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Author: sparavigna
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Phys. Rev. B (8)
Phys. Rev. E (3)
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1. Phonons in conventional and auxetic honeycomb lattices
A. Sparavigna
Abstract
The modes of vibrations of conventional and auxetic honeycomb
structures are studied by means of models where lattices are
represented by planar networks, in which rodlike particles are
linked by strings. In these structures, the translational and
rotational degrees of freedom are strongly coupled. The auxetic
network is obtained by modifying a model proposed by Evans in 1991
[Nature (London) 353, 124 (1991)], and is used to explain the
negative Poisson’s ratio of auxetic materials. Auxetics are
materials with a negative Poisson elastic parameter, meaning that
they have a lateral extension, instead of shrinking, when they are
stretched. The phonon dispersions obtained in the case of the
auxetic model are compared with those of a conventional honeycomb
network, where rigid rodlike particles are inserted. The behavior of
the rotational dispersions can explain some experimental
observations on the properties of sound propagation in the auxetic
materials.Phys. Rev. B 76, 134302 (2007)
2. Role of nonpairwise interactions on phonon thermal transport
A. Sparavigna
Hide Abstract
In this paper, the phonon system for a perfect silicon lattice is
obtained by means of a model considering a phenomenological
potential that includes both two- and three-body contributions.
Phonon dispersions are discussed, and anharmonic contributions to
the phonon Hamiltonian are evaluated. The model is compared with a
model involving a pairwise potential, previously used by the author
in the calculation of silicon thermal conductivity. The equation of
motion is solved for both models, obtaining phonon dispersions
practically indistinguishable and in good agreement with the
experimental data. The role of nonpairwise interactions in
phonon-phonon–scattering processes, relevant for the calculation of
thermal conductivity, is then discussed. The thermal conductivity
obtained with the present model including two- and three-body
interactions has a good agreement with the experimental data, better
than the one previously achieved with the model involving a central
potential.Phys. Rev. B 67, 144305 (2003)
3. Lattice thermal conductivity in cubic silicon carbide
A. Sparavigna
Abstract
The lattice thermal conductivity of cubic silicon carbide is
evaluated by means of a microscopic model considering the discrete
nature of the lattice and its Brillouin zone for phonon dispersions
and scattering mechanisms. The phonon Boltzmann equation is solved
iteratively, with the three-phonon normal and umklapp collisions
rigorously treated, avoiding relaxation-time approximations. Good
agreement with the experimental data is obtained. Moreover, the role
of point defects, such as antisites, on the lattice thermal
conductivity is discussed.Phys. Rev. B 66, 174301 (2002)
4. Influence of isotope scattering on the thermal conductivity of
diamond
A. Sparavigna
Abstract
The thermal conductivity of diamond crystals with different isotope
contents is evaluated in the framework of a microscopic model that
considers acoustic- and optical-phonon branches. The phonon
Boltzmann equation is solved iteratively, with the phonon wave
vectors taken in the real Brillouin zone and the three-phonon normal
and umklapp collisions, with the isotope scattering, rigorously
treated. As a consequence, the evaluation of the thermal
conductivity is done avoiding the relaxation-time approximation for
the scattering mechanisms. Good agreement with the experimental data
is obtained. The calculation reveals a fundamental role of the
optical phonons in determining the thermal resistivity of diamond.
