20,174 research outputs found
Design of Anisotropic Diffusion Hardware Fiber Phantoms
A gold standard for the validation of diffusion weighted magnetic resonance imaging (DW-MRI) in brain white matter (WM) is essential for clinical purposes but still not available. Synthetic anisotropic fiber bundles are proposed as phantoms for the validation of DW-MRI because of their well-known structure, their long preservability and the possibility
to create complex geometries such as curved and fiber crossings. A crucial question is how the different material properties and size of the fiber phantoms influence the outcome of the DW-MRI experiment. Several fiber materials are compared in this study. The effect of surface
relaxation and internal gradients on the SNR is evaluated. In addition, the dependency of the fiber density and fiber radius on the diffusion properties is investigated
Driven polymer translocation through a nanopore: a manifestation of anomalous diffusion
We study the translocation dynamics of a polymer chain threaded through a
nanopore by an external force. By means of diverse methods (scaling arguments,
fractional calculus and Monte Carlo simulation) we show that the relevant
dynamic variable, the translocated number of segments , displays an {\em
anomalous} diffusive behavior even in the {\em presence} of an external force.
The anomalous dynamics of the translocation process is governed by the same
universal exponent , where is the Flory
exponent and - the surface exponent, which was established recently
for the case of non-driven polymer chain threading through a nanopore. A closed
analytic expression for the probability distribution function , which
follows from the relevant {\em fractional} Fokker - Planck equation, is derived
in terms of the polymer chain length and the applied drag force . It is
found that the average translocation time scales as . Also the corresponding time dependent
statistical moments, and reveal unambiguously the anomalous nature of the translocation
dynamics and permit direct measurement of in experiments. These
findings are tested and found to be in perfect agreement with extensive Monte
Carlo (MC) simulations.Comment: 6 pages, 4 figures, accepted to Europhys. Lett; some references were
supplemented; typos were correcte
Dielectric tensor of monoclinic GaO single crystals in the spectral range eV
The dielectric tensor of -GaO was determined by generalized
spectroscopic ellipsometry in a wide spectral range from to
as well as by calculation including quasiparticle bands and
excitonic effects. The dielectric tensors obtained by both methods are in
excellent agreement with each other and the observed transitions in the
dielectric function are assigned to the corresponding valence bands. It is
shown that the off-diagonal element of the dielectric tensor reaches values up
to and cannot be neglected. Even in the
transparent spectral range where it is quite small () it causes a rotation of the dielectric axes around the symmetry axis of up
to
A two-species continuum model for aeolian sand transport
Starting from the physics on the grain scale, we develop a simple continuum
description of aeolian sand transport. Beyond popular mean-field models, but
without sacrificing their computational efficiency, it accounts for both
dominant grain populations, hopping (or "saltating") and creeping (or
"reptating") grains. The predicted stationary sand transport rate is in
excellent agreement with wind tunnel experiments simulating wind conditions
ranging from the onset of saltation to storms. Our closed set of equations thus
provides an analytically tractable, numerically precise, and computationally
efficient starting point for applications addressing a wealth of phenomena from
dune formation to dust emission.Comment: 23 pages, 9 figure
Grassmann Variables and Pseudoclassical Nuclear Magnetic Resonance
The concept of a propagator is useful and is a well-known object in diffusion
NMR experiments. Here, we investigate the related concept; the propagator for
the magnetisation or the Green's function of the Torrey-Bloch equations. The
magnetisation propagator is constructed by defining functions such as the
Hamiltonian and Lagrangian and using these to define a path integral. It is
shown that the equations of motion derived from the Lagrangian produce
complex-valued trajectories (classical paths) and it is conjectured that the
end-points of these trajectories are real-valued. The complex nature of the
trajectories also suggests that the spin degrees of freedom are also encoded
into the trajectories and this idea is explored by explicitly modeling the spin
or precessing magnetisation by anticommuting Grassmann variables. A
pseudoclassical Lagrangian is constructed by combining the diffusive (bosonic)
Lagrangian with the Grassmann (fermionic) Lagrangian, and performing the path
integral over the Grassmann variables recovers the original Lagrangian that was
used in the construction of the propagator for the magnetisation. The
trajectories of the pseudoclassical model also provide some insight into the
nature of the end-points.Comment: 25 page
