1,166 research outputs found
Microscopic origin of nonlinear non-affine deformation and stress overshoot in bulk metallic glasses
The atomic theory of elasticity of amorphous solids, based on the nonaffine
response formalism, is extended into the nonlinear stress-strain regime by
coupling with the underlying irreversible many-body dynamics. The latter is
implemented in compact analytical form using a qualitative method for the
many-body Smoluchowski equation. The resulting nonlinear stress-strain
(constitutive) relation is very simple, with few fitting parameters, yet
contains all the microscopic physics. The theory is successfully tested against
experimental data on metallic glasses, and it is able to reproduce the
ubiquitous feature of stress-strain overshoot upon varying temperature and
shear rate. A clear atomic-level interpretation is provided for the stress
overshoot, in terms of the competition between the elastic instability caused
by nonaffine deformation of the glassy cage and the stress buildup due to
viscous dissipation.Comment: Physical Review B Rapid Comm., in pres
Low-energy optical phonons induce glassy-like vibrational and thermal anomalies in ordered crystals
It is widely accepted that structural glasses and disordered crystals exhibit
anomalies in the their thermal, mechanical and acoustic properties as
manifestations of the breakdown of the long-wavelength approximation in a
disordered dissipative environment. However, the same type of glassy-like
anomalies (i.e. boson peak in the vibrational density of states (VDOS) above
the Debye level, peak in the normalized specific heat at etc)
have been recently observed also in perfectly ordered crystals, including
thermoelectric compounds. Here we present a theory that predicts these
surprising effects in perfectly ordered crystals as a result of low-lying
(soft) optical phonons. In particular, it is seen that a strong boson peak
anomaly (low-energy excess of modes) in the VDOS can be due almost entirely to
the presence of low-energy optical phonons, provided that their energy is
comparable to that of the acoustic modes at the Brillouin zone boundary. The
boson peak is predicted also to occur in the heat capacity at low . In
presence of strong damping (which might be due to anharmonicities in the
ordered crystals), these optical phonons contribute to the low- deviation
from Debye's law, producing a linear-in- behavior which is typical
of glasses, even though no assumptions of disorder whatsoever are made in the
model. These findings are relevant for understanding and tuning thermal
transport properties of thermoelectric compounds, and possibly for the
enhancement of electron-phonon superconductivity
Theory of molecular crowding in Brownian hard-sphere liquids with application to the polymer coil-globule transition
We derive an analytical pair potential of mean force for Brownian molecules
in the liquid-state. Our approach accounts for many-particle correlations of
crowding particles of the liquid, and for diffusive transport across the
spatially modulated local density of crowders in the dense environment.
Specializing on the limit of equal-size particles, we show that this diffusive
transport leads to additional density- and structure-dependent terms in the
interaction potential, and to a much stronger attraction (by a factor ~4 at
average volume fraction of crowders 0.25) than in the standard depletion
interaction where the diffusive effects are neglected. As an illustration of
the theory, we use it to study the size of a polymer chain in a solution of
inert crowders. Even in the case of athermal background solvent, when a
classical chain should be fully swollen, we find a sharp coil-globule
transition of the ideal chain collapsing at a critical value of the crowder
volume fraction ~0.145
Rheology of Hard Glassy Materials
Glassy solids may undergo a fluidization (yielding) transition upon
deformation whereby the material starts to flow plastically. It has been a
matter of debate whether this process is controlled by a specific time scale,
from among different competing relaxation/kinetic processes. Here, two
constitutive models of cage relaxation are examined within the microscopic
model of nonaffine elasto-plasticity. One (widely used) constitutive model
implies that the overall relaxation rate is dominated by the fastest between
the structural () relaxation rate and the shear-induced relaxation
rate. A different model is formulated here which, instead, assumes that the
slowest (global) relaxation process controls the overall relaxation. We show
that the first model is not compatible with the existence of finite elastic
shear modulus for quasistatic (low-frequency) deformation, while the second
model is able to describe all key features of deformation of `hard' glassy
solids, including the yielding transition, the nonaffine-to-affine plateau
crossover, and the rate-stiffening of the modulus. The proposed framework
provides an operational way to distinguish between `soft' glasses and `hard'
glasses based on the shear-rate dependence of the structural relaxation time
Parameter-free predictions of the viscoelastic response of glassy polymers from non-affine lattice dynamics
We study the viscoelastic response of amorphous polymers using theory and
simulations. By accounting for internal stresses and considering instantaneous
normal modes (INMs) within athermal non-affine theory, we make parameter-free
predictions of the dynamic viscoelastic moduli obtained in coarse-grained
simulations of polymer glasses at non-zero temperatures. The theoretical
results show very good correspondence with rheology data collected from
molecular dynamics simulations over five orders of magnitude in frequency, with
some instabilities that accumulate in the low-frequency part on approach to the
glass transition. These results provide evidence that the mechanical glass
transition itself is continuous and thus represents a crossover rather than a
true phase transition. The relatively sharp drop of the low-frequency storage
modulus across the glass transition temperature can be explained
mechanistically within the proposed theory: the proliferation of
low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy
excitations) is directly responsible for the rapid growth of a negative
non-affine contribution to the storage modulus.Comment: 10 pages, 7 figure
Theory of the phonon spectrum in host-guest crystalline solids with avoided crossing
We develop an analytical model to describe the phonon dispersion relations of host-guest lattices with heavy guest atoms (rattlers). Crucially, the model also accounts for phonon damping arising from anharmonicity. The spectrum of low-energy states contains acousticlike and (soft) optical-like modes, which display the typical avoided crossing, and which can be derived analytically by considering the dynamical coupling between host lattice and guest rattlers. Inclusion of viscous anharmonic damping in the model allows us to compute the vibrational density of states (VDOS) and the specific heat, unveiling the presence of a boson peak (BP) linked to an anharmonicity-smeared van Hove singularity. Upon increasing the coupling strength between the host and the guest dynamics, and by decreasing the energy of the soft optical modes, the BP anomaly becomes stronger and it moves towards lower frequencies. Moreover, we find a robust linear correlation between the BP frequency and the energy of the soft optical-like modes. This framework provides a useful model for tuning the thermal properties of host-guest lattices by controlling the VDOS, which is crucial for optimizing thermal conductivity and hence the energy conversion efficiency in these materials
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