2,384 research outputs found
Prospects for Interstellar Propulsion
In recognition of the increasing prospects for Earth-like exoplanet discoveries and its significance for spurring future interstellar voyages of discovery, the United States Congress recently directed NASA to undertake an interstellar mission technology assessment report. In response to this legislative charge to action, NASA has undertaken a series of extramural interstellar workshops aimed at identifying and evaluating technology concepts for enabling an interstellar scientific probe mission, associated technical challenges, technology readiness level assessments, risks, potential near-term milestones, and funding requirements. This paper summarizes these activities and discusses the scientific and technical rationale for a long-term program consisting of incremental, staged technical developments that are extensible for interstellar travel to a nearby star system over many decades
Induced p-wave superfluidity in strongly interacting imbalanced Fermi gases
The induced interaction among the majority spin species, due to the presence
of the minority species, is computed for the case of a population-imbalanced
resonantly-interacting Fermi gas. It is shown that this interaction leads to an
instability, at low temperatures, of the recently observed polaron Fermi liquid
phase of strongly imbalanced Fermi gases to a p-wave superfluid state. We find
that the associated transition temperature, while quite small in the weakly
interacting BCS regime, is experimentally accessible in the strongly
interacting unitary regime.Comment: Published versio
BEC-BCS crossover in "magnetized" Feshbach-resonantly paired superfluids
We map out the detuning-magnetization phase diagram for a ``magnetized''
(unequal number of atoms in two pairing hyperfine states) gas of fermionic
atoms interacting via an s-wave Feshbach resonance (FR). For large positive FR
detuning a normal magnetized Fermi gas is stable above an exponentially small
value of the population difference. Below this critical value the phase diagram
is dominated by coexistence of a magnetized normal gas and a singlet paired
superfluid with the latter exhibiting a BCS-Bose Einstein condensate crossover
with reduced detuning. On the BCS side of strongly overlapping Cooper pairs, a
sliver of finite-momentum paired Fulde-Ferrell-Larkin-Ovchinnikov magnetized
phase intervenes between the phase separated and normal states. In contrast,
for large negative detuning a uniform, polarized superfluid, that is a coherent
mixture of singlet Bose-Einstein-condensed molecules and fully magnetized
single-species Fermi-sea, is a stable ground state.Comment: 4 RevTeX pages, 2 figures. Minor changes from previous versio
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Parental perceptions of fifth grade physical education : a case study.
Throughout the professional literature in physical education there is considerable rhetoric about the need to recruit and maintain the support of parents. Among those who teach a vulnerable and sometimes marginal special subject, there is a widely held belief that parental approval and support matters a great deal, in fact, that parental dispositions are critical to the future of the subject in the public schools. Despite this article of professional faith, there are few studies that describe the attitudes of parents about physical education. None of those employed the strategy of probing the communication of program information and subsequent parental responses to program operation at a single site--that is, rich case analyses simply are not available. The purpose of this study was to describe parents\u27 perceptions of and attitudinal dispositions toward a physical education program at a single public school site. The primary sources of data were (a) a parent background questionnaire; (b) open-ended interviews with participants (27 parents of fifth grade children, the physical education teacher who taught those children, nine classroom teachers at the same school, and the principal); and (c) observations at the site. Results indicated that many parents possessed inaccurate information about the physical program--information that typically was acquired from their children. Few parents attempted to obtain information from other sources. Further, all parents drew on their own, often negative, recollections of gym classes when describing perceptions of and dispositions toward their child\u27s program. Although anxious to obtain parental support, the teacher did not attempt either to communicate more than minimal information about the program, or to solicit more than trivial forms of parental support and cooperation. Her strong sense of the importance of teacher autonomy, and school norms restricting parental participation in educational matters, appeared to limit her efforts to improve relations with parents. This study concludes with a discussion of the communication process between the physical education teacher and the parents, and a comparison of that with what occurred between the classroom teachers and parents. This is followed by implications for both teacher preparation and school practice
Field boundary habitats and their contribution to the area of semi-natural habitats on lowland farms in east Galway, western Ireland
peer-reviewedSustainable agriculture and the provision of environmental public goods are key deliverables for European farming and food production. Farmland biodiversity, cultural landscapes, soil functionality and climate stability are among the environmental public goods provided through agriculture.
