4,498 research outputs found
Non-equilibrium dynamics of a system with Quantum Frustration
Using flow equations, equilibrium and non-equilibrium dynamics of a two-level
system are investigated, which couples via non-commuting components to two
independent oscillator baths. In equilibrium the two-level energy splitting is
protected when the TLS is coupled symmetrically to both bath. A critical
asymmetry angle separates the localized from the delocalized phase.
On the other hand, real-time decoherence of a non-equilibrium initial state
is for a generic initial state faster for a coupling to two baths than for a
single bath.Comment: 22 pages, 9 figure
Quantitative analysis of pedestrian counterflow in a cellular automaton model
Pedestrian dynamics exhibits various collective phenomena. Here we study
bidirectional pedestrian flow in a floor field cellular automaton model. Under
certain conditions, lane formation is observed. Although it has often been
studied qualitatively, e.g., as a test for the realism of a model, there are
almost no quantitative results, neither empirically nor theoretically. As basis
for a quantitative analysis we introduce an order parameter which is adopted
from the analysis of colloidal suspensions. This allows to determine a phase
diagram for the system where four different states (free flow, disorder, lanes,
gridlock) can be distinguished. Although the number of lanes formed is
fluctuating, lanes are characterized by a typical density. It is found that the
basic floor field model overestimates the tendency towards a gridlock compared
to experimental bounds. Therefore an anticipation mechanism is introduced which
reduces the jamming probability.Comment: 11 pages, 12 figures, accepted for publication in Phys. Rev.
A simple, efficient, and general treatment of the singularities in Hartree-Fock and exact-exchange Kohn-Sham methods for solids
We present a general scheme for treating the integrable singular terms within
exact exchange (EXX) Kohn-Sham or Hartree-Fock (HF) methods for periodic
solids. We show that the singularity corrections for treating these
divergencies depend only on the total number and the positions of k-points and
on the lattice vectors, in particular the unit cell volume, but not on the
particular positions of atoms within the unit cell. The method proposed here to
treat the singularities constitutes a stable, simple to implement, and general
scheme that can be applied to systems with arbitrary lattice parameters within
either the EXX Kohn-Sham or the HF formalism. We apply the singularity
correction to a typical symmetric structure, diamond, and to a more general
structure, trans-polyacetylene. We consider the effect of the singularity
corrections on volume optimisations and k-point convergence. While the
singularity corrections clearly depends on the total number of k-points, it
exhibits a remarkably small dependence upon the choice of the specific
arrangement of the k-points.Comment: 24 pages, 5 Figures, re-submitted to Phys. Rev. B after revision
Nonequilibrium Spin Dynamics in the Ferromagnetic Kondo Model
Motivated by recent experiments on molecular quantum dots we investigate the
relaxation of pure spin states when coupled to metallic leads. Under suitable
conditions these systems are well described by a ferromagnetic Kondo model.
Using two recently developed theoretical approaches, the time-dependent
numerical renormalization group and an extended ow equation method, we
calculate the real-time evolution of a Kondo spin into its partially screened
steady state. We obtain exact analytical results which agree well with
numerical implementations of both methods. Analytical expressions for the
steady state magnetization and the dependence of the long-time relaxation on
microscopic parameters are established. We find the long-time relaxation
process to be much faster in the regime of anisotropic Kondo couplings. The
steady state magnetization is found to deviate significantly from its thermal
equilibrium value.Comment: 4 pages, 3 figures, final version as accepted by Physical Review
Letter
Biased Information Search in Homogeneous Groups: Confidence as a Moderator for the Effect of Anticipated Task Requirements
When searching for information, groups that are homogeneous regarding their members’ prediscussion decision preferences show a strong bias for information that supports rather than conflicts with the prevailing opinion (confirmation bias). The present research examined whether homogeneous groups blindly search for information confirming their beliefs irrespective of the anticipated task or whether they are sensitive to the usefulness of new information for this forthcoming task. Results of three experiments show that task sensitivity depends on the groups’ confidence in the correctness of their decision: Moderately confident groups displayed a strong confirmation bias when they anticipated having to give reasons for their decision but showed a balanced information search or even a disconfirmation bias (i.e., predominately seeking conflicting information) when they anticipated having to refute unterarguments. In contrast, highly confident groups demonstrated a strong confirmation bias independent of the anticipated task requirements
