985 research outputs found
Force calculation on walls and embedded particles in multiparticle collision dynamics simulations
Colloidal solutions posses a wide range of time and length scales, so that it
is unfeasible to keep track of all of them within a single simulation. As a
consequence some form of coarse-graining must be applied. In this work we use
the Multi-Particle Collision Dynamics scheme. We describe a particular
implementation of no-slip boundary conditions upon a solid surface, capable of
providing correct force s on the solid bypassing the calculation of the
velocity profile or the stre ss tensor in the fluid near the surface. As an
application we measure the friction on a spherical particle, when it is placed
in a bulk fluid and when it is confined in a slit. We show that the
implementation of the no-slip boundary conditions leads to an enhanced Ensko g
friction, which can be understood analytically. Because of the long-range
nature of hydrodynamic interactions, the Stokes friction obtained from the
simulations is sensitive of the simulation box size. We address this topic for
the slit geometry, showing that that the dependence on the system size differs
very much from what is expected in a 3D system, where periodic boundary
conditions are used in all directions.Comment: To appear in Physical Review
Effect of external magnetic field on electron spin dephasing induced by hyperfine interaction in quantum dots
We investigate the influence of an external magnetic field on spin phase
relaxation of single electrons in semiconductor quantum dots induced by the
hyperfine interaction. The basic decay mechanism is attributed to the
dispersion of local effective nuclear fields over the ensemble of quantum dots.
The characteristics of electron spin dephasing is analyzed by taking an average
over the nuclear spin distribution. We find that the dephasing rate can be
estimated as a spin precession frequency caused primarily by the mean value of
the local nuclear magnetic field. Furthermore, it is shown that the hyperfine
interaction does not fully depolarize electron spin. The loss of initial spin
polarization during the dephasing process depends strongly on the external
magnetic field, leading to the possibility of effective suppression of this
mechanism.Comment: 10 pages, 2 figure
Effects of an in-plane magnetic field on c-axis sum rule and superfluid density in high- cuprates
In layered cuprates, the application of an in-plane magnetic field changes the c-axis optical sum rule and superfluid density . For
pure incoherent c-axis coupling, has no effect on either quantities
but it does if an additional coherent component is present. For the coherent
contribution, different characteristic variations on and on
temperature result from the constant part of the hopping matrix
element and from the part which has zero on the diagonal of the
Brillouin zone. Only the constant part leads to a dependence on
the direction of as well as on its magnitude.Comment: 3 figure
Transport properties in the d-density wave state: Wiedemann-Franz law
We study the Wiedemann-Franz (WF) law in the d-density wave (DDW) model. Even
though the opening of the DDW gap profoundly modifies the electronic
density of states and makes it dependent on energy, the value of the WF ratio
at zero temperature (T=0) remains unchanged. However, neither electrical nor
thermal conductivity display universal behavior. For finite temperature, with T
greater than the value of the impurity scattering rate at zero frequency
i.e. , the usual WF ratio is obtained only in
the weak scattering limit. For strong scattering there are large violations of
the WF law.Comment: 1 figur
Observation of Apparently Zero-Conductance States in Corbino Samples
Using Corbino samples we have observed oscillatory conductance in a
high-mobility two-dimensional electron system subjected to crossed microwave
and magnetic fields. On the strongest of the oscillation minima the conductance
is found to be vanishingly small, possibly indicating an insulating state
associated with these minima.Comment: 4 pages, 3 figures, RevTex
A conceptual basis for surveying fouling communities at exposed and protected sites at sea: Feasible designs with exchangeable test bodies for in-situ biofouling collection
The enhanced inertia load caused by biofouling on device components, such as the foundations of wind turbines or other structures at sea, modifies the hydrodynamic properties, and increases the stress to structures, predominantly in upper water layers with high impact from wave dynamics. This compromises the stability, functioning, operation as well as the durability of these devices especially in exposed environments. A main challenge is the quantification of the impact of hydrodynamic forces on irregular bodies being overgrown by soft- and hard-bodied biofouling organisms. Therefore, test bodies from the upper 1–5 m water depth and thus exposed to the strongest wave actions close to the surface shall be overgrown by biofouling and used in measurement trials in a wave and current flume. These measurements shall shed light on the varying roughness and its influence on the load bearing capacity of foundation piles. Consequently, the main aims of the present work were the development of two independent test stations as holding devices for artificial test bodies for the collection of biofouling organisms during field studies: a carrying unit floating at the surface in an exposed area (System A) and a sampling device with access from a land-based facility (System B). Both systems are relatively easy to access, exhibit straightforward handling, and are reasonable cost-effective. A Test Body Support Unit (TBSU, System A) was designed and mounted on a spare buoy to carry the test bodies (cylinders), which serve as substrate for the fouling. The system was sufficiently robust to withstand several periods of rough sea conditions over the first two years. This system can only be accessed by vessels. System B (MareLift) provided the robustness and functionality needed for areas exhibiting harsh conditions but can be operated from land. The here used test bodies (steel panels) exhibited a sound basis for the monitoring of succession processes in the biofouling development. System B offered the possibility to analyse two habitats (intertidal and subtidal) and revealed clear differences in the composition and development of their fouling communities. Overall, both systems provide advantages in obtaining standardized biofouling samples compared to previous approaches. Such test stations play an important role in the risk management of marine sectors as they could help characterising biofouling communities over different geographical areas. System A and B provide a sound basis for biofouling research but potentially also for other potential research approaches in exposed areas as they provide space for future developments
Radiation induced oscillatory Hall effect in high mobility GaAs/AlGaAs devices
We examine the radiation induced modification of the Hall effect in high
mobility GaAs/AlGaAs devices that exhibit vanishing resistance under microwave
excitation. The modification in the Hall effect upon irradiation is
characterized by (a) a small reduction in the slope of the Hall resistance
curve with respect to the dark value, (b) a periodic reduction in the magnitude
of the Hall resistance, , that correlates with an increase in the
diagonal resistance, , and (c) a Hall resistance correction that
disappears as the diagonal resistance vanishes.Comment: 4 pages text, 4 color figure
Interlayer coupling and the c-axis quasiparticle transport in high- cuprates
The c-axis quasiparticle conductivity shows different behavior depending on
the nature of the interlayer coupling. For coherent coupling with a constant
hopping amplitude , the conductivity at zero frequency and zero
temperature depends on the direction of the magnetic field, but
it does not for angle-dependent hopping which removes the
contribution of the nodal quasiparticles. For incoherent coupling, the
conductivity is also independent of field direction and changes only when
paramagnetic effects are included. The conductivity sum rule can be used to
determine the admixture of coherent to incoherent coupling. The value of
can be dominated by while at the same time
dominates the temperature dependence of the superfluid density.Comment: 2 figure
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