798 research outputs found
Elastic anomalies in glasses: the string theory understanding in the case of Glycerol and Silica
We present an implementation of the analytical string theory recently applied
to the description of glasses. These are modeled as continuum media with
embedded elastic string heterogeneities, randomly located and randomly
oriented, which oscillate around a straight equilibrium position with a
fundamental frequency depending on their length. The existence of a length
distribution reflects then in a distribution of oscillation frequencies which
is responsible for the Boson Peak in the glass density of states. Previously,
it has been shown that such a description can account for the elastic anomalies
reported at frequencies comparable with the Boson Peak. Here we start from the
generalized hydrodynamics to determine the dynamic correlation function
associated with the coherent, dispersive and attenuated, sound
waves resulting from a sound-string interference. Once the vibrational density
of states has been measured, we can use it for univocally fixing the string
length distribution inherent to a given glass. The density-density correlation
function obtained using such distribution is strongly constrained, and able to
account for the experimental data collected on two prototypical glasses:
glycerol and silica. The obtained string length distribution is compatible with
the typical size of elastic heterogeneities previously reported for silica and
supercooled liquids, and the atomic motion associated to the string dynamics is
consistent with the soft modes recently identified in large scale numerical
simulations as non-phonon modes responsible for the Boson Peak. The theory is
thus in agreement with the most recent advances in the understanding of the
glass specific dynamics and offers an appealing simple understanding of the
microscopic origin of the latter, while raising new questions on the
universality or material-specificity of the string distribution properties.Comment: 15 pages, 8 figure
Multiple scattering of elastic waves by pinned dislocation segments in a continuum
The coherent propagation of elastic waves in a solid filled with a random
distribution of pinned dislocation segments is studied to all orders in
perturbation theory. It is shown that, within the independent scattering
approximation, the perturbation series that generates the mass operator is a
geometric series that can thus be formally summed. A divergent quantity is
shown to be renormalizable to zero at low frequencies. At higher frequencies
said quantity can be expressed in terms of a cut-off with dimensions of length,
related to the dislocation length, and physical quantities can be computed in
terms of two parameters, to be determined by experiment. The approach used in
this problem is compared and contrasted with the scattering of de Broglie waves
by delta-function potentials as described by the Schr\"odinger equation
Scattering of dislocated wavefronts by vertical vorticity and the Aharonov-Bohm effect II: Dispersive waves
Previous results on the scattering of surface waves by vertical vorticity on
shallow water are generalized to the case of dispersive water waves. Dispersion
effects are treated perturbatively around the shallow water limit, to first
order in the ratio of depth to wavelength. The dislocation of the incident
wavefront, analogous to the Aharonov-Bohm effect, is still observed. At short
wavelengths the scattering is qualitatively similar to the nondispersive case.
At moderate wavelengths, however, there are two markedly different scattering
regimes according to wether the capillary length is smaller or larger than
times depth. The dislocation is characterized by a parameter that
depends both on phase and group velocity. The validity range of the calculation
is the same as in the shallow water case: wavelengths small compared to vortex
radius, and low Mach number. The implications of these limitations are
carefully considered.Comment: 30 pages, 11 figure
Scattering of second sound waves by quantum vorticity
A new method of detection and measurement of quantum vorticity by scattering
second sound off quantized vortices in superfluid Helium is suggested.
Theoretical calculations of the relative amplitude of the scattered second
sound waves from a single quantum vortex, a vortex ring, and bulk vorticity are
presented. The relevant estimates show that an experimental verification of the
method is feasible. Moreover, it can even be used for the detection of a single
quantum vortex.Comment: Latex file, 9 page
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