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Mechanical spectroscopy of retina explants at the protein level employing nanostructured scaffolds
Development of neuronal tissue, such as folding of the brain, and formation of the fovea centralis in the human retina are intimately connected with the mechanical properties of the underlying cells and the extracellular matrix. In particular for neuronal tissue as complex as the vertebrate retina, mechanical properties are still a matter of debate due to their relation to numerous diseases as well as surgery, where the tension of the retina can result in tissue detachment during cutting. However, measuring the elasticity of adult retina wholemounts is difficult and until now only the mechanical properties at the surface have been characterized with micrometer resolution. Many processes, however, such as pathological changes prone to cause tissue rupture and detachment, respectively, are reflected in variations of retina elasticity at smaller length scales at the protein level. In the present work we demonstrate that freely oscillating cantilevers composed of nanostructured TiO2 scaffolds can be employed to study the frequency-dependent mechanical response of adult mammalian retina explants at the nanoscale. Constituting highly versatile scaffolds with strong tissue attachment for long-term organotypic culture atop, these scaffolds perform damped vibrations as fingerprints of the mechanical tissue properties that are derived using finite element calculations. Since the tissue adheres to the nanostructures via constitutive proteins on the photoreceptor side of the retina, the latter are stretched and compressed during vibration of the underlying scaffold. Probing mechanical response of individual proteins within the tissue, the proposed mechanical spectroscopy approach opens the way for studying tissue mechanics, diseases and the effect of drugs at the protein level
Geometric potential and transport in photonic topological crystals
We report on the experimental realization of an optical analogue of a quantum
geometric potential for light wave packets constrained on thin dielectric
guiding layers fabricated in silica by the femtosecond laser writing
technology. We further demonstrate the optical version of a topological
crystal, with the observation of Bloch oscillations and Zener tunneling of
purely geometric nature
Radial-velocity jitter of stars as a function of observational timescale and stellar age
Stars show various amounts of radial velocity (RV) jitter due to varying
stellar activity levels. The typical amount of RV jitter as a function of
stellar age and observational timescale has not yet been systematically
quantified, although it is often larger than the instrumental precision of
modern high-resolution spectrographs used for Doppler planet detection and
characterization. We aim to empirically determine the intrinsic stellar RV
variation for mostly G and K dwarf stars on different timescales and for
different stellar ages independently of stellar models. We also focus on young
stars ( 30 Myr), where the RV variation is known to be large. We use
archival FEROS and HARPS RV data of stars which were observed at least 30 times
spread over at least two years. We then apply the pooled variance (PV)
technique to these data sets to identify the periods and amplitudes of
underlying, quasiperiodic signals. We show that the PV is a powerful tool to
identify quasiperiodic signals in highly irregularly sampled data sets. We
derive activity-lag functions for 20 putative single stars, where lag is the
timescale on which the stellar jitter is measured. Since the ages of all stars
are known, we also use this to formulate an activity--age--lag relation which
can be used to predict the expected RV jitter of a star given its age and the
timescale to be probed. The maximum RV jitter on timescales of decades
decreases from over 500 m/s for 5 Myr-old stars to 2.3 m/s for stars with ages
of around 5 Gyr. The decrease in RV jitter when considering a timescale of only
1 d instead of 1 yr is smaller by roughly a factor of 4 for 5 Myr old stars,
and a factor of 1.5 for stars with an age of 5 Gyr. The rate at which the RV
jitter increases with lag strongly depends on stellar age and ranges from a few
days for a few 10 Myr old stars to presumably decades for stars with an age of
a few gigayears.Comment: 15 pages, 7 Figures; Changelog v2: Updated link to CDS for table E.1;
rearranged Fig. 2 to match journal layou
Biatrial Recurrence of Two Independently Growing Cardiac Myxoma in a Patient with Multiple Tumor Disease
We report the case of a 56-year-old female patient with biatrial recurrence of cardiac myxoma and extensive comorbidities. In the literature, only few cases of biatrial myxoma can be found and they generally describe a single tumor reaching both atria. We found two independently growing cardiac myxomas of both atria
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