49 research outputs found
X-ray Absorption Linear Dichroism at the Ti K-edge of TiO2 anatase single crystal
Anatase TiO2 (a-TiO2) exhibits a strong X-ray absorption linear dichroism
with the X-ray incidence angle in the pre-edge, the XANES and the EXAFS at the
titanium K-edge. In the pre-edge region the behaviour of the A1-A3 and B peaks,
originating from the 1s-3d transitions, is due to the strong -orbital
polarization and strong orbital mixing. An unambiguous assignment of the
pre-edge peak transitions is made in the monoelectronic approximation with the
support of ab initio finite difference method calculations and spherical tensor
analysis in quantitative agreement with the experiment. It is found that A1 is
mostly an on-site 3d-4p hybridized transition, while peaks A3 and B are
non-local transitions, with A3 being mostly dipolar and influence by the 3d-4p
intersite hybridization, while B is due to interactions at longer range.
Finally, peak A2 which was previously assigned to a transition involving
pentacoordinated titanium atoms exhibits a quadrupolar angular evolution with
incidence angle. These results pave the way to the use of the pre-edge peaks at
the K-edge of a-TiO2 to characterize the electronic structure of related
materials and in the field of ultrafast XAS where the linear dichroism can be
used to compare the photophysics along different axes.Comment: 43 pages, 19 figure
Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature
We present high-resolution capacitance dilatometry studies from 5 - 500 K of
untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large
contributions to the thermal expansivities due to O-ordering are found for x ~
6.95, which disappear below a kinetic glass-like transition near room
temperature. The kinetics at this glass transition is governed by an energy
barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering
studies. Using thermodynamic arguments, we show that O-ordering in the Y123
system is particularly sensitive to uniaxial pressure (stress) along the chain
axis and that the lack of well-ordered chains in Nd123 and La123 is most likely
a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR
Statistical properties of microcracking in polyurethane foams under tensile test, influence of temperature and density
We report tensile failure experiments on polyurethane (PU) foams. Experiments
have been performed by imposing a constant strain rate. We work on
heterogeneous materials for whom the failure does not occur suddenly and can
develop as a multistep process through a succession of microcracks that end at
pores. The acoustic energy and the waiting times between acoustic events follow
power-law distributions. This remains true while the foam density is varied.
However, experiments at low temperatures (PU foams more brittle) have not
yielded power-laws for the waiting times. The cumulative acoustic energy has no
power law divergence at the proximity of the failure point which is
qualitatively in agreement with other experiments done at imposed strain. We
notice a plateau in cumulative acoustic energy that seems to occur when a
single crack starts to propagate
Low temperature relaxations associated to quantum tunnelling of H in Sc and Y
Acoustic spectroscopy and electrical resistivity measurements have been employed to investigate the two relaxation processes occurring at liquid He temperature in very dilute YâH and ScâH alloys, and ascribed to H tunnelling systems near single or paired O traps. At very low H concentration, the peak due to the OâH tunnelling system becomes extremely narrow and is describable in terms of incoherent tunnelling with phonon assisted transitions