1,793 research outputs found
Spatial filtering in multichannel magnetoencephalography
Partial differential equations in boundary-value problems have been studied in order to estimate the influence of several geometrical and physical parameters involved in the outward transmission of the brain's magnetic field. Explicit Green kernels are used to obtain integral forms of generalized solutions which can be deduced from each other, as expressed over concentric spherical surfaces. That leads to numerical applications dealing with the radial component of the magnetic field. From this study, a new spatial filtering is proposed as a possible improvement in two-dimensional magnetoencephalographic mapping using large multisensors
The origin of very wide binary systems
The majority of stars in the Galactic field and halo are part of binary or
multiple systems. A significant fraction of these systems have orbital
separations in excess of thousands of astronomical units, and systems wider
than a parsec have been identified in the Galactic halo. These binary systems
cannot have formed through the 'normal' star-formation process, nor by capture
processes in the Galactic field. We propose that these wide systems were formed
during the dissolution phase of young star clusters. We test this hypothesis
using N-body simulations of evolving star clusters and find wide binary
fractions of 1-30%, depending on initial conditions. Moreover, given that most
stars form as part of a binary system, our theory predicts that a large
fraction of the known wide 'binaries' are, in fact, multiple systems.Comment: 4 pages, 1 figure, to appear in the proceedings of IAU Symposium 266,
eds. R. de Grijs & J.R.D. Lepin
Binaries and the dynamical mass of star clusters
The total mass of a distant star cluster is often derived from the virial
theorem, using line-of-sight velocity dispersion measurements and half-light
radii, under the implicit assumption that all stars are single (although it is
known that most stars form part of binary systems). The components of binary
stars exhibit orbital motion, which increases the measured velocity dispersion,
resulting in a dynamical mass overestimation. In this article we quantify the
effect of neglecting the binary population on the derivation of the dynamical
mass of a star cluster. We find that the presence of binaries plays an
important role for clusters with total mass M < 10^5 Msun; the dynamical mass
can be significantly overestimated (by a factor of two or more). For the more
massive clusters, with Mcl > 10^5 Msun, binaries do not affect the dynamical
mass estimation significantly, provided that the cluster is significantly
compact (half-mass radius < 5 pc).Comment: Comments: 2 pages. Conference proceedings for IAUS246 'Dynamical
Evolution of Dense Stellar Systems', ed. E. Vesperini (Chief Editor), M.
Giersz, A. Sills, Capri, Sept. 200
Frequency-dependent transport through a quantum dot in the Kondo regime
We study the AC conductance and equilibrium current fluctuations of a Coulomb
blockaded quantum dot. A relation between the equilibrium spectral function and
the linear AC conductance is derived which is valid for frequencies well below
the charging energy of the quantum dot. Frequency-dependent transport
measurements can thus give experimental access to the Kondo peak in the
equilibrium spectral function of a quantum dot. We illustrate this in detail
for typical experimental parameters using the numerical renormalization group
method in combination with the Kubo formalism.Comment: 4 pages, 4 figure
The dynamical fate of planetary systems in young star clusters
We carry out N-body simulations to examine the effects of dynamical
interactions on planetary systems in young open star clusters. We explore how
the planetary populations in these star clusters evolve, and how this evolution
depends on the initial amount of substructure, the virial ratio, the cluster
mass and density, and the initial semi-major axis of the planetary systems. The
fraction of planetary systems that remains intact as a cluster member, fbps, is
generally well-described by the functional form fbps=f0(1+[a/a0]^c)^-1, where
(1-f0) is the fraction of stars that escapes from the cluster, a0 the critical
semi-major axis for survival, and c a measure for the width of the transition
region. The effect of the initial amount of substructure over time can be
quantified as fbps=A(t)+B(D), where A(t) decreases nearly linearly with time,
and B(D) decreases when the clusters are initially more substructured. Provided
that the orbital separation of planetary systems is smaller than the critical
value a0, those in clusters with a higher initial stellar density (but
identical mass) have a larger probability of escaping the cluster intact. These
results help us to obtain a better understanding of the difference between the
observed fractions of exoplanets-hosting stars in star clusters and in the
Galactic field. It also allows us to make predictions about the free-floating
planet population over time in different stellar environments.Comment: 14 pages, 9 figures, accepted for publication in MNRA
Electron Pair Resonance in the Coulomb Blockade
We study many-body corrections to the cotunneling current via a localized
state with energy at large bias voltages . We show that the
transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the
localized level, results in ionization resonance peaks in the third derivative
of the current with respect to , centered at . Our
results predict the existence of previously unnoticed structure within
Coulomb-blockade diamonds.Comment: 5 pages, 4 figure
Quantum Phase Transition in a Multi-Level Dot
We discuss electronic transport through a lateral quantum dot close to the
singlet-triplet degeneracy in the case of a single conduction channel per lead.
By applying the Numerical Renormalization Group, we obtain rigorous results for
the linear conductance and the density of states. A new quantum phase
transition of the Kosterlitz-Thouless type is found, with an exponentially
small energy scale close to the degeneracy point. Below , the
conductance is strongly suppressed, corresponding to a universal dip in the
density of states. This explains recent transport measurements.Comment: 4 pages, 5 eps figures, published versio
Non-invasive detection of molecular bonds in quantum dots
We performed charge detection on a lateral triple quantum dot with star-like
geometry. The setup allows us to interpret the results in terms of two double
dots with one common dot. One double dot features weak tunnel coupling and can
be understood with atom-like electronic states, the other one is strongly
coupled forming molecule-like states. In nonlinear measurements we identified
patterns that can be analyzed in terms of the symmetry of tunneling rates.
Those patterns strongly depend on the strength of interdot tunnel coupling and
are completely different for atomic- or molecule-like coupled quantum dots
allowing the non-invasive detection of molecular bonds.Comment: 4 pages, 4 figure
Negative differential conductance in quantum dots in theory and experiment
Experimental results for sequential transport through a lateral quantum dot
in the regime of spin blockade induced by spin dependent tunneling are compared
with theoretical results obtained by solving a master equation for independent
electrons. Orbital and spin effects in electron tunneling in the presence of a
perpendicular magnetic field are identified and discussed in terms of the
Fock-Darwin spectrum with spin. In the nonlinear regime, a regular pattern of
negative differential conductances is observed. Electrical asymmetries in
tunnel rates and capacitances must be introduced in order to account for the
experimental findings. Fast relaxation of the excited states in the quantum dot
have to be assumed, in order to explain the absence of certain structures in
the transport spectra.Comment: 4 pages, 4 figure
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