758 research outputs found
Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite
The semiconductor-insulator phase transition of the single-layer manganite
La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray
powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude
that a concomitant structural transition from tetragonal I4/mmm to orthorhombic
Cmcm phases drives this electronic transition. A detailed symmetry-mode
analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)
and X1+(A)- acting on the oxygen atoms accounts for the structural
transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in
the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes
produce an overall contraction of the other Mn site (Mn2) and expansion of the
Mn1 one. The X1+ modes are responsible for the tetragonal superlattice
(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two
different Mn sites. A strong enhancement of the scattered intensity has been
observed for these superlattice reflections close to the Mn K edge, which could
be ascribed to some degree of charge disproportion between the two Mn sites of
about 0.15 electrons. We also found that the local geometrical anisotropy of
the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone
perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type
reflections without invoking any orbital ordering.Comment: 3 tables and 10 figures; accepted in Phys. Rev.
X-ray polarization: General formalism and polarization analysis
The polarization of x-rays plays an outstanding role in experimental
techniques such as non-resonant magnetic x-ray scattering and resonant x-ray
scattering of magnetic and multipolar order. Different instrumental methods
applied to synchrotron light can transform its natural polarization into an
arbitrary polarization state. Several synchrotron applications, in particular
in the field of magnetic and resonant scattering rely on the improvement in the
signal/noise ratio or the deeper insight into the ordered state and the
scattering process made possible through these polarization techniques. Here,
we present the mathematical framework for the description of fully and
partially polarized x-rays, with some applications such as linear x-ray
polarization analysis for the determination of the scattered beam's
polarization, and the Ge K-edge resonant scattering.Comment: 12 pages, 6 figures and 1 table. To be published in proceedings of
the workshop "Resonant Elastic X-ray Scattering", Aussois, France (2011
5d-5f Electric-multipole Transitions in Uranium Dioxide Probed by Non-resonant Inelastic X-ray Scattering
Non-resonant inelastic x ray scattering (NIXS) experiments have been
performed to probe the 5d-5f electronic transitions at the uranium O(4,5)
absorption edges in uranium dioxide. For small values of the scattering vector
q, the spectra are dominated by dipole-allowed transitions encapsulated within
the giant resonance, whereas for higher values of q the multipolar transitions
of rank 3 and 5 give rise to strong and well-defined multiplet structure in the
pre-edge region. The origin of the observed non-dipole multiplet structures is
explained on the basis of many-electron atomic spectral calculations. The
results obtained demonstrate the high potential of NIXS as a bulk-sensitive
technique for the characterization of the electronic properties of actinide
materials.Comment: Submitted to Physical Review Letters on 31 December 200
Cell-surface binding domains from Clostridium cellulovorans can be used for surface display of cellulosomal scaffoldins in Lactococcus lactis
Engineering microbial strains combining efficient lignocellulose metabolization and high-value chemical production is a cutting-edge strategy towards cost-sustainable 2nd generation biorefining. Here, protein components of the Clostridium cellulovorans cellulosome were introduced in Lactococcus lactis IL1403, one of the most efficient lactic acid producers but unable to directly ferment cellulose. Cellulosomes are protein complexes with high cellulose depolymerization activity whose synergistic action is supported by scaffolding protein(s) (i.e., scaffoldins). Scaffoldins are involved in bringing enzymes close to each other and often anchor the cellulosome to the cell surface. In this study, three synthetic scaffoldins were engineered by using domains derived from the main scaffoldin CbpA and the Endoglucanase E (EngE) of the C. cellulovorans cellulosome. Special focus was on CbpA X2 and EngE S-layer homology (SLH) domains possibly involved in cell-surface anchoring. The recombinant scaffoldins were successfully introduced in and secreted by L. lactis. Among them, only that carrying the three EngE SLH modules was able to bind to the L. lactis surface although these domains lack the conserved TRAE motif thought to mediate binding with secondary cell wall polysaccharides. The synthetic scaffoldins engineered in this study could serve for assembly of secreted or surface-displayed designer cellulosomes in L. lactis
Cell-surface binding domains from Clostridium cellulovorans can be used for surface display of cellulosomal scaffoldins in Lactococcus lactis
Disentangling multipole resonances through a full x-ray polarization analysis
Complete polarization analysis applied to resonant x-ray scattering at the Cr
K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted
into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994
eV). The physical mechanism behind this phenomenon is a subtle interference
effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions,
leading to a phase shift between the respective scattering amplitudes. This
effect may be exploited to disentangle two close-lying resonances that appear
as a single peak in a conventional energy scan, in this way allowing to single
out and identify the different multipole order parameters involved.Comment: 6 pages, 6 figure
Petrographic & microstratigraphic analysis of mortar-based building materials from the Temple of Venus, Pompeii.
Recipes for mortar-based building materials may change over time and differ in
various construction and restoration phases. They normally reflect craftsmen’s
knowledge, availability of raw materials, and also the importance of the building in
which they are found. The present research focuses on mortar-based materials from
several construction and renovation phases of the Temple of Venus, at Pompeii, Italy,
in order to identify any changes over time in production recipes
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