164 research outputs found
Very fast relaxation in polycarbonate glass
Low-frequency Raman and inelastic neutron scattering of amorphous bis-phenol
A polycarbonate is measured at low temperature, and compared. The vibrational
density of states and light-vibration coupling coefficient are determined. The
frequency dependences of these parameters are explained by propagating
vibration modes up to an energy of about 1 meV, and fracton-like modes in more
cohesive domains at higher energies. The vibrational dynamics is in agreement
with a disorder in the glass, which is principally of bonding or of elasticity
instead of density.Comment: 15 pages, 6 figures, to be pub. in EPJ
Low-energy vibrational density of states of plasticized poly(methyl methacrylate)
The low-energy vibrational density of states (VDOS)of hydrogenated or
deuterated poly(methyl methacrylate)(PMMA)plasticized by dibutyl phtalate (DBP)
is determined by inelastic neutron scattering.From experiment, it is equal to
the sum of the ones of the PMMA and DBP components.However, a partition of the
total low-energy VDOS among PMMA and DBP was observed.Contrary to Raman
scattering, neutron scattering does not show enhancement of the boson peak due
to plasticization.Comment: 9 pages, 2 figures (Workshop on Disordered Systems, Andalo
Vibrations of amorphous, nanometric structures: When does continuum theory apply?
Structures involving solid particles of nanometric dimensions play an
increasingly important role in material sciences. These structures are often
characterized through the vibrational properties of their constituent
particles, which can be probed by spectroscopic methods. Interpretation of such
experimental data requires an extension of continuum elasticity theory down to
increasingly small scales. Using numerical simulation and exact diagonalization
for simple models, we show that continuum elasticity, applied to disordered
system, actually breaks down below a length scale of typically 30 to 50
molecular sizes. This length scale is likely related to the one which is
generally invoked to explain the peculiar vibrational properties of glassy
systems.Comment: 4 pages, 5 figures, LATEX, Europhysics Letters accepte
Phonons and Colossal Thermal Expansion Behavior of Ag3Co(CN)6 and Ag3Fe(CN)6
Recently colossal positive volume thermal expansion has been found in the
framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been
measured using the inelastic neutron scattering technique as a function of
temperature and pressure. The data has been analyzed using ab-initio
calculations. We find that the bonding is very similar in both compounds. At
ambient pressure modes in the intermediate frequency part of the vibrational
spectra in the Co compound are shifted to slightly higher energies as compared
to the Fe compound. The temperature dependence of the phonon spectra gives
evidence for large explicit anharmonic contribution to the total anharmonicity
for low-energy modes below 5 meV. We found that modes are mainly affected by
the change in the size of unit cell, which in turn changes the bond lengths and
vibrational frequencies. Thermal expansion has been calculated via the volume
dependence of phonon spectra. Our analysis indicates that Ag phonon modes in
the energy range from 2 to 5 meV are strongly anharmonic and major contributors
to thermal expansion in both compounds. The application of pressure hardens the
low-energy part of the phonon spectra involving Ag vibrations and confirms the
highly anharmonic nature of these modes.Comment: 19 pages, 14 figures and one tabl
Specific heat of the quantum Bragg Glass
We study the thermodynamics of the vibrational modes of a lattice pinned by
impurity disorder in the absence of topological defects (Bragg glass phase).
Using a replica variational method we compute the specific heat in the
quantum regime and find at low temperatures in dimension
three and two. The prefactor is controlled by the pinning length. The non
trivial cancellation of the linear term in arises from the so-called
marginality condition and has important consequences for other mean field
models.Comment: 5 pages, RevTex, strongly revised versio
"m=1" coatings for neutron guides
A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section
"m=1" coatings for neutron guides
A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section
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