26 research outputs found
Phonon anomalies and lattice dynamics in superconducting oxychlorides CaCuOCl
We present a comprehensive study of the phonon dispersion in an underdoped,
superconducting CaCuOCl crystal. We interpret the results using
lattice dynamical calculations based on a shell model, and we compare the
results, to other hole-doped cuprates, in particular to the ones isomorphic to
LaSrCuO (LSCO). We found that an anomalous dip in the Cu-O bond
stretching dispersion develops in oxychlorides with a simultaneous marked
broadening of the mode. The broadening is maximum at that corresponds to the charge-modulations propagation vector. Our analysis
also suggests that screening effects in calculations may cause an apparent
cosine-shaped bending of the Cu-O bond-stretching dispersion along both the
( 0 0) and ( 0) directions, that is not observed on the data close to
optimal doping. This observation suggests that the discrepancy between
experimental data and \textit{ab-initio} calculations on this mode originates
from an overestimation of the doping effects on the mode
Non-adiabatic effects in the phonon dispersion of Mg 1--x Al x B 2
Superconducting MgB shows an E zone center phonon, as measured
by Raman spectroscopy, that is very broad in energy and temperature dependent.
The Raman shift and lifetime show large differences with the values elsewhere
in the Brillouin Zone measured by Inelastic X-ray Scattering (IXS), where its
dispersion can be accounted for by standard harmonic phonon theory, adding only
a moderate electron-phonon coupling. Here we show that the effects rapidly
disappear when electron-phonon coupling is switched off by Al substitution on
the Mg sites. Moreover, using IXS with very high wave-vector resolution in
MgB, we can follow the dispersion connecting the Raman and the IXS signal,
in agreement with a theory using only electron-phonon coupling but without
strong anharmonic terms. The observation is important in order to understand
the effects of electron-phonon coupling on zone center phonons modes in
MgB, but also in all metals characterized by a small Fermi velocity in a
particular direction, typical for layered compounds
Paramagnon dispersion and damping in doped NaCaCuOCl
Using Resonant Inelastic X-ray Scattering, we measure the paramagnon
dispersion and damping of undoped, antiferromagnetic CaCuOCl as
well as doped, superconducting NaCaCuOCl. Our estimation
of the spin-exchange parameter and width of the paramagnon peak at the zone
boundary confirms that no simple relation can be drawn between
these parameters and the critical temperature . Consistently with
other cuprate compounds, we show that upon doping there is a slight softening
at , but not at the zone boundary . In combination with these
measurements we perform calculations of the dynamical spin structure factor of
the one-band Hubbard model using cluster dynamical mean-field theory. The
calculations are in excellent agreement with the experiment in the undoped
case, both in terms of energy position and width. While the increase in width
is also captured upon doping, the dynamical spin structure factor shows a
sizable softening at , which provides insightful information on the
length-scale of the spin fluctuations in doped cuprates.Comment: 11 pages, 5 figures, 2 tables, V2 typo corrected in title and
reference
Bulk charge density wave and electron-phonon coupling in superconducting copper oxychlorides
Bulk charge density waves (CDWs) are now reported in nearly all
high-temperature superconducting (HTS) cuprates, with the noticeable exception
of one particular family: the copper oxychlorides. Here, we used resonant
inelastic X-ray scattering (RIXS) to reveal a bulk CDW in these materials.
Combining RIXS with non-resonant IXS, we investigate the interplay between the
lattice excitations and the CDW, and evidence bond-stretching (BS) phonon
anomalies at the CDW wave-vector. We propose that such electron-phonon
anomalies occur in the presence of dispersive charge excitations emanating from
the CDW and interacting with the BS phonon. Our observations in a structurally
simple cuprate promises to better connect bulk and surface properties and
bridge the gap between theory and experiment
Various Magnetic States for Novel Layered Cobalt Oxides CaCo 6 O 11 and BaCo 6 O 11
International audienc
Long-wavelength dispersion of transverse acoustic phonons in untwinned YBa2Cu3O7− single crystals
International audienceIn order to study the possibility of enhancing the electron-phonon couplin
Ca 2 CuO 2 Cl 2 , a redetermination from single-crystal X-ray diffraction data
International audienc
Effect of light elements on the sound velocities in solid iron: Implications for the composition of Earth's core
International audienceWe measured compressional sound velocities in light element alloys of iron (FeO, FeSi, FeS, and FeS2) at high-pressure by inelastic X-ray scattering. This dataset provides new mineralogical constraints on the composition of Earth's core, and completes the previous sets formed by the pressure–density systematics for these compounds. Based on the combination of these datasets and their comparison with radial seismic models, we propose an average composition model of the Earth's core. We show that the incorporation of small amounts of silicon or oxygen is compatible with geophysical observations and geochemical abundances. The effect of nickel on the calculated light element contents is shown to be negligible. The preferred core model derived from our measurements is an inner core which contains 2.3 wt.% silicon and traces of oxygen, and an outer core containing 2.8 wt.% silicon and around 5.3 wt.% oxygen
Anharmonic effects in MgB2? A comparative inelastic X-ray scattering and Raman study
11 pages, 10 figuresInternational audienceWe study anharmonic effects in MgB2 by comparing Inelastic X-ray and Ramanscattering together with ab-initio calculations. Using high statistics and high q resolution measurements we show that the E2g mode linewidth is independent of temperature along Gamma-A. We show, contrary to previous claims, that the Raman-peak energy decreases as a function of increasing temperature, a behaviour inconsistent with all the anharmonic ab-initio calculations of the E2g mode at Gamma available in literature. These findings and the excellent agreement between the X-ray measured and ab-initio calculated phonon spectra suggest that anharmonicity is not the main mechanism determining the temperature behaviour of the Raman-peak energy. The Raman E2g peak position and linewidth can be explained by large dynamical effects in the phonon self-energy. In light of the present findings, the commonly accepted explanation of the reduced isotope effect in terms of anharmonic effects needs to be reconsidered