528 research outputs found
Methods for suspensions of passive and active filaments
Flexible filaments and fibres are essential components of important complex
fluids that appear in many biological and industrial settings. Direct
simulations of these systems that capture the motion and deformation of many
immersed filaments in suspension remain a formidable computational challenge
due to the complex, coupled fluid--structure interactions of all filaments, the
numerical stiffness associated with filament bending, and the various
constraints that must be maintained as the filaments deform. In this paper, we
address these challenges by describing filament kinematics using quaternions to
resolve both bending and twisting, applying implicit time-integration to
alleviate numerical stiffness, and using quasi-Newton methods to obtain
solutions to the resulting system of nonlinear equations. In particular, we
employ geometric time integration to ensure that the quaternions remain unit as
the filaments move. We also show that our framework can be used with a variety
of models and methods, including matrix-free fast methods, that resolve low
Reynolds number hydrodynamic interactions. We provide a series of tests and
example simulations to demonstrate the performance and possible applications of
our method. Finally, we provide a link to a MATLAB/Octave implementation of our
framework that can be used to learn more about our approach and as a tool for
filament simulation
The exceptional Herbig Ae star HD101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star
We obtained high-resolution, high signal-to-noise UVES and a few lower
quality HARPS spectra revealing the presence of resolved magnetically split
lines. HD101412 is the first Herbig Ae star for which the rotational Doppler
effect was found to be small in comparison to the magnetic splitting. The
measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean
quadratic field was found to vary in the range of 3.5 to 4.8kG. To determine
the period of variations, we used radial velocity, equivalent width, line
width, and line asymmetry measurements of variable spectral lines of several
elements, as well as magnetic field measurements. The most pronounced
variability was detected for spectral lines of He I and the iron peak elements,
whereas the spectral lines of CNO elements are only slightly variable. From
spectral variations and magnetic field measurements we derived a potential
rotation period P_rot=13.86d, which has to be proven in future studies with a
larger number of observations. It is the first time that the presence of
element spots is detected on the surface of a Herbig Ae/Be star. Our previous
study of Herbig Ae stars revealed a trend towards stronger magnetic fields for
younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to
a few other (non-statistical) studies claiming that magnetic Herbig Ae stars
are progenitors of the magnetic Ap stars. New developments in MHD theory show
that the measured magnetic field strengths are compatible with a current-driven
instability of toroidal fields generated by differential rotation in the
stellar interior. This explanation for magnetic intermediate-mass stars could
be an alternative to a frozen-in fossil field.Comment: 7 pages, 6 figures, 1 table, to appear in Astronomische Nachrichte
Exact results for nonlinear ac-transport through a resonant level model
We obtain exact results for the transport through a resonant level model
(noninteracting Anderson impurity model) for rectangular voltage bias as a
function of time. We study both the transient behavior after switching on the
tunneling at time t = 0 and the ensuing steady state behavior. Explicit
expressions are obtained for the ac-current in the linear response regime and
beyond for large voltage bias. Among other effects, we observe current ringing
and PAT (photon assisted tunneling) oscillations.Comment: 7 page
Charge transport through single molecules, quantum dots, and quantum wires
We review recent progresses in the theoretical description of correlation and
quantum fluctuation phenomena in charge transport through single molecules,
quantum dots, and quantum wires. A variety of physical phenomena is addressed,
relating to co-tunneling, pair-tunneling, adiabatic quantum pumping, charge and
spin fluctuations, and inhomogeneous Luttinger liquids. We review theoretical
many-body methods to treat correlation effects, quantum fluctuations,
nonequilibrium physics, and the time evolution into the stationary state of
complex nanoelectronic systems.Comment: 48 pages, 14 figures, Topical Review for Nanotechnolog
Real-Time-RG Analysis of the Dynamics of the Spin-Boson Model
Using a real-time renormalization group method we determine the complete
dynamics of the spin-boson model with ohmic dissipation for coupling strengths
. We calculate the relaxation and dephasing time, the
static susceptibility and correlation functions. Our results are consistent
with quantum Monte Carlo simulations and the Shiba relation. We present for the
first time reliable results for finite cutoff and finite bias in a regime where
perturbation theory in or in tunneling breaks down. Furthermore, an
unambigious comparism to results from the Kondo model is achieved.Comment: 4 pages, 5 figures, 1 tabl
Frequency-Dependent Current Noise through Quantum-Dot Spin Valves
We study frequency-dependent current noise through a single-level quantum dot
connected to ferromagnetic leads with non-collinear magnetization. We propose
to use the frequency-dependent Fano factor as a tool to detect single-spin
dynamics in the quantum dot. Spin precession due to an external magnetic and/or
a many-body exchange field affects the Fano factor of the system in two ways.
First, the tendency towards spin-selective bunching of the transmitted
electrons is suppressed, which gives rise to a reduction of the low-frequency
noise. Second, the noise spectrum displays a resonance at the Larmor frequency,
whose lineshape depends on the relative angle of the leads' magnetizations.Comment: 12 pages, 15 figure
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