325 research outputs found
Atomic and itinerant effects at the transition metal x-ray absorption K-pre-edge exemplified in the case of VO
X-ray absorption spectroscopy is a well established tool for obtaining
information about orbital and spin degrees of freedom in transition metal- and
rare earth-compounds. For this purpose usually the dipole transitions of the L-
(2p to 3d) and M- (3d to 4f) edges are employed, whereas higher order
transitions such as quadrupolar 1s to 3d in the K-edge are rarely studied in
that respect. This is due to the fact that usually such quadrupolar transitions
are overshadowed by dipole allowed 1s to 4p transitions and, hence, are visible
only as minor features in the pre-edge region. Nonetheless, these features
carry a lot of valuable information, similar to the dipole L-edge transition,
which is not accessible in experiments under pressure due to the absorption of
the diamond anvil pressurecell. We recently performed a theoretical and
experimental analysis of such a situation for the metal insulator transition of
(V(1-x)Crx)2O3. Since the importance of the orbital degrees of freedom in this
transition is widely accepted, a thorough understanding of quadrupole
transitions of the vanadium K-pre-edge provides crucial information about the
underlying physics. Moreover, the lack of inversion symetry at the vanadium
site leads to onsite mixing of vanadium 3d- and 4p- states and related quantum
mechanical interferences between dipole and quadrupole transitions. Here we
present a theoretical analysis of experimental high resolution x-ray absorption
spectroscopy at the V pre-K edge measured in partial fluorescence yield mode
for single crystals. We carried out density functional as well as configuration
interaction calculations in order to capture effects coming from both,
itinerant and atomic limits
Understanding the complex phase diagram of uranium: the role of electron-phonon coupling
We report an experimental determination of the dispersion of the soft phonon
mode along [1,0,0] in uranium as a function of pressure. The energies of these
phonons increase rapidly, with conventional behavior found by 20 GPa, as
predicted by recent theory. New calculations demonstrate the strong pressure
(and momentum) dependence of the electron-phonon coupling, whereas the
Fermi-surface nesting is surprisingly independent of pressure. This allows a
full understanding of the complex phase diagram of uranium, and the interplay
between the charge-density wave and superconductivity
Resonant inelastic x-ray scattering in single-crystal superconducting PrFeAsO0.7
Resonant inelastic x-ray scattering (RIXS) spectra at the Fe K-edge were
measured for a single crystal of the iron oxypnictide superconductor PrFeAsO0.7
(Tc=42 K). They disclose a weak, broad feature centered around 4.5 eV energy
loss, which is slightly resonantly enhanced when the incident energy is tuned
in the vicinity of the 4p white line. We tentatively ascribe it to the
charge-transfer excitation between As 4p and Fe 3d.Comment: 2 pages, 2 figure
Inequivalent routes across the Mott transition in V2O3 explored by X-ray absorption
The changes in the electronic structure of V2O3 across the metal-insulator
transition induced by temperature, doping and pressure are identified using
high resolution x-ray absorption spectroscopy at the V pre K-edge. Contrary to
what has been taken for granted so far, the metallic phase reached under
pressure is shown to differ from the one obtained by changing doping or
temperature. Using a novel computational scheme, we relate this effect to the
role and occupancy of the a1g orbitals. This finding unveils the inequivalence
of different routes across the Mott transition in V2O
The First Ultra-cool Brown Dwarf Discovered by the Wide-field Infrared Survey Explorer
We report the discovery of the first new ultra-cool brown dwarf (BDs) found with the Wide-field Infrared Survey
Explorer (WISE). The object’s preliminary designation is WISEPC J045853.90+643451.9. Follow-up spectroscopy
with the LUCIFER instrument on the Large Binocular Telescope indicates that it is a very late-type T dwarf with a
spectral type approximately equal to T9. Fits to an IRTF/SpeX 0.8–2.5 μm spectrum to the model atmospheres of
Marley and Saumon indicate an effective temperature of approximately 600 K as well as the presence of vertical
mixing in its atmosphere. The new BD is easily detected by WISE, with a signal-to-noise ratio of ~36 at 4.6 μm.
Current estimates place it at a distance of 6–10 pc. This object represents the first in what will likely be hundreds of
nearby BDs found by WISE that will be suitable for follow-up observations, including those with the James Webb
Space Telescope. One of the two primary scientific goals of the WISE mission is to find the coolest, closest stars to
our Sun; the discovery of this new BD proves that WISE is capable of fulfilling this objective
a tool to disentangle overlapping core-excited states
We have measured resonant-Auger decay following Cl 1s−1 excitations in HCl and
CH3Cl molecules, and extracted the pseudo-cross sections of different Cl 2p−2
final states. These cross sections show clear evidence of shake processes as
well as contributions of electronic state-lifetime interference (ELI). To
describe the spectra we developed a fit approach that takes into account ELI
contributions and ultrafast nuclear dynamics in dissociative core-excited
states. Using this approach we utilized the ELI contributions to obtain the
intensity ratios of the overlapping states Cl 1s−14pπ/1s−14pσ in HCl and Cl
1s−14pe/1s−14pa1 in CH3Cl. The experimental value for HCl is compared with
theoretical results showing satisfactory agreement
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