97,633 research outputs found
Air cushion vehicles: A briefing
Experience and characteristics; the powering, uses, and implications of large air cushion vehicles (ACV); and the conceptual design and operation of a nuclear powered ACV freighter and supporting facilities are described
Effective Hamiltonian for fermions in an optical lattice across Feshbach resonance
We derive the Hamiltonian for cold fermionic atoms in an optical lattice
across a broad Feshbach resonance, taking into account of both multiband
occupations and neighboring-site collisions. Under typical configurations, the
resulting Hamiltonian can be dramatically simplified to an effective
single-band model, which describes a new type of resonance between the local
dressed molecules and the valence bond states of fermionic atoms at neighboring
sites. On different sides of such a resonance, the effective Hamiltonian is
reduced to either a t-J model for the fermionic atoms or an XXZ model for the
dressed molecules. The parameters in these models are experimentally tunable in
the full range, which allows for observation of various phase transitions.Comment: 5 pages, 2 figure
Continuously observing a dynamically decoupled spin-1 quantum gas
We continuously observe dynamical decoupling in a spin-1 quantum gas using a
weak optical measurement of spin precession. Continuous dynamical decoupling
aims to dramatically modify the character and energy spectrum of spin states to
render them insensitive to parasitic fluctuations. Continuous observation
measures this new spectrum in a single-preparation of the quantum gas. The
measured time-series contains seven tones, which spectrogram analysis parses as
splittings, coherences, and coupling strengths between the decoupled states in
real-time. With this we locate a regime where a transition between two states
is decoupled from magnetic field instabilities up to fourth order,
complementary to the parallel work at higher fields by Trypogeorgos et al.
(arXiv:1706.07876). The decoupled microscale quantum gas offers magnetic
sensitivity in a tunable band, persistent over many milliseconds: the length
scales, frequencies, and durations relevant to many applications, including
sensing biomagnetic phenomena such as neural spike trains.Comment: 5+ pages, 4 figures, 1 table; revised citation of Trypogeorgos et al.
(2017
Fermiology of Cuprates from First Principles: From Small Pockets to the Luttinger Fermi surface
Fermiology, the shape and size of the Fermi surface, underpins the
low-temperature physical properties of a metal. Recent investigations of the
Fermi surface of high-Tc superconductors, however, show a most unusual
behavior: upon addition of carriers, ``Fermi'' pockets appear around nodal
(hole doping) and antinodal (electron doping) regions of the Brillouin zone in
the ``pseudogap'' state. With progressive doping, p, these evolve into
well-defined Fermi surfaces around optimal doping (p_opt), with no pseudogap.
Correspondingly, various physical responses, including d-wave
superconductivity, evolve from highly anomalous, up to p_opt, to more
conventional beyond. Describing this evolution holds the key to understanding
high-temperature superconductivity. Here, we present ab initio quantum chemical
results for cuprates, providing a quantitative description of the evolution of
the Fermi surface with doping. Our results constitute an ab initio
justification for several, hitherto proposed semiphenomenological theories,
offering an unified basis for understanding of various, unusual physical
responses of doped cuprates
A versatile high resolution objective for imaging quantum gases
We present a high resolution objective lens made entirely from catalog
singlets that has a numerical aperture of 0.36. It corrects for aberrations
introduced by a glass window and has a long working distance of 35mm, making it
suitable for imaging objects within a vacuum system. This offers simple high
resolution imaging for many in the quantum gas community. The objective
achieves a resolution of 1.3{\mu}m at the design wavelength of 780nm, and a
diffraction-limited field of view of 360{\mu}m when imaging through a 5mm
window. Images of a resolution target and a pinhole show quantitative agreement
with the simulated lens performance. The objective is suitable for
diffraction-limited imaging on the D2 line of all the alkalis by changing only
the aperture diameter, retaining numerical apertures above 0.32. The design
corrects for window thicknesses of up to 15mm if the singlet spacings are
modified
Fractal Cosmology in an Open Universe
The clustering of galaxies is well characterized by fractal properties, with
the presence of an eventual cross-over to homogeneity still a matter of
considerable debate. In this letter we discuss the cosmological implications of
a fractal distribution of matter, with a possible cross-over to homogeneity at
an undetermined scale R_{homo}. Contrary to what is generally assumed, we show
that, even when R_{homo} -> \infty, this possibility can be treated
consistently within the framework of the expanding universe solutions of
Friedmann. The fractal is a perturbation to an open cosmology in which the
leading homogeneous component is the cosmic background radiation (CBR). This
cosmology, inspired by the observed galaxy distributions, provides a simple
explanation for the recent data which indicate the absence of deceleration in
the expansion (q_o \approx 0). Correspondingly the `age problem' is also
resolved. Further we show that the model can be extended back from the
curvature dominated arbitrarily deep into the radiation dominated era, and we
discuss qualitatively the modifications to the physics of the anisotropy of the
CBR, nucleosynthesis and structure formation.Comment: 7 pages, no figures, to appear in Europhysics Letter
Competition between Antiferromagnetism and Superconductivity in High Cuprates
Using variational cluster perturbation theory we study the competition
between d-wave superconductivity (dSC) and antiferromagnetism (AF) in the the
t-t'-t''-U Hubbard model. Large scale computer calculations reproduce the
overall ground state phase diagram of the high-temperature superconductors as
well as the one-particle excitation spectra for both hole- and electron-doping.
