343 research outputs found
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
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
Temperature and pressure-induced spin-state transitions in LaCoO3
We report the continuous variation of the spin moment of cobalt in LaCoO3
across its temperature and pressure-induced spin transitions evidenced with
K\beta emission spectra. The first thermal transition is best described by a
transition to an orbitally nondegenerate intermediate spin (S=1) state. In
parallel, continuous redistribution of the 3d electrons is also indicated by
partial fluorescence yield X-ray absorption spectra. At high pressure, our
study confirms that the material becomes low spin between 40 and 70 kbar at
room temperature
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
Inelastic X-ray Scattering by Electronic Excitations in Solids at High Pressure
Investigating electronic structure and excitations under extreme conditions
gives access to a rich variety of phenomena. High pressure typically induces
behavior such as magnetic collapse and the insulator-metal transition in 3d
transition metals compounds, valence fluctuations or Kondo-like characteristics
in -electron systems, and coordination and bonding changes in molecular
solids and glasses. This article reviews research concerning electronic
excitations in materials under extreme conditions using inelastic x-ray
scattering (IXS). IXS is a spectroscopic probe of choice for this study because
of its chemical and orbital selectivity and the richness of information it
provides. Being an all-photon technique, IXS has a penetration depth compatible
with high pressure requirements. Electronic transitions under pressure in 3d
transition metals compounds and -electron systems, most of them strongly
correlated, are reviewed. Implications for geophysics are mentioned. Since the
incident X-ray energy can easily be tuned to absorption edges, resonant IXS,
often employed, is discussed at length. Finally studies involving local
structure changes and electronic transitions under pressure in materials
containing light elements are briefly reviewed.Comment: submitted to Rev. Mod. Phy
Hard X-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction
The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, nondestructive, in-situ measurement ideal for this system. We have thus applied hard X-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few kilo-electron-volt energy range to probe the structure of an epitaxially grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO(001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 ± 4 Å, with the minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and X-ray reflectivity measurements. Through the STEM measurements, we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 Å. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision
Auger resonant-Raman decay after Xe L-edge photoexcitation
We have investigated resonant Auger decay of xenon following photoexcitation
of each of the three L edges under resonant-Raman conditions, which allowed us
to characterize several higher Rydberg transitions. Relative intensities for
spectator final states reached after L1−, L2−, and L3-edge excitations are
studied in detail. Thanks to state-of-the-art experimental arrangements, our
results not only reproduce the previously calculated 3d−25d and nd(n>5) state
cross sections after L3 excitation, but also allow extracting the 3d−26d
spectator state energy position and revealing its resonant behavior, blurred
by the insufficient experimental resolution in previous data sets. The 3d−26p
and 3d−27p states reached after L1 excitation as well as the 3d−25d and 3d−26d
states reached after L2 excitation are also investigated and their relative
intensities are reported and compared to ab initio Dirac-Hartree-Fock
configuration-interaction calculations. We found the signature of electronic-
state-lifetime interference effects between several coherently excited
intermediate states, due to large lifetime broadening. Electron recapture
processes are also identified above all three photoionization thresholds
Phonon anomalies at the valence transition of SmS : An inelasticX-ray scattering study under pressure
The phonon dispersion curve of SmS under pressure was studied by inelastic
x-ray scattering around the pressure-induced valence transition. A significant
softening of the longitudinal acoustic modes propagating along the [111]
direction was observed spanning a wide region from
() up to the zone boundary as
SmS becomes metallic. The largest softening occurs at the zone boundary and
stays stable up to the highest measured pressure of 80 kbar while a gradual
hardening of the low modes simultaneously appears. This phonon spectrum
indicates favorable conditions for the emergence of pressure-induced
superconductivity in SmS.Comment: 4 pages, 3 figure
Rare earth contributions to the X-ray magnetic circular dichroism at the Co K edge in rare earth-cobalt compounds investigated by multiple-scattering calculations
The X-ray magnetic circular dichroism (XMCD) has been measured at the Co K
edge in Co-hcp and R-Co compounds (R=La, Tb, Dy). The structure of the
experimental XMCD spectra in the near-edge region has been observed to be
highly sensitive to the magnetic environment of the absorbing site.
Calculations of the XMCD have been carried out at the Co K edge in Co metal,
LaCo and TbCo within the multiple-scattering framework including the
spin-orbit coupling. In the three systems, the XMCD spectra in the near-edge
region are well reproduced. The possibility to separate and quantitatively
estimate the local effects from those due to the neighboring atoms in the XMCD
cross section makes possible a more physical understanding of the spectra. The
present results emphasize the major role played by the states of the Tb
ions in the XMCD spectrum at the Co K edge in the TbCo compound.Comment: 34 pages, revtex, 10 eps figures included with epsf, after referee
revie
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