377 research outputs found
The role of oxygen vacancies on the structure and the density of states of iron doped zirconia
In this paper we study, both with theoretical and experimental approach, the
effect of iron doping in zirconia. Combining density functional theory (DFT)
simulations with the experimental characterization of thin films, we show that
iron is in the Fe3+ oxidation state and accordingly that the films are rich in
oxygen vacancies (VO). VO favor the formation of the tetragonal phase in doped
zirconia (ZrO2:Fe) and affect the density of state at the Fermi level as well
as the local magnetization of Fe atoms. We also show that the Fe(2p) and Fe(3p)
energy levels can be used as a marker for the presence of vacancies in the
doped system. In particular the computed position of the Fe(3p) peak is
strongly sensitive to the VO to Fe atoms ratio. A comparison of the theoretical
and experimental Fe(3p) peak position suggests that in our films this ratio is
close to 0.5. Besides the interest in the material by itself, ZrO2:Fe
constitutes a test case for the application of DFT on transition metals
embedded in oxides. In ZrO2:Fe the inclusion of the Hubbard U correction
significantly changes the electronic properties of the system. However the
inclusion of this correction, at least for the value U = 3.3 eV chosen in the
present work, worsen the agreement with the measured photo-emission valence
band spectra.Comment: 24 pages, 8 figure
Balanced superprojective varieties
We first review the definition of superprojective spaces from the functor-of-points perspective. We derive the relation between superprojective spaces and supercosets in the framework of the theory of sheaves. As an application of the geometry of superprojective spaces, we extend Donaldson\u2019s definition of balanced manifolds to supermanifolds and we derive the new conditions of a balanced supermanifold. We apply the construction to superpoints viewed as submanifolds of superprojective spaces. We conclude with a list of open issues and interesting problems that can be addressed in the present context
Cech and de Rham Cohomology of Integral Forms
We present a study on the integral forms and their Cech/de Rham cohomology.
We analyze the problem from a general perspective of sheaf theory and we
explore examples in superprojective manifolds. Integral forms are fundamental
in the theory of integration in supermanifolds. One can define the integral
forms introducing a new sheaf containing, among other objects, the new basic
forms delta(dtheta) where the symbol delta has the usual formal properties of
Dirac's delta distribution and acts on functions and forms as a Dirac measure.
They satisfy in addition some new relations on the sheaf. It turns out that the
enlarged sheaf of integral and "ordinary" superforms contains also forms of
"negative degree" and, moreover, due to the additional relations introduced,
its cohomology is, in a non trivial way, different from the usual superform
cohomology.Comment: 20 pages, LaTeX, we expanded the introduction, we add a complete
analysis of the cohomology and we derive a new duality between cohomology
group
Stabilization of tetragonal/cubic phase in Fe doped Zirconia grown by atomic layer deposition
Achieving high temperature ferromagnetism by doping transition metals thin
films is seen as a viable approach to integrate spin-based elements in
innovative spintronic devices. In this work we investigated the effect of Fe
doping on structural properties of ZrO2 grown by atomic layer deposition (ALD)
using Zr(TMHD)4 for Zr and Fe(TMHD)3 for Fe precursors and ozone as oxygen
source. The temperature during the growth process was fixed at 350{\deg}C. The
ALD process was tuned to obtain Fe doped ZrO2 films with uniform chemical
composition, as seen by time of flight secondary ion mass spectrometry. The
control of Fe content was effectively reached, by controlling the ALD precursor
pulse ratio, as checked by X-ray photoemission spectroscopy (XPS) and
spectroscopic ellipsometry. From XPS, Fe was found in Fe3+ chemical state,
which maximizes the magnetization per atom. We also found, by grazing incidence
X-ray diffraction, that the inclusion of Fe impurities in ZrO2 induces
amorphization in thin ZrO2 films, while stabilizes the high temperature
crystalline tetragonal/cubic phase after rapid thermal annealing at 600{\deg}C.Comment: 11 pages, 7 figures, 1 Tabl
Measurement of the temperature of an ultracold ion source using time-dependent electric fields
We report on a measurement of the characteristic temperature of an ultracold
rubidium ion source, in which a cloud of laser-cooled atoms is converted to
ions by photo-ionization. Extracted ion pulses are focused on a detector with a
pulsed-field technique. The resulting experimental spot sizes are compared to
particle-tracking simulations, from which a source temperature
mK and the corresponding transversal reduced emittance m rad are determined. We find that this result is
likely limited by space charge forces even though the average number of ions
per bunch is 0.022.Comment: 8 pages, 11 figure
Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals
The femtosecond optical pump-probe technique was used to study dynamics of
photoexcited electrons and coherent optical phonons in transition metals Zn and
Cd as a function of temperature and excitation level. The optical response in
time domain is well fitted by linear combination of a damped harmonic
oscillation because of excitation of coherent phonon and a
subpicosecond transient response due to electron-phonon thermalization. The
electron-phonon thermalization time monotonically increases with temperature,
consistent with the thermomodulation scenario, where at high temperatures the
system can be well explained by the two-temperature model, while below
50 K the nonthermal electron model needs to be applied. As the
lattice temperature increases, the damping of the coherent phonon
increases, while the amplitudes of both fast electronic response and the
coherent phonon decrease. The temperature dependence of the damping of
the phonon indicates that population decay of the coherent optical
phonon due to anharmonic phonon-phonon coupling dominates the decay process. We
present a model that accounts for the observed temperature dependence of the
amplitude assuming the photoinduced absorption mechanism, where the signal
amplitude is proportional to the photoinduced change in the quasiparticle
density. The result that the amplitude of the phonon follows the
temperature dependence of the amplitude of the fast electronic transient
indicates that under the resonant condition both electronic and phononic
responses are proportional to the change in the dielectric function.Comment: 10 pages, 9 figures, to appear in Physical Review
Anharmonic Decay of Vibrational States in Amorphous Silicon
Anharmonic decay rates are calculated for a realistic atomic model of
amorphous silicon. The results show that the vibrational states decay on
picosecond timescales and follow the two-mode density of states, similar to
crystalline silicon, but somewhat faster. Surprisingly little change occurs for
localized states. These results disagree with a recent experiment.Comment: 10 pages, 4 Postscript figure
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