Comparison of the theoretical results with the recent experimental
data for germanium and silicon is also proposed.Phys. Rev. B 65,
064305 (2002)
5. Role of grain boundaries as phonon diffraction gratings in the
theory of thermal conductivity
M. Omini and A. Sparavigna
Abstract
The picture of a grain boundary as a periodic array of dislocations
implies the occurrence of phonon scattering processes that the
Klemens theory of thermal conductivity does not account for. A grain
boundary works similar to a diffraction grating, producing
diffraction spectra of various orders: each order number n is
associated with a class of scattering processes contributing to
thermal resistance. The Klemens theory corresponds to n=0: it is
shown that processes with n≠0 are essential to explain the heat
transport properties of a specimen containing grain boundaries. The
theory is used to explain the behavior of thermal conductivity, both
in the range below 5 K and in the region of the conductivity peak,
as observed in crystals of lithium fluoride, alumina, and quartz. It
is also applied to the conductivity curve of fused silica, in the
frame of a model where a glass is pictured as a solid with a
high-density distribution of grain boundaries.Phys. Rev. B 61, 6677
(2000)
6. Thermal conductivity of solid neon: An iterative analysis
A. Sparavigna
Abstract
In this paper, the thermal conductivity of neon is obtained by means
of a recently proposed iterative solution of the phonon Boltzmann
equation. The potential used for the calculation is an effective
Lennard-Jones potential to include quantum effects. Good agreement
with the experimental data is obtained.Phys. Rev. B 56, 7775 (1997)
7. Quasistatic domains in planar nematic liquid crystals around the
dielectric inversion point
V. G. Chigrinov, A. Sparavigna, and A. Strigazzi
Abstract
A simple viscoelastic approach is proposed to describe the periodic
patterns, characterized by static walls and splay-bend distortion,
which appear in samples of nematic liquid crystals having dielectric
anisotropy ɛa dependent on the frequency. The modulated structure,
resulting from a steady velocity field v coupled with a steady
director field n, is achieved when an electric field is applied
normally to the plates of a planar unidirectional nematic cell. Such
a kind of quasistatic domain is theoretically investigated not only
in the frequency region, where the usual aperiodic Fréedericksz
effect becomes unfavorable, Re(ɛa) still being positive, but also
where Re(ɛa)<0, favoring in principle the initial orientation. Both
previous situations are considered in the vicinity of the sign
reversal point. The present model describes the dielectric loss near
the reversal point in terms of the appearance of the corresponding
effective space charge, which interacts with the effective electric
field, causing a steady electrohydrodynamic motion of very small
amplitude inside the nematic liquid crystal layer. As a result, a
quasistatic tilted modulated structure emerges, with wave vector
parallel to the initial planar orientation of the nematic cell. ©
1996 The American Physical Society.Phys. Rev. E 53, 4918 (1996)
8. Beyond the isotropic-model approximation in the theory of thermal
conductivity
M. Omini and A. Sparavigna
Abstract
By the use of an iterative method the linearized phonon-Boltzmann
equation for a dielectric solid subjected to a thermal gradient is
solved in the frame of three-phonon interactions. In this way it is
possible to calculate the thermal conductivity of rare-gas solids
starting from the pair potential and accounting for the real
Brillouin zone of the lattice. The numerical results are in full
agreement with experiment and represent a considerable improvement
with respect to those previously deduced for an isotropic
solid.Phys. Rev. B 53, 9064 (1996)
9. Magnetic field effect on periodic stripe domains in nematic
liquid crystals
A. Sparavigna, O. D. Lavrentovich, and A. Strigazzi
Abstract
Hybrid aligned nematic films placed onto an isotropic fluid
substrate exhibit an unusual periodic stripe domain structure that
appears only when the thickness of the film is smaller than a few
tenths of a micrometer. We investigated the effect of a magnetic
field on the threshold between the periodic stripe domains and the
aperiodic deformed structure of the director. As experimentally
observed, a magnetic field applied along the stripe domains favors a
nonperiodic state with the director undistorted in the horizontal
plane. The experimental findings are confirmed by a theory that
takes into account not only the usual type of the elastic
distortions, but also the so-called saddle-splay elasticity. A
comparison of the experimental and theoretical data allows one to
estimate that the saddle-splay elastic constant K24 is of the same
order of magnitude as the bulk elastic constants; this result agrees
with independent studies of confined liquid crystal systems.Phys.
Rev. E 51, 792 (1995)
10. Electric-field effects on the spin-density wave in magnetic
ferroelectrics
A. Sparavigna, A. Strigazzi, and A. Zvezdin
Abstract
A profound analogy exists between the modulate structures in
magnetic materials and in nematic liquid crystals, especially for
the behavior in an external field. Starting from this point, we
study the influence of an electric field on the spatially modulated
spin structure (spin-density-wave state) of the magnetic
ferroelectric BiFeO3, discovering and investigating the possibility
of a transition between the spin-density-wave state into a
homogeneous antiferromagnetic configuration.Phys. Rev. B 50, 2953
(1994)
11. Periodic stripe domains and hybrid-alignment regime in nematic
liquid crystals: Threshold analysis
A. Sparavigna, O. D. Lavrentovich, and A. Strigazzi
Abstract
Recently we investigated the occurrence of static periodic stripes
in a hybrid aligned nematic cell. Assuming that the tilt anchoring
was stronger at the planar wall than at the homeotropic wall, we
have found the critical thickness of the cell for the transition
from planar to periodic alignment as a function of the surface
energy in the presence of a magnetic field. Here we study, for the
same kind of cell, the critical thickness between the periodic and
the aperiodic deformed structure by means of an appropriate
numerical technique. As expected, such a threshold was found to be
greater than the asymptotic threshold between planar and aperiodic
structures. We performed an experiment, which allowed us to give an
estimate of the surfacelike elastic constant K24.Phys. Rev. E 49,
1344 (1994)