613 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
Excitonic quasiparticles in a spin-orbit Mott insulator
In condensed matter systems, out of a large number of interacting degrees of
freedom emerge weakly coupled particles, in terms of which most physical
properties are described. For example, Landau quasiparticles (QP) determine all
electronic properties of a normal metal. The lack of identification of such QPs
is major barrier for understanding myriad exotic properties of correlated
electrons, such as unconventional superconductivity and non-Fermi liquid
behaviours. Here, we report the observation of a composite particle in a Mott
insulator Sr2IrO4---and exciton dressed with magnons---that propagates with the
canonical characteristics of a QP: a finite QP residue and a lifetime longer
than the hopping time scale. The dynamics of this charge-neutral bosonic
excitation mirrors the fundamental process of the analogous one-hole
propagation in the background of ordered spins, for which a well-defined QP has
never been observed. The much narrower linewidth of the exciton reveals the
same intrinsic dynamics that is obscured for the hole and is intimately related
to the mechanism of high temperature superconductivity.Comment: submitted versio
Cognitive Load Theory: Advances in Research on Worked Examples, Animations, and Cognitive Load Measurement
The contributions to this special issue document some recent advances of cognitive load theory, and are based on contributions to the Third International Cognitive Load Theory Conference (2009), Heerlen, The Netherlands. The contributions focus on developments in example-based learning, amongst others on the effects of integrating worked examples in cognitive tutoring systems; specify the effects of transience on cognitive load and why segmentation may help counteract these effects in terms of the role of time in working memory load; and discuss the possibilities offered by electroencephalography (EEG) to provide a continuous and objective measure of cognitive load. This article provides a short introduction to the contributions in this issue
Quartz-based flat-crystal resonant inelastic x-ray scattering spectrometer with sub-10 meV energy resolution
Continued improvement of the energy resolution of resonant inelastic x-ray
scattering (RIXS) spectrometers is crucial for fulfilling the potential of this
technique in the study of electron dynamics in materials of fundamental and
technological importance. In particular, RIXS is the only alternative tool to
inelastic neutron scattering capable of providing fully momentum resolved
information on dynamic spin structures of magnetic materials, but is limited to
systems whose magnetic excitation energy scales are comparable to the energy
resolution. The state-of-the-art spherical diced crystal analyzer optics
provides energy resolution as good as 25 meV but has already reached its
theoretical limit. Here, we demonstrate a novel sub-10meV RIXS spectrometer
based on flat-crystal optics at the Ir-L absorption edge (11.215 keV)
that achieves an analyzer energy resolution of 3.9meV, very close to the
theoretical value of 3.7meV. In addition, the new spectrometer allows
efficient polarization analysis without loss of energy resolution. The
performance of the instrument is demonstrated using longitudinal acoustical and
optical phonons in diamond, and magnon in SrIrO. The novel
sub-10meV RIXS spectrometer thus provides a window into magnetic
materials with small energy scales
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
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
Doping dependence of charge-transfer excitations in La_{2-x}Sr_xCuO_4
We report a resonant inelastic x-ray scattering (RIXS) study of the doping
dependence of charge-transfer excitations in . The mome
ntum dependence of these charge excitations are studied over the whole
Brillouin zone in underdoped (x=0.05) and optimally doped (x=0.17) samples, and
compared with that of the undoped (x=0) sample. We observe a large change in
the RIXS spectra between the x=0 and x=0.17 sample, while the RIXS spectra of
the x=0.05 sample are similar to that of the x=0 sample. The most prominent
effect of doped-holes on the charge excitation spectra is the appearance of a
continuum of intensity, which exhibits a strong momentum-dependence below 2 eV.
For the x=0.17 sample, some of the spectral weight from the lowest-lying
charge-transfer excitation of the undoped compound is transferred to the
continuum intensity below the gap, in agreement with earlier optical studies.
However, the higher energy charge-transfer excitation carries significant
spectral weight even for the x=0.17 sample. The doping dependence of the
dispersion of this charge-transfer excitation is also discussed and compared
with recent theoretical calculations.Comment: 7 pages, 6 figures, to appear in Phys. Rev.
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