5,162 research outputs found
Development of an ultra-low-shock separation nut
The technical problems encountered in the development of an advanced separation nut design are described. The nut is capable of sustaining a large preload and releasing that load with a low level of induced pyrotechnic shock, while demonstrating a tolerance for extremely high shock imposed by other pyrotechnic devices. The analysis of the separation nut was performed to acquire additional understanding of the phenomena affecting operation of the nut and to provide quantitative evaluation of design modification for aerospace applications
Composite fermions in bands with N-fold rotational symmetry
We study the effect of band anisotropy with discrete rotational symmetry
(where ) in the quantum Hall regime of two-dimensional electron
systems. We focus on the composite Fermi liquid (CFL) at half filling of the
lowest Landau level. We find that the magnitude of anisotropy transferred to
the composite fermions decreases very rapidly with . We demonstrate this by
performing density matrix normalization group calculations on the CFL, and
comparing the anisotropy of the composite fermion Fermi contour with that of
the (non-interacting) electron Fermi contour at zero magnetic field. We also
show that the effective interaction between the electrons after projecting into
a single Landau level is much less anisotropic than the band, a fact which does
not depend on filling and thus has implications for other quantum Hall states
as well. Our results confirm experimental observations on anisotropic bands
with warped Fermi contours, where the only detectable effect on the composite
Fermi contour is an elliptical distortion ().Comment: 6 pages + bibliography, 5 figure
Connection between Fermi contours of zero-field electrons and composite fermions in two-dimensional systems
We investigate the relation between the Fermi sea (FS) of zero-field carriers
in two-dimensional systems and the FS of the corresponding composite fermions
which emerge in a high magnetic field at filling , as the
kinetic energy dispersion is varied. We study cases both with and without
rotational symmetry, and find that there is generally no straightforward
relation between the geometric shapes and topologies of the two FSs. In
particular, we show analytically that the composite Fermi liquid (CFL) is
completely insensitive to a wide range of changes to the zero-field dispersion
which preserve rotational symmetry, including ones that break the zero-field FS
into multiple disconnected pieces. In the absence of rotational symmetry, we
show that the notion of `valley pseudospin' in many-valley systems is
generically not transferred to the CFL, in agreement with experimental
observations. We also discuss how a rotationally symmetric band structure can
induce a reordering of the Landau levels, opening interesting possibilities of
observing higher-Landau-level physics in the high-field regime.Comment: 7 pages + references, 7 figures. Added many-body DMRG calculatio
Geometry of flux attachment in anisotropic fractional quantum Hall states
Fractional quantum Hall (FQH) states are known to possess an internal metric
degree of freedom that allows them to minimize their energy when contrasting
geometries are present in the problem (e.g., electron band mass and dielectric
tensor). We investigate the internal metric of several incompressible FQH
states by probing its response to band mass anisotropy using infinite DMRG
simulations on a cylinder geometry. We test and apply a method to extract the
internal metric of a FQH state from its guiding center structure factor. We
find that the response to band mass anisotropy is approximately the same for
states in the same Jain sequence, but changes substantially between different
sequences. We provide a theoretical explanation of the observed behavior of
primary states at filling in terms of a minimal microscopic model
of flux attachment.Comment: 12 pages including references, 14 figure
Nonequilibrium Response from the dissipative Liouville Equation
The problem of response of nonequilibrium systems is currently under intense
investigation. We propose a general method of solution of the Liouville
Equation for thermostatted particle systems subjected to external forces which
retains only the slow degrees of freedom, by projecting out the majority of
fast variables. Response formulae, extending the Green-Kubo relations to
dissipative dynamics are provided, and comparison with numerical data is
presented
The Milky Way as a High Redshift Galaxy: The Importance of Thick Disk Formation in Galaxies
We compare the star-formation history and dynamics of the Milky Way (MW) with
the properties of distant disk galaxies. During the first ~4 Gyr of its
evolution, the MW formed stars with a high star-formation intensity (SFI),
Sigma_SFR~0.6 Msun/yr/kpc2 and as a result, generated outflows and high
turbulence in its interstellar medium. This intense phase of star formation
corresponds to the formation of the thick disk. The formation of the thick disk
