83,130 research outputs found
Phase behaviour of the confined lattice gas Lebwohl-Lasher model
The phase behaviour of the Lebwohl-Lasher lattice gas model (one of the
simplest representations of a nematogenic fluid) confined in a slab is
investigated by means of extensive Monte Carlo simulations. The model is known
to yield a first order gas-liquid transition in both the 2D and 3D limits, that
is coupled with an orientational order-disorder transition. This latter
transition happens to be first order in the 3D limit and it shares some
characteristic features with the continuous defect mediated
Berezinskii-Kosterlitz-Thouless transition in 2D. In this work we will analyze
in detail the behaviour of this system taking full advantage of the lattice
nature of the model and the particular symmetry of the interaction potential,
which allows for the use of efficient cluster algorithms.Comment: 6 pages, 5 figure
Confirming what we know: Understanding questionable research practices in intro physics labs
Many institutions are changing the focus of their introductory physics labs
from verifying physics content towards teaching students about the skills and
nature of science. As instruction shifts, so too will the ways students
approach and behave in the labs. In this study, we evaluated students' lab
notes from an early activity in an experimentation-focused lab course. We found
that about 30% of student groups (out of 107 groups at three institutions)
recorded questionable research practices in their lab notes, such as subjective
interpretations of results or manipulating equipment and data. The large
majority of these practices were associated with confirmatory goals, which we
suspect stem from students' prior exposure to verification labs. We propose
ways for experimentation-focused labs to better engage students in the
responsible conduct of research and authentic scientific practice.Comment: 4 pages, 4 figure
Bright solitary waves of atomic Bose-Einstein condensates under rotation
We analyse the rotation of bright solitary waves formed of atomic
Bose-Einstein condensates with attractive atomic interactions. By employing a
variational technique and assuming an irrotational quadrupolar flow field, we
map out the variational solutions in the rotating frame. In particular, we show
that rotation has a considerable stabilising effect on the system,
significantly raising the critical threshold for collapse of the bright
solitary waves.Comment: 4 pages, 3 figure
Instabilities leading to vortex lattice formation in rotating Bose-Einstein condensates
We present a comprehensive theoretical study of vortex lattice formation in
atomic Bose-Einstein condensates confined by a rotating elliptical trap. We
consider rotating solutions of the classical hydrodynamic equations, their
response to perturbations, as well as time-dependent simulations. We
discriminate three distinct, experimentally testable, regimes of instability:
{\em ripple}, {\em interbranch}, and {\em catastrophic}. Under
symmetry-breaking perturbations these instabilities lead to lattice formation
even at zero temperature. While our results are consistent with previous
theoretical and experimental results, they shed new light on lattice formation.Comment: 5 pages, 2 figure
A near zero velocity dispersion stellar component in the Canes Venatici dwarf spheroidal galaxy
We present a spectroscopic survey of the newly-discovered Canes Venatici
dwarf galaxy using the Keck/DEIMOS spectrograph. Two stellar populations of
distinct kinematics are found to be present in this galaxy: an extended,
metal-poor component, of half-light radius 7'.8(+2.4/-2.1), which has a
velocity dispersion of 13.9(+3.2/-2.5) km/s, and a more concentrated
(half-light radius 3'.6(+1.1/-0.8) metal-rich component of extremely low
velocity dispersion. At 99% confidence, the upper limit to the central velocity
dispersion of the metal-rich population is 1.9 km/s. This is the lowest
velocity dispersion ever measured in a galaxy. We perform a Jeans analysis on
the two components, and find that the dynamics of the structures can only be
consistent if we adopt extreme (and unlikely) values for the scale length and
velocity dispersion of the metal-poor population. With a larger radial velocity
sample and improved measurements of the density profile of the two populations,
we anticipate that it will be possible to place strong constraints on the
central distribution of the dark matter in this galaxy.Comment: 5 pages, 7 figures, accepted by MNRA
Understanding and misunderstanding of neuroimaging: some data from first year undergraduates
• In recent years, neuroimaging research has become a popular and exciting source of news in the press and media but claims are often exaggerated and people’s understanding of the techniques appears poor.
• In this study, 207 first year undergraduates in psychology completed a True/False questionnaire in which 28 statements about neuroimaging were presented
• Respondents showed a very skeptical attitude to some of the claims made for neuroimaging but showed poor understanding of the methods
• 82% correctly judged that neuroimaging could not be used to read minds; 88% correctly thought neuroimaging could detect brain abnormalities; and 77% thought that newspapers and media did not report neuroimaging studies accurately
• However, 76% incorrectly thought that neuroimaging allowed us to see behaviour in the brain as and when it happens; 80% incorrectly thought that babies, children and adults could be studied with all types of imaging; 84% incorrectly thought neuroimaging could identify a person suffering from mental illnes
Structure formation during the collapse of a dipolar atomic Bose-Einstein condensate
We investigate the collapse of a trapped dipolar Bose-Einstein condensate.
This is performed by numerical simulations of the Gross-Pitaevskii equation and
the novel application of the Thomas-Fermi hydrodynamic equations to collapse.
We observe regimes of both global collapse, where the system evolves to a
highly elongated or flattened state depending on the sign of the dipolar
interaction, and local collapse, which arises due to dynamically unstable
phonon modes and leads to a periodic arrangement of density shells, disks or
stripes. In the adiabatic regime, where ground states are followed, collapse
can occur globally or locally, while in the non-adiabatic regime, where
collapse is initiated suddenly, local collapse commonly occurs. We analyse the
dependence on the dipolar interactions and trap geometry, the length and time
scales for collapse, and relate our findings to recent experiments.Comment: In this version (the published version) we have slightly rewritten
the manuscript in places and have corrected some typos. 15 pages and 13
figure
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