919 research outputs found
Studies Of Vibrational Surface Modes In Ionic Crystals. I. Detailed Shell-model Studies For The Unrelaxed (001) Face Of Seven Crystals Having The Rocksalt Structure
We present a comprehensive, unified account of the lattice dynamics of unrelaxed (001) faces of seven crystals having the rock salt structure, three of which have an absolute gap between the acoustical and optical bulk bands (RbF, RbCl, and NaI) and four of which have overlap between these bulk bands (NaF, NaCl, LiF, and MgO). The results are obtained from calculations of the normal modes of a thin crystal film in which the ionic interactions are described by the rigid-shell model. An outline of the formulation of the shell model for the film is given, and the rapid convergence of surface-phonon frequencies with increasing film thickness is demonstrated. Surface- and pseudo surface-phonon dispersion curves and bulk bands are displayed in the form of the dispersion curves of a 15-layer film over the two-dimensional surface Brillouin zone. The surface-excess phonon density of states fs(ω) is given, and its peaks and valleys are correlated with features in the film dispersion curves. The domains of existence and the vibrational character of the more prominent surface-phonon bands are discussed. We establish the relation between the surface modes of the lattice models and the macroscopic surface waves of the dielectric and elastic continuum theories (Fuchs-Kliewer and Rayleigh waves, respectively). Comparison with experiments is made; the agreement with low-energy atom scattering from single-crystal surfaces of LiF and with inelastic neutron scattering from micro crystallite samples of MgO is particularly notable, although the latter comparison also displays some significant discrepancies. © 1977 The American Physical Society
Valency of rare earths in RIn3 and RSn3: Ab initio analysis of electric-field gradients
In RIn3 and RSn3 the rare earth (R) is trivalent, except for Eu and Yb, which
are divalent. This was experimentally determined in 1977 by perturbed angular
correlation measurements of the electric-field gradient on a 111Cd impurity. At
that time, the data were interpreted using a point charge model, which is now
known to be unphysical and unreliable. This makes the valency determination
potentially questionable. We revisit these data, and analyze them using ab
initio calculations of the electric-field gradient. From these calculations,
the physical mechanism that is responsible for the influence of the valency on
the electric-field gradient is derived. A generally applicable scheme to
interpret electric-field gradients is used, which in a transparent way
correlates the size of the field gradient with chemical properties of the
system.Comment: 10 page
Possible Competing Order-Induced Fermi Arcs in Cuprate Superconductors
We investigate the scenario of competing order (CO) induced Fermi arcs and
pseudogap in cuprate superconductors. For hole-type cuprates, both phenomena as
a function of temperature and doping level can be accounted for if the CO
vanishes at above the superconducting transition and the CO
wave-vector Q is parallel to the antinodal direction. In contrast, the absence
of these phenomena and the non-monotonic d-wave gap in electron-type cuprates
may be attributed to and a CO wave-vector Q parallel to the
nodal direction.Comment: 6 pages and 5 figures, with one figure added in the updated version.
Accepted for publication in Solid State Communications, Fast Communications.
Corresponding author: Nai-Chang Yeh ([email protected]
Hierarchical Hough all-sky search for periodic gravitational waves in LIGO S5 data
We describe a new pipeline used to analyze the data from the fifth science
run (S5) of the LIGO detectors to search for continuous gravitational waves
from isolated spinning neutron stars. The method employed is based on the Hough
transform, which is a semi-coherent, computationally efficient, and robust
pattern recognition technique. The Hough transform is used to find signals in
the time-frequency plane of the data whose frequency evolution fits the pattern
produced by the Doppler shift imposed on the signal by the Earth's motion and
the pulsar's spin-down during the observation period. The main differences with
respect to previous Hough all-sky searches are described. These differences
include the use of a two-step hierarchical Hough search, analysis of
coincidences among the candidates produced in the first and second year of S5,
and veto strategies based on a test.Comment: 7 pages, 2 figures, Amaldi08 proceedings, submitted to JPC
The Electron-Phonon Interaction in the Presence of Strong Correlations
We investigate the effect of strong electron-electron repulsion on the
electron-phonon interaction from a Fermi-liquid point of view: the strong
interaction is responsible for vertex corrections, which are strongly dependent
on the ratio. These corrections generically lead to a strong
suppression of the effective coupling between quasiparticles mediated by a
single phonon exchange in the limit. However, such effect
is not present when . Analyzing the Landau stability
criterion, we show that a sizable electron-phonon interaction can push the
system towards a phase-separation instability. A detailed analysis is then
carried out using a slave-boson approach for the infinite-U three-band Hubbard
model. In the presence of a coupling between the local hole density and a
dispersionless optical phonon, we explicitly confirm the strong dependence of
the hole-phonon coupling on the transferred momentum versus frequency ratio. We
also find that the exchange of phonons leads to an unstable phase with negative
compressibility already at small values of the bare hole-phonon coupling. Close
to the unstable region, we detect Cooper instabilities both in s- and d-wave
channels supporting a possible connection between phase separation and
