2,007 research outputs found
Electron-phonon interaction in n-doped cuprates: an Inelastic X-ray Scattering study
Inelastic x-ray scattering (IXS) with very high (meV) energy resolution has
become a valuable spectroscopic tool, complementing the well established
coherent inelastic neutron scattering (INS) technique for phonon dispersion
investigations. In the study of crystalline systems IXS is a viable alternative
to INS, especially in cases where only small samples are available. Using IXS,
we have measured the phonon dispersion of Nd_{1.86}Ce_{0.14}CuO_{4+\delta}
along the [x,0,0] and [x,x,0] in-plane directions. Compared to the undoped
parent compound, the two highest longitudinal optical (LO) phonon branches are
shifted to lower energies because of Coulomb-screening effects brought about by
the doped charge carriers. An additional anomalous softening of the highest
branch is observed around q=(0.2,0,0). This anomalous softening, akin to what
has been observed in other compounds, provides evidence for a strong
electron-phonon coupling in the electron-doped high-temperature
superconductors.Comment: Proceedings of the SATT11 conference, Vietri sul Mare - Italy (March
2002); accepted for publication on Int. J. Mod. Phys.
High frequency acoustic modes in liquid gallium at the melting point
The microscopic dynamics in liquid gallium (l-Ga) at melting (T=315 K) has
been studied by inelastic x-ray scattering. We demonstrate the existence of
collective acoustic-like modes up to wave-vectors above one half of the first
maximum of the static structure factor, at variance with earlier results from
inelastic neutron scattering data [F.J. Bermejo et al. Phys. Rev. E 49, 3133
(1994)]. Despite the structural (an extremely rich polymorphism and rather
complex phase diagram) and electronic (mixed valence) peculiarity of l-Ga, its
collective dynamics is strikingly similar to the one of Van der Walls and
alkali metals liquids. This result speaks in favor of the universality of the
short time dynamics in monatomic liquids rather than of system-specific
dynamics.Comment: LaTex format, 11 pages, 4 EncapsulatedPostScript figure
Acoustic Nature of the Boson Peak in Vitreous Silica
New temperature dependent inelastic x-ray (IXS) and Raman (RS) scattering
data are compared to each other and with existing inelastic neutron scattering
data in vitreous silica (v-SiO_2), in the 300 - 1775 K region. The IXS data
show collective propagating excitations up to Q=3.5 nm^-1. The temperature
behaviour of the excitations at Q=1.6 nm^-1 matches that of the boson peak
found in INS and RS. This supports the acoustic origin of the excess of
vibrational states giving rise to the boson peak in this glass.Comment: 10 pages and 4 figure
Elastic constant dishomogeneity and dependence of the broadening of the dynamical structure factor in disordered systems
We propose an explanation for the quadratic dependence on the momentum ,
of the broadening of the acoustic excitation peak recently found in the study
of the dynamic structure factor of many real and simulated glasses. We ascribe
the observed law to the spatial fluctuations of the local wavelength of
the collective vibrational modes, in turn produced by the dishomegeneity of the
inter-particle elastic constants. This explanation is analitically shown to
hold for 1-dimensional disordered chains and satisfatorily numerically tested
in both 1 and 3 dimensions.Comment: 4 pages, RevTeX, 5 postscript figure
Crystal-like high frequency phonons in the amorphous phases of solid water
The high frequency dynamics of low- (LDA) and high-density amorphous-ice
(HDA) and of cubic ice (I_c) has been measured by inelastic X-ray Scattering
(IXS) in the 1-15 nm^{-1} momentum transfer (Q) range. Sharp phonon-like
excitations are observed, and the longitudinal acoustic branch is identified up
to Q = 8nm^{-1} in LDA and I_c and up to 5nm^{-1} in HDA. The narrow width of
these excitations is in sharp contrast with the broad features observed in all
amorphous systems studied so far. The "crystal-like" behavior of amorphous
ices, therefore, implies a considerable reduction in the number of decay
channels available to sound-like excitations which is assimilated to low local
disorder.Comment: 4 pages, 3 figure
Theoretical analysis of the magnetic circular dichroism in the 2p3d and 2p4d x-ray emission of Gd
The 2p3d and 2p4d x-ray emission spectral shapes have been calculated using a theoretical description of spin-polarized 2p photoemission and atomic multiplet calculations of the 2p3d and 2p4d radiative decay.
Emphasis is given to the use of circular-polarized x rays for the excitation process. Good agreement with experiment is found and all visible experimental structures can be explained. It is shown that because of weak multiplet effects in the intermediate state, it is possible to use the incoherent, two-step model. The angle
between the emitted x rays and the magnetization is able to affect the magnetic circular dichroism (MCD) spectral shape observed, while the angle between the incident x ray and the magnetization only affects the intensity of the MCD
Coma with Vertical Gaze Palsy: Relevance of Angio-CT in Acute Percheron Artery Syndrome
Background: A 63-year-old woman with chronic atrial fibrillation treated with warfarin was admitted to emergency for coma and complete vertical gaze palsy. Investigations: Brain CT and MRI, echo-colour Doppler sonography of the supraaortic vessels, angio-CT of the intracranial vessels, EEG, transesophageal echocardiogram, biohumoral tests. Brain CT and MRI scans showed bilateral thalamic lesions with involvement of the right midbrain; EEG showed a diffuse alpha rhythm prevalent on the posterior regions; echo-colour Doppler sonography of the supraaortic vessels showed marked reduction of blood flow in the right vertebral artery; angio-CT scans showed occlusion of the right vertebral artery and a significant filling defect of the first part of the right posterior cerebral artery (P1) from which the artery of Percheron arises. A follow-up angio-CT showed a complete recanalization of P1. Diagnosis: Percheron artery syndrome
The nature of the short wavelength excitations in vitreous silica: X-Rays Brillouin scattering study
The dynamical structure factor (S(Q,E)) of vitreous silica has been measured
by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed
exchanged energy (E) - an experimental procedure that, contrary to the usual
one at constant Q, provides spectra with much better identified inelastic
features. This allows the first direct evidence of Brillouin peaks in the
S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that
excludes the possibility that the BP marks the transition from propagating to
localised dynamics in glasses.Comment: 4 pages, 3 Postscript figures. To appear in Physical Review Letter
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