydberg states are reported. These studies lead to an accurate determination of the lowest ionization threshold as 94 098.9±1 cm^(−1). Observed rotational and spin–orbit branching ratios are compared to the results of ab initio calculations. The differences between theory and experiment highlight the dominant role of rotational and spin–orbit interactions for the dynamic properties of the high‐n Rydberg states involved in the pulsed field ionization process
Electronic structure and spectroscopy of O2 and O2+
We carried out a comprehensive SCF MRD--CI ab initio study of the
electronic
structure of O and O. Potential energy curves (PECs) of
about 150
electronic states of O and
about
100 of O, as well as a number of
states of
O were computed. The cc--pVQZ basis set augmented with diffuse
functions was employed. Spectroscopic parameters
( , ,
IP, etc.) are reported.
A preliminary sample of the results will be presented. The electronic absorption
spectrum of O has proved difficult to analyze/interpret
due to the unusually large number of electronic states which arise
from
the peculiar open--shell structure of both the oxygen atomic fragments and the
O molecule. For instance, there are 62 valence molecular electronic
states which
correlate to the six lowest dissociation limits resulting from
the three valence O atom fragment states (P, D, S).
In addition, there are several Rydberg series
converging to the X ground ionic state and to the lowest
two excited states of the cation, a and A.
Furthermore, a number of interactions of various types among several electronic states result in rovibronic perturbations
which manifest themselves, e.g., as irregular vibronic structure,
hence severely complicating the
assignment of the absorption features and the analysis and
interpretation of the spectrum.
An overview of the electronic states and spectroscopy of O will be presented.
A chief motivation of this study of O was
to try to provide a theoretical insight on the nature,
energetic position, shape, and dissociation asymptotes,
of electronic states located in the 4 eV energy region
encompassed between the O ground state X (IP eV)
and the first excited state of the cation a
(IP eV).
This in order to aid in the interpretation of experimental data
related to the mechanism(s) of the neutral dissociation of the O
(Rydberg) superexcited states,
which competes with autoionization.
We are currently striving to compute PECs of relatively highly
excited states of O located in the 12--16 eV energy region which might
help to visualize possible pathways for the
neutral XUV photodissociation of the I, I and I
superexcited states of O leading to the O(P) + O(S, S) dissociation limits.Ope
Large phenotype jumps in biomolecular evolution
By defining the phenotype of a biopolymer by its active three-dimensional
shape, and its genotype by its primary sequence, we propose a model that
predicts and characterizes the statistical distribution of a population of
biopolymers with a specific phenotype, that originated from a given genotypic
sequence by a single mutational event. Depending on the ratio g0 that
characterizes the spread of potential energies of the mutated population with
respect to temperature, three different statistical regimes have been
identified. We suggest that biopolymers found in nature are in a critical
regime with g0 in the range 1-6, corresponding to a broad, but not too broad,
phenotypic distribution resembling a truncated Levy flight. Thus the biopolymer
phenotype can be considerably modified in just a few mutations. The proposed
model is in good agreement with the experimental distribution of activities
determined for a population of single mutants of a group I ribozyme.Comment: to appear in Phys. Rev. E; 7 pages, 6 figures; longer discussion in
VII, new fig.
Observation of Feshbach resonances in an ultracold gas of Cr
We have observed Feshbach resonances in elastic collisions between ultracold
Cr atoms. This is the first observation of collisional Feshbach
resonances in an atomic species with more than one valence electron. The zero
nuclear spin of Cr and thus the absence of a Fermi-contact interaction
leads to regularly-spaced resonance sequences. By comparing resonance positions
with multi-channel scattering calculations we determine the s-wave scattering
length of the lowest potentials to be
\unit[112(14)]{a_0}, \unit[58(6)]{a_0} and -\unit[7(20)]{a_0} for S=6, 4,
and 2, respectively, where a_{0}=\unit[0.0529]{nm}.Comment: 4 pages, 2 figures, 1 tabl
Standard monomial theory for wonderful varieties
A general setting for a standard monomial theory on a multiset is introduced
and applied to the Cox ring of a wonderful variety. This gives a degeneration
result of the Cox ring to a multicone over a partial flag variety. Further, we
deduce that the Cox ring has rational singularities.Comment: v3: 20 pages, final version to appear on Algebras and Representation
Theory. The final publication is available at Springer via
http://dx.doi.org/10.1007/s10468-015-9586-z. v2: 20 pages, examples added in
Section 3 and in Section
Phonons in the multiferroic langasite BaNbFeSiO : evidences for symmetry breaking
The chiral langasite BaNbFeSiO is a multiferroic
compound. While its magnetic order below T=27 K is now well characterised,
its polar order is still controversial. We thus looked at the phonon spectrum
and its temperature dependence to unravel possible crystal symmetry breaking.
We combined optical measurements (both infrared and Raman spectroscopy) with ab
initio calculations and show that signatures of a polar state are clearly
present in the phonon spectrum even at room temperature. An additional symmetry
lowering occurs below 120~K as seen from emergence of softer phonon modes in
the THz range. These results confirm the multiferroic nature of this langasite
and open new routes to understand the origin of the polar state
Formation of collective spins in frustrated clusters
Using magnetization, specific heat and neutron scattering measurements, as
well as exact calculations on realistic models, the magnetic properties of the
\lacuvo compound are characterized on a wide temperature range. At high
temperature, this oxide is well described by strongly correlated atomic =1/2
spins while decreasing the temperature it switches to a set of weakly
interacting and randomly distributed entangled pseudo spins and
. These pseudo-spins are built over frustrated clusters, similar to
the kagom\'e building block, at the vertices of a triangular superlattice, the
geometrical frustration intervening then at different scales.Comment: 10 page
On spherical twisted conjugacy classes
Let G be a simple algebraic group over an algebraically closed field of good
odd characteristic, and let theta be an automorphism of G arising from an
involution of its Dynkin diagram. We show that the spherical theta-twisted
conjugacy classes are precisely those intersecting only Bruhat cells
corresponding to twisted involutions in the Weyl group. We show how the
analogue of this statement fails in the triality case. We generalize to good
odd characteristic J-H. Lu's dimension formula for spherical twisted conjugacy
classes.Comment: proof of Lemma 6.4 polished. The journal version is available at
http://www.springerlink.com/content/k573l88256753640
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