6,637 research outputs found
Aging after shear rejuvenation in a soft glassy colloidal suspension: evidence for two different regimes
The aging dynamics after shear rejuvenation in a glassy, charged clay
suspension have been investigated through dynamic light scattering (DLS). Two
different aging regimes are observed: one is attained if the sample is
rejuvenated before its gelation and one after the rejuvenation of the gelled
sample. In the first regime, the application of shear fully rejuvenates the
sample, as the system dynamics soon after shear cessation follow the same aging
evolution characteristic of normal aging. In the second regime, aging proceeds
very fast after shear rejuvenation, and classical DLS cannot be used. An
original protocol to measure an ensemble averaged intensity correlation
function is proposed and its consistency with classical DLS is verified. The
fast aging dynamics of rejuvenated gelled samples exhibit a power law
dependence of the slow relaxation time on the waiting time.Comment: 7 pages, 6 figure
Vanishing conductivity of quantum solitons in polyacetylene
Quantum solitons or polarons are supposed to play a crucial role in the
electric conductivity of polyacetylene, in the intermediate doping regime. We
present an exact fully quantized calculation of the quantum soliton
conductivity in polyacetylene and show that it vanishes exactly. This is
obtained by applying a general method of soliton quantization, based on
order-disorder duality, to a Z(2)-symmetric complex extension of the TLM
dimerization effective field theory. We show that, in this theory,
polyacetylene solitons are sine-Gordon solitons in the phase of the complex
field.Comment: To appear in J. Phys. A: Math. Theor., 15 page
Aging of the Nonlinear Optical Susceptibility of colloidal solutions
Using Z-scan and dynamic light scattering measurements we investigate the
nonlinear optics response of a colloidal solution undergoing dynamics slowing
down with age. We study the high optical nonlinearity of an organic dye
(Rhodamine B) dispersed in a water-clay (Laponite) solution, at different clay
concentrations (2.0 wt% - 2.6 wt%), experiencing the gelation process. We
determine the clay platelets self diffusion coefficient and, by its comparison
with the structural relaxation time, we conclude that the gelation process
proceeds through the structuring of interconnecting clay platelets network
rather than through clusters growth and aggregation.Comment: 4 figures, 4 page
Thermodynamic limits of sperm swimming precision
Sperm swimming is crucial to fertilise the egg, in nature and in assisted
reproductive technologies. Modelling the sperm dynamics involves elasticity,
hydrodynamics, internal active forces, and out-of-equilibrium noise. Here we
demonstrate experimentally the relevance of energy dissipation for sperm
beating fluctuations. For each motile cell, we reconstruct the time-evolution
of the two main tail's spatial modes, which together trace a noisy limit cycle
characterised by a maximum level of precision . Our results indicate
, remarkably close to the estimated precision of a
dynein molecular motor actuating the flagellum, which is bounded by its energy
dissipation rate according to the Thermodynamic Uncertainty Relation. Further
experiments under oxygen deprivation show that decays with energy
consumption, as it occurs for a single molecular motor. Both observations can
be explained by conjecturing a high level of coordination among the
conformational changes of dynein motors. This conjecture is supported by a
theoretical model for the beating of an ideal flagellum actuated by a
collection of motors, including a motor-motor nearest neighbour coupling of
strength : when is small the precision of a large flagellum is much
higher than the single motor one. On the contrary, when is large the two
become comparable.Comment: Main Text with Appendices (14 pages, 9 figures) plus Supplementary
Information, Accepted for Publication in PRX-Lif
Vibrational origin of the fast relaxation processes in molecular glass-formers
We study the interaction of the relaxation processes with the density
fluctuations by molecular dynamics simulation of a flexible molecule model for
o-terphenyl (oTP) in the liquid and supercooled phases. We find evidence,
besides the structural relaxation, of a secondary vibrational relaxation whose
characteristic time, few ps, is slightly temperature dependent. This i)
confirms the result by Monaco et al. [Phys. Rev, E 62, 7595 (2000)] of the
vibrational nature of the fast relaxation observed in Brillouin Light
Scattering (BLS) experiments in oTP; and ii) poses a caveat on the
interpretation of the BLS spectra of molecular systems in terms of a purely
center of mass dynamics.Comment: RevTeX, 5 pages, 4 eps figure
Saddles in the energy landscape probed by supercooled liquids
We numerically investigate the supercooled dynamics of two simple model
liquids exploiting the partition of the multi-dimension configuration space in
basins of attraction of the stationary points (inherent saddles) of the
potential energy surface. We find that the inherent saddles order and potential
energy are well defined functions of the temperature T. Moreover, decreasing T,
the saddle order vanishes at the same temperature (T_MCT) where the inverse
diffusivity appears to diverge as a power law. This allows a topological
interpretation of T_MCT: it marks the transition from a dynamics between basins
of saddles (T>T_MCT) to a dynamics between basins of minima (T<T_MCT).Comment: 4 pages, 3 figures, to be published on PR
Topological Description of the Aging Dynamics in Simple Glasses
We numerically investigate the aging dynamics of a monatomic Lennard-Jones
glass, focusing on the topology of the potential energy landscape which, to
this aim, has been partitioned in basins of attraction of stationary points
(saddles and minima). The analysis of the stationary points visited during the
aging dynamics shows the existence of two distinct regimes: i) at short times,
t<t_c, the system visits basins of saddles whose energies and orders decrease
with t; ii) at long times, t>t_c, the system mainly lies in basins pertaining
to minima of slowly decreasing energy. The dynamics for t>t_c can be
represented by a simple random walk on a network of minima with a jump
probability proportional to the inverse of the waiting time.Comment: 4 pages, 5 postscript figure
Generalized fluctuation-dissipation relation and effective temperature in off-equilibrium colloids
The fluctuation-dissipation relation (FDR), a fundamental result of
equilibrium statistical physics, ceases to be valid when a system is taken out
of the equilibrium. A generalization of FDR has been theoretically proposed for
out-of-equilibrium systems: the kinetic temperature entering FDR is substituted
by a time-scale dependent effective temperature. We combine the measurements of
the correlation function of the rotational dynamics of colloidal particles
obtained via dynamic light scattering with those of the birefringence response
to study the generalized FDR in an off-equilibrium clay suspension undergoing
aging. We i) find that FDR is strongly violated in the early stage of the aging
process and is gradually recovered as the aging time increases and, ii), we
determine the aging time evolution of the effective temperature, giving support
to the proposed generalization scheme.Comment: 4 pages, 3 figure
Fractional Spin Excitations in the Infinite-Layer Cuprate CaCuO2
We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic dynamics of the infinite-layer cuprate CaCuO2. We find that close to the (1/2,0) point, the single magnon decays into a broad continuum of excitations accounting for about 80% of the total magnetic spectral weight. Polarization-resolved RIXS spectra reveal the overwhelming dominance of the spin-flip (Delta S = 1) character of this continuum with respect to the Delta S = 0 multimagnon contributions. Moreover, its incident-energy dependence is identical to that of the magnon, supporting a common physical origin. We propose that the continuum originates from the decay of the magnon into spinon pairs, and we relate it to the exceptionally high ring exchange J(c) similar to J(1) of CaCuO2. In the infinite-layer cuprates, long-range and multisite hopping integrals are very important, and they amplify the 2D quantum magnetism effects in spite of the 3D antiferromagnetic Neel order
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