529 research outputs found
Charge-transfer exciton in La2CuO4 probed with resonant inelastic x-ray scattering
We report a high-resolution resonant inelastic x-ray scattering study of
La2CuO4. A number of spectral features are identified that were not clearly
visible in earlier lower-resolution data. The momentum dependence of the
spectral weight and the dispersion of the lowest energy excitation across the
insulating gap have been measured in detail. The temperature dependence of the
spectral features was also examined. The observed charge transfer edge shift,
along with the low dispersion of the first charge transfer excitation are
attributed to the lattice motion being coupled to the electronic system. In
addition, we observe a dispersionless feature at 1.8 eV, which is associated
with a d-d crystal field excitation.Comment: 5 pages, 4 figure
Magnetic field dependence of charge stripe order in La2-xBaxCuO4 (x~1/8)
We have carried out a detailed investigation of the magnetic field dependence
of charge ordering in La2-xBaxCuO4 (x~1/8) utilizing high-resolution x-ray
scattering. We find that the charge order correlation length increases as the
magnetic field greater than ~5T is applied in the superconducting phase (T=2K).
The observed unusual field dependence of the charge order correlation length
suggests that the static charge stripe order competes with the superconducting
ground state in this sample.Comment: 4 pages, 4 figure
X-ray induced electronic structure change in CuIrS
The electronic structure of CuIrS has been investigated using various
bulk-sensitive x-ray spectroscopic methods near the Ir -edge: resonant
inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy in the partial
fluorescence yield (PFY-XAS) mode, and resonant x-ray emission spectroscopy
(RXES). A strong RIXS signal (0.75 eV) resulting from a charge-density-wave gap
opening is observed below the metal-insulator transition temperature of 230 K.
The resultant modification of electronic structure is consistent with the
density functional theory prediction. In the spin- and charge- dimer disordered
phase induced by x-ray irradiation below 50 K, we find that a broad peak around
0.4 eV appears in the RIXS spectrum.Comment: 4 pages and 4 figure
Resonant inelastic X-ray scattering study of overdoped LaSrCuO
Resonant inelastic x-ray scattering (RIXS) at the copper K absorption edge
has been performed for heavily overdoped samples of LaSrCuO
with and 0.30. We have observed the charge transfer and
molecular-orbital excitations which exhibit resonances at incident energies of
and 8.998 keV, respectively. From a comparison with previous
results on undoped and optimally-doped samples, we determine that the
charge-transfer excitation energy increases monotonically as doping increases.
In addition, the -dependences of the RIXS spectral weight and absorption
spectrum exhibit no clear peak at keV in contrast to results in
the underdoped samples. The low-energy ( eV) continuum excitation
intensity has been studied utilizing the high energy resolution of 0.13 eV
(FWHM). A comparison of the RIXS profiles at and
indicates that the continuum intensity exists even at in the
overdoped samples, whereas it has been reported only at and
for the sample. Furthermore, we also found an additional excitation on
top of the continuum intensity at the and positions.Comment: 7 pages, 7 figure
Tree Compression with Top Trees Revisited
We revisit tree compression with top trees (Bille et al, ICALP'13) and
present several improvements to the compressor and its analysis. By
significantly reducing the amount of information stored and guiding the
compression step using a RePair-inspired heuristic, we obtain a fast compressor
achieving good compression ratios, addressing an open problem posed by Bille et
al. We show how, with relatively small overhead, the compressed file can be
converted into an in-memory representation that supports basic navigation
operations in worst-case logarithmic time without decompression. We also show a
much improved worst-case bound on the size of the output of top-tree
compression (answering an open question posed in a talk on this algorithm by
Weimann in 2012).Comment: SEA 201
Properties of charge density waves in LaBaCuO
We report a comprehensive x-ray scattering study of charge density wave
(stripe) ordering in , for which the
superconducting is greatly suppressed. Strong superlattice reflections
corresponding to static ordering of charge stripes were observed in this
sample. The structural modulation at the lowest temperature was deduced based
on the intensity of over 70 unique superlattice positions surveyed. We found
that the charge order in this sample is described with one-dimensional charge
density waves, which have incommensurate wave-vectors (0.23, 0, 0.5) and (0,
0.23, 0.5) respectively on neighboring planes. The structural
modulation due to the charge density wave order is simply sinusoidal, and no
higher harmonics were observed. Just below the structural transition
temperature, short-range charge density wave correlation appears, which
develops into a large scale charge ordering around 40 K, close to the spin
density wave ordering temperature. However, this charge ordering fails to grow
into a true long range order, and its correlation length saturates at , and slightly decreases below about 15 K, which may be due to the onset
of two-dimensional superconductivity.Comment: 11 pages, 9 figure
Kinematic dynamo action in a sphere: Effects of periodic time-dependent flows on solutions with axial dipole symmetry
Choosing a simple class of flows, with characteristics that may be present in
the Earth's core, we study the ability to generate a magnetic field when the
flow is permitted to oscillate periodically in time. The flow characteristics
are parameterised by D, representing a differential rotation, M, a meridional
circulation, and C, a component characterising convective rolls. Dynamo action
is sensitive to these flow parameters and fails spectacularly for much of the
parameter space where magnetic flux is concentrated into small regions.
Oscillations of the flow are introduced by varying the flow parameters in
time, defining a closed orbit in the space (D,M). Time-dependence appears to
smooth out flux concentrations, often enhancing dynamo action. Dynamo action
can be impaired, however, when flux concentrations of opposite signs occur
close together as smoothing destroys the flux by cancellation.
It is possible to produce geomagnetic-type reversals by making the orbit
stray into a region where the steady flows generate oscillatory fields. In this
case, however, dynamo action was not found to be enhanced by the
time-dependence.
A novel approach is taken to solving the time-dependent eigenvalue problem,
where by combining Floquet theory with a matrix-free Krylov-subspace method we
avoid large memory requirements for storing the matrix required by the standard
approach.Comment: 22 pages, 12 figures. Geophys. Astrophys. Fluid Dynam., as accepted
(2004
Temperature dependent d-d excitations in manganites probed by resonant inelastic x-ray scattering
We report the observation of temperature dependent electronic excitations in
various manganites utilizing resonant inelastic x-ray scattering (RIXS) at the
Mn K-edge. Excitations were observed between 1.5 and 16 eV with temperature
dependence found as high as 10 eV. The change in spectral weight between 1.5
and 5 eV was found to be related to the magnetic order and independent of the
conductivity. On the basis of LDA+U and Wannier function calculations, this
dependence is associated with intersite d-d excitations. Finally, the
connection between the RIXS cross-section and the loss function is addressed.Comment: 5 pages, 5 figure
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