2,982 research outputs found
Co-ordinating retinal histogenesis: early cell cycle exit enhances early cell fate determination in the Xenopus retina
The laminar arrays of distinct cell types in the vertebrate retina are built by a histogenic process in which cell fate is correlated with birth order. To explore this co-ordination mechanistically, we altered the relative timing of cell cycle exit in the developing Xenopus retina and asked whether this affected the activity of neural determinants. We found that Xath5, a bHLH proneural gene that promotes retinal ganglion cell (RGC) fate, ( Kanekar, S., Perron, M., Dorsky, R., Harris, W. A., Jan, L. Y., Jan, Y. N. and Vetter, M. L. (1997) Neuron 19, 981-994), does not cause these cells to be born prematurely. To drive cells out of the cell cycle early, therefore, we misexpressed the cyclin kinase inhibitor, p27Xic1. We found that early cell cycle exit potentiates the ability of Xath5 to promote RGC fate. Conversely, the cell cycle activator, cyclin E1, which inhibits cell cycle exit, biases Xath5-expressing cells toward later neuronal fates. We found that Notch activation in this system caused cells to exit the cell cycle prematuely, and when it is misexpressed with Xath5, it also potentiates the induction of RGCs. The potentiation is counteracted by co-expression of cyclin E1. These results suggest a model of histogenesis in which the activity of factors that promote early cell cycle exit enhances the activity of factors that promote early cellular fates
Calculation of electronic properties of amorphous alloys
We describe the application of the
locally-self-consistent-multiple-scattering (LSMS)[1] method to amorphous
alloys. The LSMS algorithm is optimized for the Intel XP/S-150, a
multiple-instruction-multiple-data parallel computer with 1024 nodes and 2
compute processors per node. The electron density at each site is determined by
solving the multiple scattering equation for atoms within a specified distance
of the atom under consideration. Because this method is carried out in real
space it is ideal for treating amorphous alloys. We have adapted the code to
the calculation of the electronic properties of amorphous alloys. In these
calculations we determine the potentials in the atomic sphere approximation
self consistently at each site, unlike previous calculations[2] where we
determined the potentials self consistently at an average site. With these
self-consistent potentials, we then calculate electronic properties of various
amorphous alloy systems. We present calculated total electronic densities of
states for amorphous NiP and NiPdP with 300
atoms in a supercell.Comment: 10 pages, plain tex, 2 figures. Paper accepted for publication in
Proceedings of LAM-9 and Journal of non-Crystalline Solids. Please request
preprints from J.C. Swihart ([email protected]
Muon spin relaxation study of the magnetism in unilluminated Prussian Blue analogue photomagnets
We present longitudinal field muon spin relaxation (SR) measurements in
the unilluminated state of the photo-sensitive molecular magnetic Co-Fe
Prussian blue analogues MCo[Fe(CN)] HO, where
M=K and Rb with and , respectively. These results are
compared to those obtained in the stoichiometric limit,
Co[Fe(CN)] HO, which is not photo-sensitive. We find
evidence for correlation between the range of magnetic ordering and the value
of in the unilluminated state which can be explained using a site
percolation model.Comment: 7 pages, 12 figure
Compact spherical neutron polarimeter using high-T-c YBCO films
We describe a simple, compact device for spherical neutron polarimetry measurements at small neutron scattering angles. The device consists of a sample chamber with very low (<0.01 G) magnetic field flanked by regions within which the neutron polarization can be manipulated in a controlled manner. This allows any selected initial and final polarization direction of the neutrons to be obtained. We have constructed a prototype device using high-Tc superconducting films and mu-metal to isolate regions with different magnetic fields and tested device performance in transmission geometry. Finite-element methods were used to simulate the device’s field profile and these have been verified by experiment using a small solenoid as a test sample. Measurements are reported using both monochromatic and polychromatic neutron sources. The results show that the device is capable of extracting sample information and distinguishing small angular variations of the sample magnetic field. As a more realistic test, we present results on the characterization of a 10 μm thick Permalloy film in zero magnetic field, as well as its response to an external magnetic field
Causal explanation for observed superluminal behavior of microwave propagation in free space
In this paper we present a theoretical analysis of an experiment by Mugnai
and collaborators where superluminal behavior was observed in the propagation
of microwaves. We suggest that what was observed can be well approximated by
the motion of a superluminal X wave. Furthermore the experimental results are
also explained by the so called scissor effect which occurs with the
convergence of pairs of signals coming from opposite points of an annular
region of the mirror and forming an interference peak on the intersection axis
traveling at superluminal speed. We clarify some misunderstandings concerning
this kind of electromagnetic wave propagation in vacuum.Comment: 9 pages, 3 figures, accepted for publication in Physics Letters
Thermodynamic and Tunneling Density of States of the Integer Quantum Hall Critical State
We examine the long wave length limit of the self-consistent Hartree-Fock
approximation irreducible static density-density response function by
evaluating the charge induced by an external charge. Our results are consistent
with the compressibility sum rule and inconsistent with earlier work that did
not account for consistency between the exchange-local-field and the disorder
potential. We conclude that the thermodynamic density of states is finite, in
spite of the vanishing tunneling density of states at the critical energy of
the integer quantum Hall transition.Comment: 5 pages, 4 figures, minor revisions, published versio
Quantum information processing using Josephson junctions coupled through cavities
Josephson junctions have been shown to be a promising solid-state system for
implementation of quantum computation. The significant two-qubit gates are
generally realized by the capacitive coupling between the nearest neighbour
qubits. We propose an effective Hamiltonian to describe charge qubits coupled
through the cavity. We find that nontrivial two-qubit gates may be achieved by
this coupling. The ability to interconvert localized charge qubits and flying
qubits in the proposed scheme implies that quantum network can be constructed
using this large scalable solid-state system.Comment: 5 pages, to appear in Phys Rev A; typos corrected, solutions in last
eqs. correcte
Completely packed O(n) loop models and their relation with exactly solved coloring models.
Theoretical Physic
Planes of satellite galaxies: when exceptions are the rule
The detection of planar structures within the satellite systems of both the Milky Way (MW) and Andromeda (M31) has been reported as being in stark contradiction to the predictions of the standard cosmological model (Λ cold dark matter – ΛCDM). Given the ambiguity in defining a planar configuration, it is unclear how to interpret the low incidence of the MW and M31 planes in ΛCDM. We investigate the prevalence of satellite planes around galactic mass haloes identified in high-resolution cosmological simulations. We find that planar structures are very common, and that ∼10 per cent of ΛCDM haloes have even more prominent planes than those present in the Local Group. While ubiquitous, the planes of satellite galaxies show a large diversity in their properties. This precludes using one or two systems as small-scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation. This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the Local Group and ΛCDM predictions. In fact, ∼10 per cent of ΛCDM galactic haloes have planes of satellites that are as infrequent as the MW and M31 planes. The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and M31 systems, respectively
Critical temperature oscillations in magnetically coupled superconducting mesoscopic loops
We study the magnetic interaction between two superconducting concentric
mesoscopic Al loops, close to the superconducting/normal phase transition. The
phase boundary is measured resistively for the two-loop structure as well as
for a reference single loop. In both systems Little-Parks oscillations,
periodic in field are observed in the critical temperature Tc versus applied
magnetic field H. In the Fourier spectrum of the Tc(H) oscillations, a weak
'low frequency' response shows up, which can be attributed to the inner loop
supercurrent magnetic coupling to the flux of the outer loop. The amplitude of
this effect can be tuned by varying the applied transport current.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
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