159 research outputs found
Generalized Langevin Equation and non-Markovian fluctuation-dissipation theorem for particle-bath systems in external oscillating fields
The Generalized Langevin Equation (GLE) can be derived from a particle-bath
Hamiltonian, in both classical and quantum dynamics, and provides a route to
the (both Markovian and non-Markovian) fluctuation-dissipation theorem (FDT).
All previous studies have focused either on particle-bath systems with
time-independent external forces only, or on the simplified case where only the
tagged particle is subject to the external time-dependent oscillatory field.
Here we extend the GLE and the corresponding FDT for the more general case
where both the tagged particle and the bath oscillators respond to an external
oscillatory field. This is the example of a charged or polarisable particle
immersed in a bath of other particles that are also charged or polarizable,
under an external AC electric field. For this Hamiltonian, we find that the
ensemble average of the stochastic force is not zero, but proportional to the
AC field. The associated FDT reads as , where is
the random force, is the friction memory function, and is
a numerical prefactor
Hydrodynamics of disordered marginally-stable matter
We study the vibrational spectra and the specific heat of disordered systems
using an effective hydrodynamic framework. We consider the contribution of
diffusive modes, i.e. the 'diffusons', to the density of states and the
specific heat. We prove analytically that these new modes provide a constant
term to the vibrational density of states . This contribution is
dominant at low frequencies, with respect to the Debye propagating modes. We
compare our results with numerical simulations data and random matrix theory.
Finally, we compute the specific heat and we show the existence of a linear in
scaling at low temperatures due to the diffusive modes. We
analytically derive the coefficient in terms of the diffusion constant
of the quasi-localized modes and we obtain perfect agreement with numerical
data. The linear in behavior in the specific heat is stronger the more
localized the modes, and crosses over to a (Debye) regime at a
temperature , where is the speed of sound. Our
results suggest that the anomalous properties of glasses and disordered systems
can be understood effectively within a hydrodynamic approach which accounts for
diffusive quasi-localized modes generated via disorder-induced scattering.Comment: v2: 5 pages, 5 figures, minor revision, matching the published
version in PRResearch as Rapid Communicatio
Single-molecule force spectroscopy with photoluminescent semiconducting polymers: Harnessing entropy
We discuss implications of a recent experimental breakthrough which uses a
fluorescence-doped flexible semiconducting polymer to construct a
single-molecule sensor which can detect ultra-weak forces in the molecular
environment, with a grey scale down to 300 femtonewtons
Theory of vanishing heavy-quarks contribution to quark-gluon plasma viscosity
The shear viscosity of strongly interacting dense heavy-quarks plasma is
evaluated analytically using a methodology valid for strongly-correlated
nonequilibrium dense matter. The shear viscosity turns out to be directly
proportional to the zero-frequency limit of the spectral function. By
evaluating the latter using lattice QCD data from the literature, the vanishing
of interaction-dominated viscosity contribution of heavy quarks to the QGP
plasma is demonstrated.Comment: arXiv admin note: substantial text overlap with arXiv:2306.0577
General theory of the viscosity of liquids and solids from nonaffine particle motions
A new microscopic formula for the viscosity of liquids and solids is derived
rigorously from a first-principles (microscopically reversible) Hamiltonian for
particle-bath atomistic motion. The derivation is done within the framework of
nonaffine linear response theory. The new formula may lead to a valid
alternative to the Green-Kubo approach to describe the viscosity of condensed
matter systems from molecular simulations without having to fit long-time
tails. Furthermore, it provides a direct link between the viscosity, the
vibrational density of states of the system, and the zero-frequency limit of
the memory kernel
On a Coarse-Graining Concept in Colloidal Physics with Application to Fluid and Arrested Colloidal Suspensions in Shearing Fields
We poorly understand the macroscopic properties of complex fluids and of
amorphous bodies in general. This is mainly due to the interplay between
phenomena at different levels and length-scales. In particular, it is not
necessarily true that the microscopic level (dominated by direct interactions)
coincides with the level where the continuum description comes into play. This
is typically the case in the presence of structural inhomogeneities which are
inherent to all structurally disordered states of matter below close packing.
As a consequence, the macroscopic response to external fields of either fluid
or arrested disordered states is not well understood. In order to disentangle
this complexity, in this work we build upon a simple yet seemingly powerful
concept. This can be summarized as follows: the mesoscopic length-scale of
structural inhomogeneities is assumed to be the characteristic length-scale of
the effective building blocks, while the degrees of freedom of the primary
particles are integrated out. Theoretical results are derived, in the present
work, for the macroscopic response of fluid and dynamically arrested model
colloidal states in fields of shear. The predictions of the coarse-grained
theories and the applicability of the principle are tested in comparison with
original simulation and experimental data
Theory of activated-rate processes under shear with application to shear-induced aggregation of colloids
Using a novel approximation scheme within the convective diffusion (two body
Smoluchowski) equation framework, we unveil the shear-driven aggregation
mechanism at the origin of structure-formation in sheared colloidal systems.
The theory, verified against numerics and experiments, explains the induction
time followed by explosive (irreversible) rise of viscosity observed in
charge-stabilized colloidal and protein systems under steady shear. The
Arrhenius-type equation with shear derived here, extending Kramers theory in
the presence of shear, is the first analytical result clearly showing the
important role of shear-drive in activated-rate processes as they are
encountered in soft condensed matter
- …