5,997 research outputs found
Reversable heat flow through the carbon nanotube junctions
Microscopic mechanisms of externally controlled reversable heat flow through
the carbon nanotube junctions (NJ) are studied theoretically. Our model
suggests that the heat is transfered along the tube section by
electrons () and holes () moving ballistically in either in parallel or
in opposite directions and accelerated by the bias source-drain voltage (Peltier effect). We compute the Seebeck coefficient , electric
and thermal conductivities and find that their magnitudes
strongly depend on and . The sign reversal of
versus the sign of formerly observed experimentally is interpreted
in this work in terms of so-called chiral tunneling phenomena (Klein paradox)
Phase field theory of polycrystalline solidification in three dimensions
A phase field theory of polycrystalline solidification is presented that is
able to describe the nucleation and growth of anisotropic particles with
different crystallographic orientation in three dimensions. As opposed with the
two-dimensional case, where a single orientation field suffices, in three
dimensions, minimum three fields are needed. The free energy of grain
boundaries is assumed to be proportional to the angular difference between the
adjacent crystals expressed here in terms of the differences of the four
symmetric Euler parameters. The equations of motion for these fields are
obtained from variational principles. Illustrative calculations are performed
for polycrystalline solidification with dendritic, needle and spherulitic
growth morphologies.Comment: 7 pages, 4 figures, submitted to Europhysics Letters on 14th
February, 200
Spin dephasing and photoinduced spin diffusion in high-mobility 110-grown GaAs-AlGaAs two-dimensional electron systems
We have studied spin dephasing and spin diffusion in a high-mobility
two-dimensional electron system, embedded in a GaAs/AlGaAs quantum well grown
in the [110] direction, by a two-beam Hanle experiment. For very low excitation
density, we observe spin lifetimes of more than 16 ns, which rapidly decrease
as the pump intensity is increased. Two mechanisms contribute to this decrease:
the optical excitation produces holes, which lead to a decay of electron spin
via the Bir-Aranov-Pikus mechanism and recombination with spin-polarized
electrons. By scanning the distance between the pump and probe beams, we
observe the diffusion of spin-polarized electrons over more than 20 microns.
For high pump intensity, the spin polarization in a distance of several microns
from the pump beam is larger than at the pump spot, due to the reduced
influence of photogenerated holes.Comment: 4 pages, 3 figure
Magnetic properties of nanosized diluted magnetic semiconductors with band splitting
The continual model of the nonuniform magnetism in thin films and wires of a
diluted magnetic semiconductor is considered with taking into account the
finite spin polarization of carriers responsible for the indirect interaction
of magnetic impurities (e.g. via RKKY mechanism). Spatial distributions (across
the film thickness or the wire radius) of the magnetizaton and carrier
concentrations of different spin orientations, as well as the temperature
dependence of the average magnetization are determined as the solution of the
nonlinear integral equation
The Ultrasensitivity of Living Polymers
Synthetic and biological living polymers are self-assembling chains whose
chain length distributions (CLDs) are dynamic. We show these dynamics are
ultrasensitive: even a small perturbation (e.g. temperature jump) non-linearly
distorts the CLD, eliminating or massively augmenting short chains. The origin
is fast relaxation of mass variables (mean chain length, monomer concentration)
which perturbs CLD shape variables before these can relax via slow chain growth
rate fluctuations. Viscosity relaxation predictions agree with experiments on
the best-studied synthetic system, alpha-methylstyrene.Comment: 4 pages, submitted to Phys. Rev. Let
Limitations on the attainable intensity of high power lasers
It is shown that even a single pair created by a super strong laser
field in vacuum would cause development of an avalanche-like QED cascade which
rapidly depletes the incoming laser pulse. This confirms the old N. Bohr
conjecture that the electric field of the critical QED strength
could never be created.Comment: 4 pages, 3 figure
Detection of large magneto-anisotropy of electron spin dephasing in a high-mobility two-dimensional electron system in a GaAs/AlGaAs quantum well
In time-resolved Faraday rotation experiments we have detected an inplane
anisotropy of the electron spin-dephasing time (SDT) in an
--modulation-doped GaAs/AlGaAs single quantum well. The SDT
was measured with magnetic fields of T, applied in the and
inplane crystal directions of the GaAs quantum well. For fields
along , we have found an up to a factor of about 2 larger SDT than
in the perpendicular direction. Fully microscopic calculations, by numerically
solving the kinetic spin Bloch equations considering the D'yakonov-Perel' and
the Bir-Aronov-Pikus mechanisms, reproduce the experimental findings
quantitatively. This quantitative analysis of the data allowed us to determine
the relative strengths of Rashba and Dresselhaus terms in our sample. Moreover,
we could estimate the SDT for spins aligned in the {\em inplane}
direction to be on the order of several nanoseconds, which is up to two orders
of magnitude larger than that in the perpendicular {\em inplane} direction.Comment: 4 pages, 4 figures, to be published in PR
Testing the chemical tagging technique with open clusters
Context. Stars are born together from giant molecular clouds and, if we
assume that the priors were chemically homogeneous and well-mixed, we expect
them to share the same chemical composition. Most of the stellar aggregates are
disrupted while orbiting the Galaxy and most of the dynamic information is
lost, thus the only possibility of reconstructing the stellar formation history
is to analyze the chemical abundances that we observe today.
Aims. The chemical tagging technique aims to recover disrupted stellar
clusters based merely on their chemical composition. We evaluate the viability
of this technique to recover co-natal stars that are no longer gravitationally
bound.
Methods. Open clusters are co-natal aggregates that have managed to survive
together. We compiled stellar spectra from 31 old and intermediate-age open
clusters, homogeneously derived atmospheric parameters, and 17 abundance
species, and applied machine learning algorithms to group the stars based on
their chemical composition. This approach allows us to evaluate the viability
and efficiency of the chemical tagging technique.
Results. We found that stars at different evolutionary stages have distinct
chemical patterns that may be due to NLTE effects, atomic diffusion, mixing,
and biases. When separating stars into dwarfs and giants, we observed that a
few open clusters show distinct chemical signatures while the majority show a
high degree of overlap. This limits the recovery of co-natal aggregates by
applying the chemical tagging technique. Nevertheless, there is room for
improvement if more elements are included and models are improved.Comment: accepted for publication in Astronomy and Astrophysics. Corrected
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