324,667 research outputs found
Pure and O-Substitution
The basic properties of distributivity and deletion of pure and o-substitution are investigated. The obtained results are applied to show preservation of recognizability in a number of surprising cases. It is proved that linear and recognizable tree series are closed under o-substitution provided that the underlying semiring is commutative, continuous, and additively idempotent. It is known that, in general, pure substitution does not preserve recognizability (not even for linear target tree series), but it is shown that recognizable linear probability distributions (represented as tree series) are closed under pure substitution
Structure and gap of low- (GaIn)O alloys
We study the electronic and local structural properties of pure and
In-substituted -GaO using density functional theory (DFT). Our
main result is that the structural energetics of In in GaO causes most
sites to be essentially inaccessible to In substitution, thus reducing the
maximum In content in thi to somewhere between 12 and 25 \% in this phase. We
also find that the gap variation with doping is essentially due to "chemical
pressure", i.e. volume variations with doping.Comment: 4 pages, 3 figure
Hybridization between the conduction band and 3d orbitals in the oxide-based diluted magnetic semiconductor InVO
The electronic structure of InVO () has been
investigated using photoemission spectroscopy (PES) and x-ray absorption
spectroscopy (XAS). The V core-level PES and XAS spectra revealed
trivalent electronic state of the V ion, consistent with the substitution of
the V ion for the In site. The V 3d partial density of states obtained by the
resonant PES technique showed a sharp peak above the O band. While the O
XAS spectrum of InVO was similar to that of InO,
there were differences in the In and 3d XAS spectra between V-doped and
pure InO. The observations give clear evidence for hybridization
between the In conduction band and the V 3d orbitals in InVO.Comment: 5 pages, 4 figure
Density functional theoretical study of Cun, Aln (n = 4–31) and copper doped aluminum clusters: Electronic properties and reactivity with atomic oxygen
A DFT study of the electronic properties of copper doped aluminum clusters and their reactivity with atomic oxygen is reported. Firstly we performed calculations for the pure Cun and Aln (n = 4, 9, 10, 13, 25 and 31) clusters and we determined their atomization energy for some frozen conformations at the B3PW91 level. The calculated work functions and M–M (M = Cu, Al) bond energies of the largest clusters are comparable with experimental data. Secondly, we focused our attention on the change of the electronic properties of the systems upon the substitution of an Al atom by a Cu one. This latter stabilizes the system as the atomization energy of the 31- atoms cluster increases of 0.31 eV when the substitution is done on the surface and of 1.18 eV when it is done inside the cluster. We show that the electronic transfer from the Al cluster to the Cu atom located at the surface is large (equal to 0.7 e) while it is negligible when Cu is inserted in the Aln cluster. Moreover, the DOS of the Al31 and Al30Cu systems are compared. Finally, the chemisorption energies of atomic oxygen in threefold sites of the Al31, Cu31 and Al30Cu clusters are calculated and discussed. We show that the chemisorption energy of O is decreasing on the bimetallic systems compared to the pure aluminum cluster
Quantum Impurities and the Neutron Resonance Peak in : Ni versus Zn
The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin
dynamics of an optimally doped high temperature superconductor is compared in
two samples with almost identical superconducting transition temperatures:
YBa(CuNi)O (T=80 K) and
YBa(CuZn)O (T=78 K). In the Ni-substituted
system, the magnetic resonance peak (which is observed at E40 meV in
the pure system) shifts to lower energy with a preserved E/T ratio
while the shift is much smaller upon Zn substitution. By contrast Zn, but not
Ni, restores significant spin fluctuations around 40 meV in the normal state.
These observations are discussed in the light of models proposed for the
magnetic resonance peak.Comment: 3 figures, submitted to PR
Singling out the effect of quenched disorder in the phase diagram of cuprates
We investigate the specific influence of structural disorder on the
suppression of antiferromagnetic order and on the emergence of cuprate
superconductivity. We single out pure disorder, by focusing on a series of
YEuBaCuO samples at fixed oxygen content
, in the range . The gradual Y/Eu isovalent substitution
smoothly drives the system through the Mott-insulator to superconductor
transition from a full antiferromagnet with N\'eel transition K at
to a bulk superconductor with superconducting critical temperature
K at , YBaCuO. The electronic properties are
finely tuned by gradual lattice deformations induced by the different cationic
radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O
chain length, as well as a controlled amount of disorder in the active
Cu(2)O bilayers. We check that internal charge transfer from the basal to
the active plane is entirely responsible for the doping of the latter and we
show that superconductivity emerges with orthorhombicity. By comparing
transition temperatures with those of the isoelectronic clean system we
deterime the influence of pure structural disorder connected with the Y/Eu
alloy.Comment: 10 pages 11 figures, submitted to Journal of Physics: Condensed
Matter, Special Issue in memory of Prof. Sandro Massid
Single crystal growth and study of the magnetic properties of the mixed spin-dimer system BaSrCrO
The compounds SrCrO and BaCrO are
insulating dimerized antiferromagnets with Cr magnetic ions. These
spin- ions form hexagonal bilayers with a strong intradimer
antiferromagnetic interaction, that leads to a singlet ground state and gapped
triplet states. We report on the effect on the magnetic properties of
SrCrO by introducing chemical disorder upon replacing Sr by
Ba. Two single crystals of BaSrCrO with
(3.33\% of ) and (6.66\%) were grown in a four-mirror type
optical floating-zone furnace. The magnetic properties on these compounds were
studied by magnetization measurements. Inelastic neutron scattering
measurements on BaSrCrO were performed in order to
determine the interaction constants and the spin gap for . The
intradimer interaction constant is found to be =5.332(2) meV, about 4\%
smaller than that of pure SrCrO, while the interdimer
exchange interaction is smaller by 6.9\%. These results indicate a
noticeable change in the magnetic properties by a random substitution effect
Asymmetric synthesis : approaches via enantiomerically pure acetal and oxazoline ligands
This thesis describes the synthesis of novel ligands that include enantiomerically
pure acetal and oxazoline moieties. These ligands are utilised in a number of metalmediated
asymmetric syntheses.
All asymmetric acetals and pyridine based acetals are synthesised in good yield in a
single step from their corresponding enantiomerically pure diols. C2 symmetric
bisacetals are investigated as ligands in the organolithium and Grignard additions to
benzaldehyde with promising results. C2 symmetric bisacetals and pyridine based
acetals are tested for their ability to induce asymmetry in copper(l) catalysed
cyclopropanation of styrene using ethyl diazoacetate and the lanthanide(lII)
catalysed Diels-Alder cycloaddition involving Danishefsky's diene with little success.
Enantiomerically pure phosphinooxazoline ligands are available in good yield in two
steps from their corresponding enantiomerically pure aminoalcohols.
Enantiomerically pure acetal substituted pyridines and phosphinooxazoline ligands
are considered in the rhodium (I) catalysed hydrosilylation of ketones. Reaction
conditions for the more successful phosphinooxazoline ligands are optimised. Using
these ligands a range of enantiomerically enriched alcohols is presented in good
yield and enantiomeric excess. Novel phosphinooxazoline ligands are applied to the palladium(O) catalysed
allylic substitution reaction with excellent enantioselectivities of the substitution
product
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