2,764 research outputs found
Multichannel pulse height analyzer is inexpensive, features low power requirements
Consumption multichannel pulse height analyzer performs balloon and rocket investigations of solar neutrons with energies greater than 10 MeV. The lightweight unit can operate in a temperature range of minus 30 degrees to plus 70 degrees C and withstand storage temperatures from minus 50 degrees to plus 90 degrees C
Mitofusins Mfn1 and Mfn2 coordinately regulate mitochondrial fusion and are essential for embryonic development
Mitochondrial morphology is determined by a dynamic equilibrium between organelle fusion and fission, but the significance of these processes in vertebrates is unknown. The mitofusins, Mfn1 and Mfn2, have been shown to affect mitochondrial morphology when overexpressed. We find that mice deficient in either Mfn1 or Mfn2 die in midgestation. However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal. Embryonic fibroblasts lacking Mfn1 or Mfn2 display distinct types of fragmented mitochondria, a phenotype we determine to be due to a severe reduction in mitochondrial fusion. Moreover, we find that Mfn1 and Mfn2 form homotypic and heterotypic complexes and show, by rescue of mutant cells, that the homotypic complexes are functional for fusion. We conclude that Mfn1 and Mfn2 have both redundant and distinct functions and act in three separate molecular complexes to promote mitochondrial fusion. Strikingly, a subset of mitochondria in mutant cells lose membrane potential. Therefore, mitochondrial fusion is essential for embryonic development, and by enabling cooperation between mitochondria, has protective effects on the mitochondrial population
Cell Size Control in Yeast
Cell size is an important adaptive trait that influences nearly all aspects of cellular physiology. Despite extensive characterization of the cell-cycle regulatory network, the molecular mechanisms coupling cell growth to division, and thereby controlling cell size, have remained elusive. Recent work in yeast has reinvigorated the size control field and suggested provocative mechanisms for the distinct functions of setting and sensing cell size. Further examination of size-sensing models based on spatial gradients and molecular titration, coupled with elucidation of the pathways responsible for nutrient-modulated target size, may reveal the fundamental principles of eukaryotic cell size control
Systematic {\it ab initio} study of the magnetic and electronic properties of all 3d transition metal linear and zigzag nanowires
It is found that all the zigzag chains except the nonmagnetic (NM) Ni and
antiferromagnetic (AF) Fe chains which form a twisted two-legger ladder, look
like a corner-sharing triangle ribbon, and have a lower total energy than the
corresponding linear chains. All the 3d transition metals in both linear and
zigzag structures have a stable or metastable ferromagnetic (FM) state. The
electronic spin-polarization at the Fermi level in the FM Sc, V, Mn, Fe, Co and
Ni linear chains is close to 90% or above. In the zigzag structure, the AF
state is more stable than the FM state only in the Cr chain. It is found that
the shape anisotropy energy may be comparable to the electronic one and always
prefers the axial magnetization in both the linear and zigzag structures. In
the zigzag chains, there is also a pronounced shape anisotropy in the plane
perpendicular to the chain axis. Remarkably, the axial magnetic anisotropy in
the FM Ni linear chain is gigantic, being ~12 meV/atom. Interestingly, there is
a spin-reorientation transition in the FM Fe and Co linear chains when the
chains are compressed or elongated. Large orbital magnetic moment is found in
the FM Fe, Co and Ni linear chains
Field-induced structure transformation in electrorheological solids
We have computed the local electric field in a body-centered tetragonal (BCT)
lattice of point dipoles via the Ewald-Kornfeld formulation, in an attempt to
examine the effects of a structure transformation on the local field strength.
For the ground state of an electrorheological solid of hard spheres, we
identified a novel structure transformation from the BCT to the face-centered
cubic (FCC) lattices by changing the uniaxial lattice constant c under the hard
sphere constraint. In contrast to the previous results, the local field
exhibits a non-monotonic transition from BCT to FCC. As c increases from the
BCT ground state, the local field initially decreases rapidly towards the
isotropic value at the body-centered cubic lattice, decreases further, reaching
a minimum value and increases, passing through the isotropic value again at an
intermediate lattice, reaches a maximum value and finally decreases to the FCC
value. An experimental realization of the structure transformation is
suggested. Moreover, the change in the local field can lead to a generalized
Clausius-Mossotti equation for the BCT lattices.Comment: Submitted to Phys. Rev.