Future Common Agricultural Policy (CAP) direct payments are intended to be more targeted at the provision of these agricultural deliverables. Field boundaries are an example of such deliverables.
They are widespread features that have both environmental and aesthetic functions in farmed landscapes. However, research on their variety, density and contribution to semi-natural habitat cover on farms in Ireland is lacking. This study investigates the diversity and density of all field
boundary habitat types on 32 lowland farms in east County Galway, western Ireland. A total of 286km of field boundaries were surveyed across six study sites. Five types of field boundary habitats were recorded. The density of field boundaries on the farms studied was high and could have
positive implications for delivery of environmental public goods and sustainable farming metrics. In more intensively farmed areas, field boundaries were the only remaining semi-natural habitat on
some farms highlighting the need to retain, and improve the ecological quality, of these features. The condition of one field boundary type (hedgerows) was also investigated in further detail. While
the density of field boundaries was high on many of the surveyed farms, we found that the
hedgerows on these farms were not necessarily in good condition for wildlife
Pairing of a trapped resonantly-interacting fermion mixture with unequal spin populations
We consider the phase separation of a trapped atomic mixture of fermions with
unequal spin populations near a Feshbach resonance. In particular, we determine
the density profile of the two spin states and compare with the recent
experiments of Partridge et al. (cond-mat/0511752). Overall we find quite good
agreement. We identify the remaining discrepancies and pose them as open
problems.Comment: 4 figures, 4+ pages, revtex
Observation of Vortex Pinning in Bose-Einstein Condensates
We report the observation of vortex pinning in rotating gaseous Bose-Einstein
condensates (BEC). The vortices are pinned to columnar pinning sites created by
a co-rotating optical lattice superimposed on the rotating BEC. We study the
effects of two different types of optical lattice, triangular and square. With
both geometries we see an orientation locking between the vortex and the
optical lattices. At sufficient intensity the square optical lattice induces a
structural cross-over in the vortex lattice.Comment: 4 pages, 6 figures. Replaced by final version to appear in Phys. Rev.
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Induced superfluidity of imbalanced Fermi gases near unitarity
The induced intraspecies interactions among the majority species, mediated by
the minority species, is computed for a population-imbalanced two-component
Fermi gas. Although the Feshbach-resonance mediated interspecies interaction is
dominant for equal populations, leading to singlet s-wave pairing, we find that
in the strongly imbalanced regime the induced intraspecies interaction leads to
p-wave pairing and superfluidity of the majority species. Thus, we predict that
the observed spin-polaron Fermi liquid state in this regime is unstable to
p-wave superfluidity, in accordance with the results of Kohn and Luttinger,
below a temperature that, near unitarity, we find to be within current
experimental capabilities. Possible experimental signatures of the p-wave state
using radio-frequency spectroscopy as well as density-density correlations
after free expansion are presented.Comment: 15 pages, 13 figures, submitted to Phys. Rev.
Oscillatory pairing of fermions in spin-split traps
As a means of realizing oscillatory pairing between fermions, we study
superfluid pairing between two fermion "spin" species that are confined to
adjustable spin-dependent trapping potentials. Focusing on the one-dimensional
limit, we find that with increasing separation between the spin-dependent traps
the fermions exhibit distinct phases, including a fully paired phase, a
spin-imbalanced phase with oscillatory pairing, and an unpaired fully
spin-polarized phase. We obtain the phase diagram of fermions in such a
spin-split trap and discuss signatures of these phases in cold-atom
experiments.Comment: 5 pages, 5 figures, RevTe
Atom-molecule coherence in a one-dimensional system
We study a model of one-dimensional fermionic atoms that can bind in pairs to
form bosonic molecules. We show that at low energy, a coherence develops
between the molecule and fermion Luttinger liquids. At the same time, a gap
opens in the spin excitation spectrum. The coherence implies that the order
parameters for the molecular Bose-Einstein Condensation and the atomic BCS
pairing become identical. Moreover, both bosonic and fermionic charge density
wave correlations decay exponentially, in contrast with a usual Luttinger
liquid. We exhibit a Luther-Emery point where the systems can be described in
terms of noninteracting pseudofermions. At this point, we provide closed form
expressions for the density-density response functions.Comment: 5 pages, no figures, Revtex 4; (v2) added a reference to
cond-mat/0505681 where related results are reported; (v3) Expression of
correlation functions given in terms of generalized hypergeometric function
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