superconductivity in strongly correlated systems.Comment: LateX 3.14, 04.11.1994 Preprint no.101
Pacifism in Fin-de-Siècle Austria: The Politics and Limits of Peace Activism
The late Habsburg Monarchy produced two of the most renowned peace activists of their day: Bertha von Suttner and Alfred Fried. In comparison to these two Nobel Peace laureates, the main association of Austro-pacifism – the Österreichische Friedensgesellschaft (ÖFG) – is less well known. The article concentrates on this organization, which had been founded in 1891, and it draws attention to the political and intellectual environment in which it operated. The ÖFG originated in the milieu of Austro-German liberalism, but had an ambivalent rapport with liberal politics. The Austro-pacifists' focus on supranational principles and dynastic loyalty sat uneasily with the national dimensions of Cisleithanian politics. The obstacles encountered by the ÖFG illustrate wider aspects of the political culture of fin-de-siècle Austria, ranging from the question of militarism in Austrian society to the challenges created by socialist and nationalist movements. As a whole, the article highlights the inherent limitations of Austro-pacifism, as reflected in its quest for respectability and its acceptance of the social and political order
Hensel y Gretel
Empresa: Juan A. PamiasOrquestra Simfònica del Gran Teatre del Liceu, director Siegfried MeikPrograma de la representació de Hänsel und Gretel, òpera en tres actes d'Engelbert Humperdinck amb llibret d'Adelheid WetteDe cada obra s'ha digitalitzat un programa sencer. De la resta s'han digitalitzat les parts que són diferents
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Effect of powder metallurgy synthesis parameters for pure aluminium on resultant mechanical properties
In this work, pure aluminium powders of different average particle size were compacted, sintered into discs and tested for mechanical strength at different strain rates. The effects of average particle size (15, 19, and 35 μm), sintering rate (5 and 20 °C/min) and sample indentation test speed (0.5, 0.7, and 1.0 mm/min) were examined. A compaction pressure of 332 MPa with a holding time of six minutes was used to produce the green compacted discs. The consolidated green specimens were sintered with a holding time of 4 h, a temperature of 600 °C in an argon atmosphere. The resulting sintered samples contained higher than 85% density. The mechanical properties and microstructure were characterized using indentation strength measurement tests and SEM analysis respectively. After sintering, the aluminium grain structure was observed to be of uniform size within the fractured samples. The indentation test measurements showed that for the same sintering rate, the 35 μm powder particle size provided the highest radial and tangential strength while the 15 μm powder provided the lowest strengths. Another important finding from this work was the increase in sintered sample strength which was achieved using the lower sinter heating rate, 5 °C/min. This resulted in a tangential stress value of 365 MPa which was significantly higher than achieved, 244 MPa, using the faster sintering heating rate, 20 °C/min
Search for Gravitational Wave Bursts from Soft Gamma Repeaters
We present the results of a LIGO search for short-duration gravitational
waves (GWs) associated with Soft Gamma Repeater (SGR) bursts. This is the first
search sensitive to neutron star f-modes, usually considered the most efficient
GW emitting modes. We find no evidence of GWs associated with any SGR burst in
a sample consisting of the 27 Dec. 2004 giant flare from SGR 1806-20 and 190
lesser events from SGR 1806-20 and SGR 1900+14 which occurred during the first
year of LIGO's fifth science run. GW strain upper limits and model-dependent GW
emission energy upper limits are estimated for individual bursts using a
variety of simulated waveforms. The unprecedented sensitivity of the detectors
allows us to set the most stringent limits on transient GW amplitudes published
to date. We find upper limit estimates on the model-dependent isotropic GW
emission energies (at a nominal distance of 10 kpc) between 3x10^45 and 9x10^52
erg depending on waveform type, detector antenna factors and noise
characteristics at the time of the burst. These upper limits are within the
theoretically predicted range of some SGR models.Comment: 6 pages, 1 Postscript figur
A Joint Search for Gravitational Wave Bursts with AURIGA and LIGO
The first simultaneous operation of the AURIGA detector and the LIGO
observatory was an opportunity to explore real data, joint analysis methods
between two very different types of gravitational wave detectors: resonant bars
and interferometers. This paper describes a coincident gravitational wave burst
search, where data from the LIGO interferometers are cross-correlated at the
time of AURIGA candidate events to identify coherent transients. The analysis
pipeline is tuned with two thresholds, on the signal-to-noise ratio of AURIGA
candidate events and on the significance of the cross-correlation test in LIGO.
The false alarm rate is estimated by introducing time shifts between data sets
and the network detection efficiency is measured with simulated signals with
power in the narrower AURIGA band. In the absence of a detection, we discuss
how to set an upper limit on the rate of gravitational waves and to interpret
it according to different source models. Due to the short amount of analyzed
data and to the high rate of non-Gaussian transients in the detectors noise at
the time, the relevance of this study is methodological: this was the first
joint search for gravitational wave bursts among detectors with such different
spectral sensitivity and the first opportunity for the resonant and
interferometric communities to unify languages and techniques in the pursuit of
their common goal.Comment: 18 pages, IOP, 12 EPS figure
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