4,766 research outputs found
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
Observation of Umklapp processes in non-crystalline materials
Umklapp processes are known to exist in cristalline materials, where they
control important properties such as thermal conductivity, heat capacity and
electrical conductivity. In this work we report the provocative observation of
Umklapp processes in a non-periodical system, namely liquid Lithium. The lack
of a well defined periodicity seems then not to prevent the existence of these
scattering processes mechanisms provided that the local order of the systems
i.e. the maxima of the static structure factor supply the equivalent of a
reciprocal lattice vector in the case of cristalline materials.Comment: 13 pages P
Non-dynamic origin of the acoustic attenuation at high frequency in glasses
The sound attenuation in the THz region is studied down to T=16 K in glassy
glycerol by inelastic x-ray scattering. At striking variance with the decrease
found below 100 K in the GHz data, the attenuation in the THz range does not
show any T dependence. This result i) indicates the presence of two different
attenuation mechanisms, active respectively in the high and low frequency
limits; ii) demonstrates the non-dynamic origin of the attenuation of THz sound
waves, and confirms a similar conclusion obtained in SiO2 glass by molecular
dynamics; and iii) supports the low frequency attenuation mechanism proposed by
Fabian and Allen (Phys.Rev.Lett. 82, 1478 (1999)).Comment: 3 pages, 5 Figures, To be published in PR
Frustration and sound attenuation in structural glasses
Three classes of harmonic disorder systems (Lennard-Jones like glasses,
percolators above threshold, and spring disordered lattices) have been
numerically investigated in order to clarify the effect of different types of
disorder on the mechanism of high frequency sound attenuation. We introduce the
concept of frustration in structural glasses as a measure of the internal
stress, and find a strong correlation between the degree of frustration and the
exponent alpha that characterizes the momentum dependence of the sound
attenuation . In particular, alpha decreases from
about d+1 in low-frustration systems (where d is the spectral dimension), to
about 2 for high frustration systems like the realistic glasses examined.Comment: Revtex, 4 pages including 4 figure
Roadmap of ultrafast x-ray atomic and molecular physics
X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Ångstrom, and HHG provides unprecedented time resolution (~50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ~280 eV (44 Ångstroms) and the bond length in methane of ~1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science
The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica
Spectroscopic results on low frequency excitations of densified silica are
presented and related to characteristic thermal properties of glasses. The end
of the longitudinal acoustic branch is marked by a rapid increase of the
Brillouin linewidth with the scattering vector. This rapid growth saturates at
a crossover frequency Omega_co which nearly coincides with the center of the
boson peak. The latter is clearly due to additional optic-like excitations
related to nearly rigid SiO_4 librations as indicated by hyper-Raman
scattering. Whether the onset of strong scattering is best described by
hybridization of acoustic modes with these librations, by their elastic
scattering (Rayleigh scattering) on the local excitations, or by soft
potentials remains to be settled.Comment: 14 pages, 6 figures, to be published in a special issue of J. Phys.
Condens. Matte
Vibrational spectrum of topologically disordered systems
The topological nature of the disorder of glasses and supercooled liquids
strongly affects their high-frequency dynamics. In order to understand its main
features, we analytically studied a simple topologically disordered model,
where the particles oscillate around randomly distributed centers, interacting
through a generic pair potential. We present results of a resummation of the
perturbative expansion in the inverse particle density for the dynamic
structure factor and density of states. This gives accurate results for the
range of densities found in real systems.Comment: Completely rewritten version, accepted in Physical Review Letter
Hamas: da movimento politico-religioso a referente istituzionale della causa nazionale palestinese.
Nella presente tesi di dottorato ho analizzato l'evoluzione ideologico-politica del movimento di resistenza islamica (Hamas) sin dalle sue origini. A tal proposito si è reso necessario uno studio di carattere filologico di alcuni documenti chiave del movimento stesso (statuto, programma elettorale e documento della concordia nazionale), attingendo alle fonti primarie in lingua araba. Tale analisi è stata compiuta focalizzando sia il contesto storico-politico nel quale Hamas si è consolidato, che i rapporti che il movimento ha tenuto con le altre formazioni politiche palestinesi, in particolar modo con Fatah. Inoltre l'analisi dello statuto stilato nel 1988 mi ha consentito di evidenziare la ferma posizione che il movimento Hamas ha nei riguardi sia degli ebrei (fortemente critica) che nei riguardi dello stato di Israele (ancora oggi non riconosciuto). Particolare attenzione è stata rivolta alla strategia politica adottata dal movimento, al ricorso all'uso della forza e al
le operazioni definite di martirio. Questo lavoro ha fatto riferimento alle opere dei più accreditati studiosi del settore, sia occidentali che arabi, che hanno analizzato la nascita e lo sviluppo di questo movimento nel contesto politologico delle relazioni internazionali. Si è fatto inoltre riferimento alla posizione assunta dai principali membri della comunità internazionale attraverso documenti diplomatici e posizioni ufficiali. In conclusione, dal presente studio è emersa una doppia anima di questo movimento: quella militante contraria a qualsiasi soluzione negoziata con Israele e quella moderata che invece propende a pervenire ad una soluzione pacifica del conflitto israelo-palestinese, se pur nel rispetto di quelli che sono stati definiti i diritti inalienabili del popolo palestinese. In entrambe le posizioni sussiste un forte radicamento sociale derivante dall'attività di assistenza e di sostegno alla popolazione civile, provata da decenni di stenti e di sradica
mento, che sembra aver individuato in questo movimento l'unica speranza di riconquista della patria perduta
High throughput interactome determination via sulfur anomalous scattering
We propose a novel approach to detect the binding between proteins making use
of the anomalous diffraction of natively present heavy elements inside the
molecule 3D structure. In particular, we suggest considering sulfur atoms
contained in protein structures at lower percentages than the other atomic
species. Here, we run an extensive preliminary investigation to probe both the
feasibility and the range of usage of the proposed protocol. In particular, we
(i) analytically and numerically show that the diffraction patterns produced by
the anomalous scattering of the sulfur atoms in a given direction depend
additively on the relative distances between all couples of sulfur atoms. Thus
the differences in the patterns produced by bound proteins with respect to
their non-bonded states can be exploited to rapidly assess protein complex
formation. Next, we (ii) carried out analyses on the abundances of sulfurs in
the different proteomes and molecular dynamics simulations on a representative
set of protein structures to probe the typical motion of sulfur atoms. Finally,
we (iii) suggest a possible experimental procedure to detect protein-protein
binding. Overall, the completely label-free and rapid method we propose may be
readily extended to probe interactions on a large scale even between other
biological molecules, thus paving the way to the development of a novel field
of research based on a synchrotron light source.Comment: 9 pages, 4 figure
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