1,954 research outputs found
Dynamical heterogeneity in soft particle suspensions under shear
We present experimental measurements of dynamical heterogeneities in a dense
system of microgel spheres, sheared at different rates and at different packing
fractions in a microfluidic channel, and visualized with high speed digital
video microscopy. A four-point dynamic susceptibility is deduced from video
correlations, and is found to exhibit a peak that grows in height and shifts to
longer times as the jamming transition is approached from two different
directions. In particular, the time for particle-size root-mean square relative
displacements is found to scale as where is the strain rate and
is the distance from the random close packing volume
fraction. The typical number of particles in a dynamical heterogeneity is
deduced from the susceptibility peak height and found to scale as . Exponent uncertainties are less than ten
percent. We emphasize that the same power-law behavior is found at packing
fractions above and below . Thus, our results considerably extend a
previous observation of for granular heap flow at
fixed packing below . Furthermore, the implied result compares well with expectation from mode-coupling theory and
with prior observations for driven granular systems
Interacting Dark Matter and Dark Energy
We discuss models for the cosmological dark sector in which the energy
density of a scalar field approximates Einstein's cosmological constant and the
scalar field value determines the dark matter particle mass by a Yukawa
coupling. A model with one dark matter family can be adjusted so the
observational constraints on the cosmological parameters are close to but
different from what is predicted by the Lambda CDM model. This may be a useful
aid to judging how tightly the cosmological parameters are constrained by the
new generation of cosmological tests that depend on the theory of structure
formation. In a model with two families of dark matter particles the scalar
field may be locked to near zero mass for one family. This can suppress the
long-range scalar force in the dark sector and eliminate evolution of the
effective cosmological constant and the mass of the nonrelativistic dark matter
particles, making the model close to Lambda CDM, until the particle number
density becomes low enough to allow the scalar field to evolve. This is a
useful example of the possibility for complexity in the dark sector.Comment: 15 pages, 6 figures; added a reference and a minor correctio
Microfluidic rheology of soft colloids above and below jamming
The rheology near jamming of a suspension of soft colloidal spheres is
studied using a custom microfluidic rheometer that provides stress versus
strain rate over many decades. We find non-Newtonian behavior below the jamming
concentration and yield stress behavior above it. The data may be collapsed
onto two branches with critical scaling exponents that agree with expectations
based on Hertzian contacts and viscous drag. These results support the
conclusion that jamming is similar to a critical phase transition, but with
interaction-dependent exponents.Comment: 4 pages, experimen
Magnetic Order Beyond RKKY in the Classical Kondo Lattice
We study the Kondo lattice model of band electrons coupled to classical
spins, in three dimensions, using a combination of variational calculation and
Monte Carlo. We use the weak coupling `RKKY' window and the strong coupling
regime as benchmarks, but focus on the physically relevant intermediate
coupling regime. Even for modest electron-spin coupling the phase boundaries
move away from the RKKY results, the non interacting Fermi surface no longer
dictates magnetic order, and weak coupling `spiral' phases give way to
collinear order. We use these results to revisit the classic problem of 4f
magnetism and demonstrate how both electronic structure and coupling effects
beyond RKKY control the magnetism in these materials.Comment: 6 pages, 4 figs. Improved figures, expanded captions. To appear in
Europhys. Let
Effect of Al mole fraction on carrier diffusion lengths and lifetimes in AlxGa1−xAs
The ambipolar diffusion length and carrier lifetime are measured in AlxGa1−xAs for several mole fractions in the interval 0<x<0.38. These parameters are found to have significantly higher values in the higher mole fraction samples. These increases are attributed to occupation of states in the indirect valleys, and supporting calculations are presented
A provably stable and high-order accurate finite difference approximation for the incompressible boundary layer equations
In this article we develop a high order accurate method to solve the incompressible boundary layer equations in a provably stable manner. We first derive continuous energy estimates, and then proceed to the discrete setting. We formulate the discrete approximation using high-order finite difference methods on summation-by-parts form and implement the boundary conditions weakly using the simultaneous approximation term method. By applying the discrete energy method and imitating the continuous analysis, the discrete estimate that resembles the continuous counterpart is obtained proving stability. We also show that these newly derived boundary conditions removes the singularities associated with the null-space of the nonlinear discrete spatial operator. Numerical experiments that verifies the high-order accuracy of the scheme and coincides with the theoretical results are presented. The numerical results are compared with the well-known Blasius similarity solution as well as that resulting from the solution of the incompressible Navier Stokes equation
Dynamical Compactification and Inflation in Einstein-Yang-Mills Theory with Higher Derivative Coupling
We study cosmology of the Einstein-Yang-Mills theory in ten dimensions with a
quartic term in the Yang-Mills field strength. We obtain analytically a class
of cosmological solutions in which the extra dimensions are static and the
scale factor of the four-dimensional Friedmann-Lemaitre-Robertson-Walker metric
is an exponential function of time. This means that the model can explain
inflation. Then we look for solutions that describe dynamical compactification
of the extra dimensions. The effective cosmological constant in the
four-dimensional universe is determined from the gravitational coupling,
ten-dimensional cosmological constant, gauge coupling and higher derivative
coupling. By numerical integration, the solution with is found to
behave as a matter-dominated universe which asymptotically approaches flat
space-time, while the solution with a non-vanishing approaches de
Sitter space-time in the asymptotic future.Comment: 30 pages, 7 figure
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