Nonequilibrium field-induced phase separation in single-file diffusion
Using an analytically tractable lattice model for reaction-diffusion
processes of hard-core particles we demonstrate that under nonequilibrium
conditions phase coexistence may arise even if the system is effectively
one-dimensional as e.g. in the channel system of some zeolites or in artificial
optical lattices. In our model involving two species of particles a
steady-state particle current is maintained by a density gradient between the
channel boundaries and by the influence of an external driving force. This
leads to the development of a fluctuating but always microscopically sharp
interface between two domains of different densities which are fixed by the
boundary chemical potentials. The internal structure of the interface becomes
very simple for strong driving force. We calculate the drift velocity and
diffusion coefficient of the interface in terms of the microscopic model
parameters.Comment: 38 pages, 2 figure
Autocatalytic reaction-diffusion processes in restricted geometries
We study the dynamics of a system made up of particles of two different
species undergoing irreversible quadratic autocatalytic reactions: . We especially focus on the reaction velocity and on the average time at
which the system achieves its inert state. By means of both analytical and
numerical methods, we are also able to highlight the role of topology in the
temporal evolution of the system
Magnetic Resonance Imaging of Gas Hydrate Formation and Conversion at Sub-Seafloor Conditions
The production of natural gas from sub-seafloor gas hydrates is one possible strategy to
meet the world’s growing demand for energy. On the other hand, climate warming scenarios
call for the substitution of fossil energy resources by sustainable energy concepts. Burning
natural gas from gas hydrates could be emission neutral if it was combined with a safe storage
of the emitted CO2. Laboratory experiments, that address corresponding strategies, need to be
performed under high pressures and low temperatures to meet the thermodynamic conditions
of the sub-seafloor environment. In this paper, we present a high-pressure flow-through
sample cell that is suitable for nuclear magnetic resonance (NMR) experiments at realistic
marine environmental conditions, i.e. pressures up to 15 MPa and temperatures from 5 to 20
°C, and we demonstrate its suitability in applied gas hydrate research
Identification of a possible superconducting transition above room temperature in natural graphite crystals
Measuring with high precision the electrical resistance of highly ordered
natural graphite samples from a Brazil mine, we have identified a transition at
350~K with 40~K transition width. The step-like change in
temperature of the resistance, its magnetic irreversibility and time dependence
after a field change, consistent with trapped flux and flux creep, and the
partial magnetic flux expulsion obtained by magnetization measurements, suggest
the existence of granular superconductivity below 350~K. The zero-field virgin
state can only be reached again after zero field cooling the sample from above
the transition. Paradoxically, the extraordinarily high transition temperature
we found for this and several other graphite samples is the reason why this
transition remained undetected so far. The existence of well ordered
rhombohedral graphite phase in all measured samples has been proved by x-rays
diffraction measurements, suggesting its interfaces with the Bernal phase as a
possible origin for the high-temperature superconductivity, as theoretical
studies predicted. The localization of granular superconductivity at these two
dimensional interfaces prevents the observation of a zero resistance state or
of a full Meissner state.Comment: 14 pages with 21 figure
The effects of halogen elements on the opening of an icosahedral B12 framework
The fully halogenated or hydrogenated B12X12 (X = H, F, Cl, Br and I)
clusters are confirmed to be icosahedral. On the other hand, the bare B12
cluster is shown to have a planar structure. A previous study showed that a
transformation from an icosahedron to a plane happens when 5 to 7 iodine atoms
are substituted. Later, the transition was confirmed to be seven iodine
substitutions based on an infrared spectroscopy study. In this study, we
investigated the effects of different halogen atoms on the opening of the B12
icosahedral cage by means of density functional theory calculations. We found
that the halogen elements do not have significant effects on the geometries of
the clusters. The computed IR spectra show similar representative peaks for all
halogen substituted clusters. Interestingly, we found a blue-shift in the IR
spectra with the increase in the mass of the halogen atoms. Further, we
compared the Gibbs free energies at different temperatures for different
halogen atoms. The results show that the Gibbs free energy differences between
open and close structures of B12X7 become larger when heavier halogen atoms are
present. This interesting finding was subsequently investigated by energy
decomposition analysis
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