Effects of geometric anisotropy on local field distribution: Ewald-Kornfeld formulation
We have applied the Ewald-Kornfeld formulation to a tetragonal lattice of
point dipoles, in an attempt to examine the effects of geometric anisotropy on
the local field distribution. The various problems encountered in the
computation of the conditionally convergent summation of the near field are
addressed and the methods of overcoming them are discussed. The results show
that the geometric anisotropy has a significant impact on the local field
distribution. The change in the local field can lead to a generalized
Clausius-Mossotti equation for the anisotropic case.Comment: Accepted for publications, Journal of Physics: Condensed Matte
Boundary conditions for interfaces of electromagnetic (photonic) crystals and generalized Ewald-Oseen extinction principle
The problem of plane-wave diffraction on semi-infinite orthorhombic
electromagnetic (photonic) crystals of general kind is considered. Boundary
conditions are obtained in the form of infinite system of equations relating
amplitudes of incident wave, eigenmodes excited in the crystal and scattered
spatial harmonics. Generalized Ewald-Oseen extinction principle is formulated
on the base of deduced boundary conditions. The knowledge of properties of
infinite crystal's eigenmodes provides option to solve the diffraction problem
for the corresponding semi-infinite crystal numerically. In the case when the
crystal is formed by small inclusions which can be treated as point dipolar
scatterers with fixed direction the problem admits complete rigorous analytical
solution. The amplitudes of excited modes and scattered spatial harmonics are
expressed in terms of the wave vectors of the infinite crystal by closed-form
analytical formulae. The result is applied for study of reflection properties
of metamaterial formed by cubic lattice of split-ring resonators.Comment: 15 pages, 8 figures, submitted to PR
Statistical-mechanical theory of the overall magnetic properties of mesocrystals
The mesocrystal showing both electrorheological and magnetorheological
effects is called electro-magnetorheological (EMR) solids. Prediction of the
overall magnetic properties of the EMR solids is a challenging task due to the
coexistence of the uniaxially anisotropic behavior and structural transition as
well as long-range interaction between the suspended particles. To consider the
uniaxial anisotropy effect, we present an anisotropic Kirkwood-Fr\"{o}hlich
equation for calculating the effective permeabilities by adopting an explicit
characteristic spheroid rather than a characteristic sphere used in the
derivation of the usual Kirkwood-Fr\"{o}hlich equation. Further, by applying an
Ewald-Kornfeld formulation we are able to investigate the effective
permeability by including the structural transition and long-range interaction
explicitly. Our theory can reduce to the usual Kirkwood-Fr\"{o}hlich equation
and Onsager equation naturally. To this end, the numerical simulation shows the
validity of monitoring the structure of EMR solids by detecting their effective
permeabilities.Comment: 14 pages, 1 figur
Two-Dimensional Wigner Crystal in Anisotropic Semiconductor
We investigate the effect of mass anisotropy on the Wigner crystallization
transition in a two-dimensional (2D) electron gas. The static and dynamical
properties of a 2D Wigner crystal have been calculated for arbitrary 2D Bravais
lattices in the presence of anisotropic mass, as may be obtainable in Si
MOSFETs with (110) surface. By studying the stability of all possible lattices,
we find significant change in the crystal structure and melting density of the
electron lattice with the lowest ground state energy.Comment: 4 pages, revtex, 4 figure
Subtle competition between ferromagnetic and antiferromagnetic order in a Mn(II) - free radical ferrimagnetic chain
The macroscopic magnetic characterization of the Mn(II) - nitronyl nitroxide
free radical chain (Mn(hfac)2(R)-3MLNN) evidenced its transition from a
1-dimensional behavior of ferrimagnetic chains to a 3-dimensional ferromagnetic
long range order below 3 K. Neutron diffraction experiments, performed on a
single crystal around the transition temperature, led to a different conclusion
: the magnetic Bragg reflections detected below 3 K correspond to a canted
antiferromagnet where the magnetic moments are mainly oriented along the chain
axis. Surprisingly in the context of other compounds in this family of magnets,
the interchain coupling is antiferromagnetic. This state is shown to be very
fragile since a ferromagnetic interchain arrangement is recovered in a weak
magnetic field. This peculiar behavior might be explained by the competition
between dipolar interaction, shown to be responsible for the antiferromagnetic
long range order below 3 K, and exchange interaction, the balance between these
interactions being driven by the strong intrachain spin correlations. More
generally, this study underlines the need, in this kind of molecular compounds,
to go beyond macroscopic magnetization measurements.Comment: 12 pages, 10 figures, submitted to Phys. Rev.
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