21,994 research outputs found
Pressure-control purge panel for automatic butt welding
Modification of a purge panel for use in an automatic butt weld reduces the drop in pressure between the regulators and the weld head and tube purge fitting. The invention affects air regulators for plants, regulating circuits for pneumatic valves, and automatic welding machines
Optimizing information flow in small genetic networks. II: Feed forward interactions
Central to the functioning of a living cell is its ability to control the
readout or expression of information encoded in the genome. In many cases, a
single transcription factor protein activates or represses the expression of
many genes. As the concentration of the transcription factor varies, the target
genes thus undergo correlated changes, and this redundancy limits the ability
of the cell to transmit information about input signals. We explore how
interactions among the target genes can reduce this redundancy and optimize
information transmission. Our discussion builds on recent work [Tkacik et al,
Phys Rev E 80, 031920 (2009)], and there are connections to much earlier work
on the role of lateral inhibition in enhancing the efficiency of information
transmission in neural circuits; for simplicity we consider here the case where
the interactions have a feed forward structure, with no loops. Even with this
limitation, the networks that optimize information transmission have a
structure reminiscent of the networks found in real biological systems
Helicity amplitudes and electromagnetic decays of hyperon resonances
We present results for the helicity amplitudes of the lowest-lying hyperon
resonances Y*, computed within the framework of the Bonn constituent-quark
model, which is based on the Bethe-Salpeter approach. The seven parameters
entering the model were fitted to the best known baryon masses. Accordingly,
the results for the helicity amplitudes are genuine predictions. Some hyperon
resonances are seen to couple more strongly to a virtual photon with finite Q^2
than to a real photon. Other Y*'s, such as the S_{01}(1670) Lambda resonance or
the S_{11}(1620) Sigma resonance, couple very strongly to real photons. We
present a qualitative argument for predicting the behaviour of the helicity
asymmetries of baryon resonances at high Q^2.Comment: 20 pages, 26 figures, uses svjour.cls style, submitted to the
European Physical Journal
Spin states of the first four holes in a silicon nanowire quantum dot
We report measurements on a silicon nanowire quantum dot with a clarity that
allows for a complete understanding of the spin states of the first four holes.
First, we show control of the hole number down to one. Detailed measurements at
perpendicular magnetic fields reveal the Zeeman splitting of a single hole in
silicon. We are able to determine the ground-state spin configuration for one
to four holes occupying the quantum dot and find a spin filling with
alternating spin-down and spin-up holes, which is confirmed by
magnetospectroscopy up to 9T. Additionally, a so far inexplicable feature in
single-charge quantum dots in many materials systems is analyzed in detail. We
observe excitations of the zero-hole ground-state energy of the quantum dot,
which cannot correspond to electronic or Zeeman states. We show that the most
likely explanation is acoustic phonon emission to a cavity between the two
contacts to the nanowire.Comment: 24 pages, 8 figures, both including supporting informatio
Spin diffusion in doped semiconductors
The behavior of spin diffusion in doped semiconductors is shown to be
qualitatively different than in undoped (intrinsic) ones. Whereas a spin packet
in an intrinsic semiconductor must be a multiple-band disturbance, involving
inhomogeneous distributions of both electrons and holes, in a doped
semiconductor a single-band disturbance is possible. For n-doped nonmagnetic
semiconductors the enhancement of diffusion due to a degenerate electron sea in
the conduction band is much larger for these single-band spin packets than for
charge packets, and can exceed an order of magnitude at low temperatures even
for equilibrium dopings as small as 10^16 cm^-3. In n-doped ferromagnetic and
semimagnetic semiconductors the motion of spin packets polarized antiparallel
to the equilibrium carrier spin polarization is predicted to be an order of
magnitude faster than for parallel polarized spin packets. These results are
reversed for p-doped semiconductors.Comment: 8 pages, 4 figure
Scaling Theory for Migration-Driven Aggregate Growth
We give a comprehensive rate equation description for the irreversible growth
of aggregates by migration from small to large aggregates. For a homogeneous
rate K(i;j) at which monomers migrate from aggregates of size i to those of
size j, that is, K(ai;aj) ~ a^{lambda} K(i,j), the mean aggregate size grows
with time as t^{1/(2-lambda)} for lambda<2. The aggregate size distribution
exhibits distinct regimes of behavior which are controlled by the scaling
properties of the migration rate from the smallest to the largest aggregates.
Our theory applies to diverse phenomena, such as the distribution of city
populations, late stage coarsening of non-symmetric binary systems, and models
for wealth exchange.Comment: 4 pages, 2-column revtex format. Revision to appear in PRL. Various
changes in response to referee comments. Figure from version 1 deleted but is
available at http://physics.bu.edu/~redne
Search for the Hypothetical pi -> mu x Decay
The KARMEN collaboration has reported the possible observation of a hitherto
unknown neutral and weakly interacting particle x, which is produced in the
decay pi -> mu + x with a mass m(x) = 33.9 MeV. We have searched for this
hypothetical decay branch by studying muons from pion decay in flight with the
LEPS spectrometer at the piE3 channel at PSI and find branching ratios BR(pi-
to mu- anti-x) < 4e-7 and BR(pi+ to mu+ x) < 7e-8 (95\% C.L.). Together with
the limit BR > 2e-8 derived in a recent theoretical paper our result would
leave only a narrow region for the existence of x if it is a heavy neutrino.Comment: 10 pages, TeX (uses epsf), 3 Postscript figures uu-encode
Quantum Dot Version of Berry's Phase: Half-Integer Orbital Angular Momenta
We show that Berry's geometrical (topological) phase for circular quantum
dots with an odd number of electrons is equal to \pi and that eigenvalues of
the orbital angular momentum run over half-integer values. The non-zero value
of the Berry's phase is provided by axial symmetry and two-dimensionality of
the system. Its particular value (\pi) is fixed by the Pauli exclusion
principle. Our conclusions agree with the experimental results of T. Schmidt
{\it at el}, \PR B {\bf 51}, 5570 (1995), which can be considered as the first
experimental evidence for the existence of a new realization of Berry's phase
and half-integer values of the orbital angular momentum in a system of an odd
number of electrons in circular quantum dots.Comment: 4 pages, 2 figure
Low Energy Analyzing Powers in Pion-Proton Elastic Scattering
Analyzing powers of pion-proton elastic scattering have been measured at PSI
with the Low Energy Pion Spectrometer LEPS as well as a novel polarized
scintillator target. Angular distributions between 40 and 120 deg (c.m.) were
taken at 45.2, 51.2, 57.2, 68.5, 77.2, and 87.2 MeV incoming pion kinetic
energy for pi+ p scattering, and at 67.3 and 87.2 MeV for pi- p scattering.
These new measurements constitute a substantial extension of the polarization
data base at low energies. Predictions from phase shift analyses are compared
with the experimental results, and deviations are observed at low energies.Comment: 15 pages, 4 figure
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