We identify clear signatures of the Mott gap as well as of AF and of dSC that
should be observable in photoemission experiments.Comment: 4 pages, 4 figure
Mopra CO Observations of the Bubble HII Region RCW120
We use the Mopra radio telescope to test for expansion of the molecular gas
associated with the bubble HII region RCW120. A ring, or bubble, morphology is
common for Galactic HII regions, but the three-dimensional geometry of such
objects is still unclear. Detected near- and far-side expansion of the
associated molecular material would be consistent with a three-dimensional
spherical object. We map the transitions of CO,
CO, CO, and CO, and detect emission from all
isotopologues. We do not detect the masing lines of
CHOH at 108.8939 GHz. The strongest CO emission is from the
photodissociation region (PDR), and there is a deficit of emission toward the
bubble interior. We find no evidence for expansion of the molecular material
associated with RCW120 and therefore can make no claims about its geometry. The
lack of detected expansion is roughly in agreement with models for the
time-evolution of an HII region like RCW120, and is consistent with an
expansion speed of . Single-position CO spectra show
signatures of expansion, which underscores the importance of mapped spectra for
such work. Dust temperature enhancements outside the PDR of RCW120 coincide
with a deficit of emission in CO, confirming that these temperature
enhancements are due to holes in the RCW120 PDR. H emission shows that
RCW120 is leaking of the ionizing photons into the interstellar
medium (ISM) through PDR holes at the locations of the temperature
enhancements. H-alpha emission also shows a diffuse "halo" from leaked photons
not associated with discrete holes in the PDR. Overall of all
ionizing photons are leaking into the nearby ISM.Comment: 35 pages, 14 figures. Accepted to Ap
First-principles calculation of the intersublattice exchange interactions and Curie temperatures of full Heusler alloys Ni2MnX (X=Ga, In, Sn, Sb)
The interatomic exchange interactions and Curie temperatures in Ni-based full
Heusler alloys Ni2MnX with X=Ga, In, Sn and Sb are studied within the framework
of the density-functional theory. The calculation of the exchange parameters is
based on the frozen-magnon approach. Despite closeness of the experimental
Curie temperatures for all four systems their magnetism appeared to differ
strongly. This difference involves both the Mn-Mn and Mn-Ni exchange
interactions. The Curie temperatures, Tc, are calculated within the mean-field
approximation by solving a matrix equation for a multi-sublattice system. Good
agreement with experiment for all four systems is obtained. The role of
different exchange interactions in the formation of Tc of the systems is
discussed.Comment: 6 pages, 4 figure
Unresolved versus resolved: testing the validity of young simple stellar population models with VLT/MUSE observations of NGC 3603
CONTEXT. Stellar populations are the building blocks of galaxies including
the Milky Way. The majority, if not all extragalactic studies are entangled
with the use of stellar population models given the unresolved nature of their
observation. Extragalactic systems contain multiple stellar populations with
complex star formation histories. However, their study is mainly based upon the
principles of simple stellar populations (SSP). Hence, it is critical to
examine the validity of SSP models. AIMS. This work aims to empirically test
the validity of SSP models. This is done by comparing SSP models against
observations of spatially resolved young stellar population in the
determination of its physical properties, i.e. age and metallicity. METHODS.
Integral field spectroscopy of a young stellar cluster in the Milky Way, NGC
3603, is used to study the properties of the cluster both as a resolved and
unresolved stellar population. The unresolved stellar population is analysed
using the H equivalent width as an age indicator, and the ratio of
strong emission lines to infer metallicity. In addition, spectral energy
distribution (SED) fitting using STARLIGHT, is used to infer these properties
from the integrated spectrum. Independently, the resolved stellar population is
analysed using the color-magnitude diagram (CMD) for age and metallicity
determination. As the SSP model represents the unresolved stellar population,
the derived age and metallicity are put to test whether they agree with those
derived from resolved stars. RESULTS. The age and metallicity estimate of NGC
3603 derived from integrated spectroscopy are confirmed to be within the range
of those derived from the CMD of the resolved stellar population, including
other estimates found in the literature. The result from this pilot study
supports the reliability of SSP models for studying unresolved young stellar
populations.Comment: 9 pages, 5 figures, accepted to A&
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