17,842 research outputs found
Incommensurate phonon anomaly and the nature of charge density waves in cuprates
While charge density wave (CDW) instabilities are ubiquitous to
superconducting cuprates, the different ordering wavevectors in various cuprate
families have hampered a unified description of the CDW formation mechanism.
Here we investigate the temperature dependence of the low energy phonons in the
canonical CDW ordered cuprate LaBaCuO. We discover
that the phonon softening wavevector associated with CDW correlations becomes
temperature dependent in the high-temperature precursor phase and changes from
a wavevector of 0.238 reciprocal space units (r.l.u.) below the ordering
transition temperature up to 0.3~r.l.u. at 300~K. This high-temperature
behavior shows that "214"-type cuprates can host CDW correlations at a similar
wavevector to previously reported CDW correlations in non-"214"-type cuprates
such as YBaCuO. This indicates that cuprate CDWs may
arise from the same underlying instability despite their apparently different
low temperature ordering wavevectors.Comment: Accepted in Phys. Rev. X; 9 pages; 5 figures; 3 pages of
supplementary materia
Bond stretching phonon softening and angle-resolved photoemission kinks in optimally doped Bi2Sr1.6La0.4Cu2O6 superconductors
We report the first measurement of the optical phonon dispersion in optimally
doped single layer Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering.
We found a strong softening of the Cu-O bond stretching phonon at about
q=(0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates.
A direct comparison with angle-resolved photoemission spectroscopy measurements
taken on the same sample, revealed an excellent agreement in terms of energy
and momentum between the ARPES nodal kink and the soft part of the bond
stretching phonon. Indeed, we find that the momentum space where a 63 meV kink
is observed can be connected with a vector q=(xi,0,0) with xi~0.22, which
corresponds exactly to the soft part of the bond stretching phonon mode. This
result supports an interpretation of the ARPES kink in terms of electron-phonon
coupling.Comment: submited to PR
Momentum-resolved lattice dynamics of parent and electron-doped SrIrO
The mixing of orbital and spin character in the wave functions of the
iridates has led to predictions of strong couplings among their lattice,
electronic and magnetic degrees of freedom. As well as realizing a novel
spin-orbit assisted Mott-insulating ground state, the perovskite iridate
SrIrO has strong similarities with the cuprate LaCuO,
which on doping hosts a charge-density wave that appears intimately connected
to high-temperature superconductivity. These phenomena can be sensitively
probed through momentum-resolved measurements of the lattice dynamics, made
possible by meV-resolution inelastic x-ray scattering. Here we report the first
such measurements for both parent and electron-doped SrIrO. We find
that the low-energy phonon dispersions and intensities in both compounds are
well described by the same nonmagnetic density functional theory calculation.
In the parent compound, no changes of the phonons on magnetic ordering are
discernible within the experimental resolution, and in the doped compound no
anomalies are apparent due to charge-density waves. These measurements extend
our knowledge of the lattice properties of (SrLa)IrO
and constrain the couplings of the phonons to magnetic and charge order.Comment: 8 pages, 6 figures (+ 12 pages, 6 figures of supplemental material
Asymmetries on red giant branch surfaces from CHARA/MIRC optical interferometry
Context. Red giant branch (RGB) stars are very bright objects in galaxies and
are often used as standard candles. Interferometry is the ideal tool to
characterize the dynamics and morphology of their atmospheres. Aims. We aim at
precisely characterising the surface dynamics of a sample of RGB stars.
Methods. We obtained interferometric observations for three RGB stars with the
MIRC instrument mounted at the CHARA interfer- ometer. We looked for
asymmetries on the stellar surfaces using limb-darkening models. Results. We
measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas,
HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas.
We detected departures from the centrosymmetric case for all three stars with
the tendency of a greater effect for lower logg of the sample. We explored the
causes of this signal and conclude that a possible explanation to the
interferometric signal is the convection-related and/or the magnetic-related
surface activity. However, it is necessary to monitor these stars with new
observations, possibly coupled with spectroscopy, in order to firmly establish
the cause.Comment: Accepted for publication as a Letter in Astronomy and Astrophysics,
section 1. Letters to the Editor. The official date of acceptance is
06/03/201
Social Effects in Science: Modelling Agents for a Better Scientific Practice
Science is a fundamental human activity and we trust its results because it
has several error-correcting mechanisms. Its is subject to experimental tests
that are replicated by independent parts. Given the huge amount of information
available, scientists have to rely on the reports of others. This makes it
possible for social effects to influence the scientific community. Here, an
Opinion Dynamics agent model is proposed to describe this situation. The
influence of Nature through experiments is described as an external field that
acts on the experimental agents. We will see that the retirement of old
scientists can be fundamental in the acceptance of a new theory. We will also
investigate the interplay between social influence and observations. This will
allow us to gain insight in the problem of when social effects can have
negligible effects in the conclusions of a scientific community and when we
should worry about them.Comment: 14 pages, 5 figure
Effect of K Doping on Phonons in Ba1-xKxFe2As2
The lattice dynamics of Ba1-xKxFe2As2 (x = 0.00, 0.27) have been studied by
inelastic X-ray scattering measurement at room temperature. K doping induces
the softening and broadening of phonon modes in the energy range E = 10-15 meV.
Analysis with a Born-von Karman force-constant model indicates that the
softening results from reduced interatomic force constants around (Ba,K) sites
following the displacement of divalent Ba by monovalent K. The phonon
broadening may be explained by the local distortions induced by the K
substitution. Extra phonon modes are observed around the wave vector q =
(0.5,0,0) at E = 16.5 meV for the x = 0.27 sample. These modes may arise either
from the local disorder induced by K doping or from electron-phonon coupling.Comment: J. Phys. Soc. Jpn. (in press
Study of Envelope Velocity Evolution of Type Ib-c Core-Collapse Supernovae from Observations of XRF 080109 / SN 2008D and GRB 060218 / SN 2006aj with BTA
Results of modeling the spectra of two supernovae SN 2008D and SN 2006aj
related to the X-ray flash XRF 080109 and gamma-ray burst GRB / XRF 060218,
respectively, are studied. The spectra were obtained with the 6-meter BTA
telescope of the Special Astrophysical Observatory of the Russian Academy of
Sciences in 6.48 and 27.61 days after the explosion of SN 2008D, and in 2.55
and 3.55 days after the explosion of SN 2006aj. The spectra were interpreted in
the Sobolev approximation with the SYNOW code. An assumption about the presence
of envelopes around the progenitor stars is confirmed by an agreement between
the velocities of lines interpreted as hydrogen and helium, and the empiric
power-law velocity drop with time for the envelopes of classic core-collapse
supernovae. Detection of a P Cyg profile of the H-beta line in the spectra of
optical afterglows of GRBs can be a determinative argument in favor of this
hypothesis.Comment: 12 pages, 6 figures, accepted for publication in Astrophysical
Bulletin
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