is a crucial phase which enables the MW to have formed approximately half of
its total stellar mass by z~1 which is similar to "MW progenitor galaxies"
selected by abundance matching. This agreement suggests that the formation of
the thick disk may be a generic evolutionary phase in disk galaxies. Using a
simple energy injection-kinetic energy relationship between the 1-D velocity
dispersion and SFI, we can reproduce the average perpendicular dispersion in
stellar velocities of the MW with age. This relationship, its inferred
evolution, and required efficiency are consistent with observations of galaxies
from z~0-3. The high turbulence generated by intense star formation naturally
resulted in a thick disk, a chemically well-mixed ISM, and is the mechanism
that links the evolution of MW to the observed characteristics of distant disk
galaxies.Comment: 5 pages, 4 figures; accepted to ApJ Letter
On the nature of the magnetic ground-state wave function of V_2O_3
After a brief historical introduction, we dwell on two recent experiments in
the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray
scattering and non-reciprocal linear dichroism, whose interpretations are in
conflict, as they require incompatible magnetic space groups. Such a conflict
is critically reviewed, in the light of the present literature, and new
experimental tests are suggested, in order to determine unambiguously the
magnetic group. We then focus on the correlated, non-local nature of the
ground-state wave function, that is at the basis of some drawbacks of the LDA+U
approach: we singled out the physical mechanism that makes LDA+U unreliable,
and indicate the way out for a possible remedy. Finally we explain, by means of
a symmetry argument related to the molecular wave function, why the magnetic
moment lies in the glide plane, even in the absence of any local symmetry at
vanadium sites.Comment: 7 pages, 1 figur
Constraining Fundamental Physics with Future CMB Experiments
The Planck experiment will soon provide a very accurate measurement of Cosmic
Microwave Background anisotropies. This will let cosmologists determine most of
the cosmological parameters with unprecedented accuracy. Future experiments
will improve and complement the Planck data with better angular resolution and
better polarization sensitivity. This unexplored region of the CMB power
spectrum contains information on many parameters of interest, including
neutrino mass, the number of relativistic particles at recombination, the
primordial Helium abundance and the injection of additional ionizing photons by
dark matter self-annihilation. We review the imprint of each parameter on the
CMB and forecast the constraints achievable by future experiments by performing
a Monte Carlo analysis on synthetic realizations of simulated data. We find
that next generation satellite missions such as CMBPol could provide valuable
constraints with a precision close to that expected in current and near future
laboratory experiments. Finally, we discuss the implications of this
intersection between cosmology and fundamental physics.Comment: 11 pages, 14 figure
A new look at the kinematics of the bulge from an N-body model
(Abridged) By using an N-body simulation of a bulge that was formed via a bar
instability mechanism, we analyse the imprints of the initial (i.e. before bar
formation) location of stars on the bulge kinematics, in particular on the
heliocentric radial velocity distribution of bulge stars. Four different
latitudes were considered: , , , and
, along the bulge minor axis as well as outside it, at
and . The bulge X-shaped structure comprises
stars that formed in the disk at different locations. Stars formed in the outer
disk, beyond the end of the bar, which are part of the boxy peanut-bulge
structure may show peaks in the velocity distributions at positive and negative
heliocentric radial velocities with high absolute values that can be larger
than 100 , depending on the observed direction. In some
cases the structure of the velocity field is more complex and several peaks are
observed. Stars formed in the inner disk, the most numerous, contribute
predominantly to the X-shaped structure and present different kinematic
characteristics. Our results may enable us to interpret the cold high-velocity
peak observed in the APOGEE commissioning data, as well as the excess of
high-velocity stars in the near and far arms of the X-shaped structure at
= and =. When compared with real data, the kinematic
picture becomes more complex due to the possible presence in the observed
samples of classical bulge and/or thick disk stars. Overall, our results point
to the existence of complex patterns and structures in the bulge velocity
fields, which are generated by the bar. This suggests that caution should be
used when interpreting the bulge kinematics: the presence of substructures,
peaks and clumps in the velocity fields is not necessarily a sign of past
accretion events.Comment: 21 pages, 18 figures. Accepted for publication in A&
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