134 research outputs found
Collapsing granular suspensions
A 2D contact dynamics model is proposed as a microscopic description of a
collapsing suspension/soil to capture the essential physical processes
underlying the dynamics of generation and collapse of the system. Our physical
model is compared with real data obtained from in situ measurements performed
with a natural collapsing/suspension soil. We show that the shear strength
behavior of our collapsing suspension/soil model is very similar to the
behavior of this collapsing suspension soil, for both the unperturbed and the
perturbed phases of the material.Comment: 7 pages, 5 figures, accepted for publication in EPJ
Particle and particle pair dispersion in turbulence modeled with spatially and temporally correlated stochastic processes
In this paper we present a new model for modeling the diffusion and relative
dispersion of particles in homogeneous isotropic turbulence. We use an
Heisenberg-like Hamiltonian to incorporate spatial correlations between fluid
particles, which are modeled by stochastic processes correlated in time. We are
able to reproduce the ballistic regime in the mean squared displacement of
single particles and the transition to a normal diffusion regime for long
times. For the dispersion of particle pairs we find a -dependence of the
mean squared separation at short times and a -dependence for long ones. For
intermediate times indications for a Richardson law are observed in
certain situations. Finally the influence of inertia of real particles on the
dispersion is investigated.Comment: 10 pages, 7 figures, 1 tabl
Electronic Scattering Effects in Europium-Based Iron Pnictides
In a comprehensive study, we investigate the electronic scattering effects in
EuFe(AsP) by using Fourier-transform infrared
spectroscopy. In spite of the fact that Eu local moments order around
\,K, the overall optical response is strikingly similar
to the one of the well-known Ba-122 pnictides. The main difference lies within
the suppression of the lower spin-density-wave gap feature. By analysing our
spectra with a multi-component model, we find that the high-energy feature
around 0.7\,eV -- often associated with Hund's rule coupling -- is highly
sensitive to the spin-density-wave ordering, this further confirms its direct
relationship to the dynamics of itinerant carriers. The same model is also used
to investigate the in-plane anisotropy of magnetically detwinned
EuFeAs in the antiferromagnetically ordered state, yielding a
higher Drude weight and lower scattering rate along the crystallographic
-axis. Finally, we analyse the development of the room temperature spectra
with isovalent phosphor substitution and highlight changes in the scattering
rate of hole-like carriers induced by a Lifshitz transition
Emergence of chaotic scattering in ultracold Er and Dy
We show that for ultracold magnetic lanthanide atoms chaotic scattering
emerges due to a combination of anisotropic interaction potentials and Zeeman
coupling under an external magnetic field. This scattering is studied in a
collaborative experimental and theoretical effort for both dysprosium and
erbium. We present extensive atom-loss measurements of their dense magnetic
Feshbach resonance spectra, analyze their statistical properties, and compare
to predictions from a random-matrix-theory inspired model. Furthermore,
theoretical coupled-channels simulations of the anisotropic molecular
Hamiltonian at zero magnetic field show that weakly-bound, near threshold
diatomic levels form overlapping, uncoupled chaotic series that when combined
are randomly distributed. The Zeeman interaction shifts and couples these
levels, leading to a Feshbach spectrum of zero-energy bound states with
nearest-neighbor spacings that changes from randomly to chaotically distributed
for increasing magnetic field. Finally, we show that the extreme temperature
sensitivity of a small, but sizeable fraction of the resonances in the Dy and
Er atom-loss spectra is due to resonant non-zero partial-wave collisions. Our
threshold analysis for these resonances indicates a large collision-energy
dependence of the three-body recombination rate
Collective Electronic Excitation Coupling between Planar Optical Lattices using Ewald's Method
Using Ewald's summation method we investigate collective electronic
excitations (excitons) of ultracold atoms in parallel planar optical lattices
including long range interactions. The exciton dispersion relation can then be
suitably rewritten and efficiently calculated for long range resonance
dipole-dipole interactions. Such in-plane excitons resonantly couple for two
identical optical lattices, with an energy transfer strength decreasing
exponentially with the distance between the lattices. This allows a restriction
of the transfer to neighboring planes and gives rise to excitons delocalized
between the lattices. In general equivalent results will hold for any planar
system containing lattice layers of optically active and dipolar materials.Comment: 6 pages, and 7 figure
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