393 research outputs found
Bistability and nonequilibrium condensation in a driven-dissipative Josephson array: a c-field model
Developing theoretical models for nonequilibrium quantum systems poses
significant challenges. Here we develop and study a multimode model of a
driven-dissipative Josephson junction chain of atomic Bose-Einstein
condensates, as realised in the experiment of Labouvie et al. [Phys. Rev. Lett.
116, 235302 (2016)]. The model is based on c-field theory, a beyond-mean-field
approach to Bose-Einstein condensates that incorporates fluctuations due to
finite temperature and dissipation. We find the c-field model is capable of
capturing all key features of the nonequilibrium phase diagram, including
bistability and a critical slowing down in the lower branch of the bistable
region. Our model is closely related to the so-called Lugiato-Lefever equation,
and thus establishes new connections between nonequilibrium dynamics of
ultracold atoms with nonlinear optics, exciton-polariton superfluids, and
driven damped sine-Gordon systems.Comment: 12 pages, 10 figure
Quantitative acoustic models for superfluid circuits
We experimentally realize a highly tunable superfluid oscillator circuit in a
quantum gas of ultracold atoms and develop and verify a simple lumped-element
description of this circuit. At low oscillator currents, we demonstrate that
the circuit is accurately described as a Helmholtz resonator, a fundamental
element of acoustic circuits. At larger currents, the breakdown of the
Helmholtz regime is heralded by a turbulent shedding of vortices and density
waves. Although a simple phase-slip model offers qualitative insights into the
circuit's resistive behavior, our results indicate deviations from the
phase-slip model. A full understanding of the dissipation in superfluid
circuits will thus require the development of empirical models of the turbulent
dynamics in this system, as have been developed for classical acoustic systems.Comment: 12 pages, 9 figure
Lifting the veil of depression and alcoholism in sport coaching: how do we care for carers?
The purpose of this article is to explore the insights of an elite sport coach living with comorbid depression and alcohol misuse. Such consideration is necessary because as coaching is increasingly repositioned as a caring activity the wellbeing of coaches themselves has rarely been considered. To address this gap, a narrative analysis methodology and a story telling approach was used to present the experiences of a case study coach (Steve). The story is derived from Steve’s own perspective and the perspective of his wife, Jane. This novel multi-voiced approach reveals the complex interplay between the sporting environment, Steve’s depression and his alcohol-related problems. In doing so, Steve’s story makes an original contribution by unveiling the issue of mental health in elite sport coaching. Steve’s story depicts the significant impact of depressive symptoms, coupled by excessive alcohol use on a coach, their personal life and their career. Using the work of Goffman and Sartre, the story alludes to how such symptoms can be hidden, to greater and lesser extents, from professional colleagues. This is an important theoretical contribution, because to care for coaches, colleagues, employers and health professionals will need to understand the needs of individuals such as Steve. Developing an open and supportive culture, which accepts that coaches are fallible, may however be a challenge within professional sport contexts
Optimizing astrophotonic spatial reformatters using simulated on-sky performance
One of the most useful techniques in astronomical instrumentation is image
slicing. It enables a spectrograph to have a more compact angular slit, whilst
retaining throughput and increasing resolving power. Astrophotonic components
like the photonic lanterns and photonic reformatters can be used to replace
bulk optics used so far. This study investigates the performance of such
devices using end-to-end simulations to approximate realistic on-sky
conditions. It investigates existing components, tries to optimize their
performance and aims to understand better how best to design instruments to
maximize their performance. This work complements the recent work in the field
and provides an estimation for the performance of the new components.Comment: Conference proceedings in SPIE 2018 Austin Texa
Physician attitude, awareness, and knowledge regarding guidelines for transcranial Doppler screening in sickle cell disease.
ObjectiveWe explored factors that may influence physician adherence to transcranial Doppler (TCD) screening guidelines among children with sickle cell disease.MethodsPediatric hematologists, neurologists, and primary care physicians (n = 706) responded to a mailed survey in May 2012 exploring factors hypothesized to influence physician adherence to TCD screening guidelines: physician (internal) barriers and physician-perceived external barriers. Responses were compared by specialty using chi-square tests.ResultsAmong 276 physicians (44%), 141 currently treated children with sickle cell disease; 72% recommend screening. Most primary care physicians (66%) did not feel well informed regarding TCD guidelines, in contrast to neurologists (25%) and hematologists (6%, P < .0001). Proportion of correct answers on knowledge questions was low (13%-35%). Distance to a vascular laboratory and low patient adherence were external barriers to receipt of TCD screening.ConclusionsAdditional research regarding physicians' lack of self-efficacy and knowledge of recommendations could help clarify their role in recommendation of TCD screening
The Closest View of a Fast Coronal Mass Ejection: How Faulty Assumptions near Perihelion Lead to Unrealistic Interpretations of PSP/WISPR Observations
We report on the closest view of a coronal mass ejection observed by the
Parker Solar Probe (PSP)/Wide-field Imager for {Parker} Solar PRobe (WISPR)
instrument on September 05, 2022, when PSP was traversing from a distance of
15.3~to~13.5~R from the Sun. The CME leading edge and an arc-shaped
{\emph{concave-up} structure near the core} was tracked in WISPR~field of view
using the polar coordinate system, for the first time. Using the impact
distance on Thomson surface, we measured average speeds of CME leading edge and
concave-up structure as 2500~~270\,km\,s and
400~~70\,km\,s with a deceleration of
20~m~s for the later. {The use of the plane-of-sky approach
yielded an unrealistic speed of more than three times of this estimate.} We
also used single viewpoint STEREO/COR-2A images to fit the Graduated
Cylindrical Shell (GCS) model to the CME while incorporating the source region
location from EUI of Solar Orbiter and estimated a 3D speed of
2700\,km\,s. We conclude that this CME exhibits the highest
speed during the ascending phase of solar cycle 25. This places it in the
category of extreme speed CMEs, which account for only 0.15\% of all CMEs
listed in the CDAW CME catalog.Comment: 13 Pages, 6 Figures; Accepted in The Astrophysical Journal Letter
Universal expansion of vortex clusters in a dissipative two-dimensional superfluid
A large ensemble of quantum vortices in a superfluid may itself be treated as
a novel kind of fluid that exhibits anomalous hydrodynamics. Here we consider
the dynamics of vortex clusters with thermal friction, and present an analytic
solution that uncovers a new universality class in the out-of-equilibrium
dynamics of dissipative superfluids. We find that the long-time dynamics of the
vorticity distribution is an expanding Rankine vortex (i.e.~top-hat
distribution) independent of initial conditions. This highlights a
fundamentally different decay process to classical fluids, where the Rankine
vortex is forbidden by viscous diffusion. Numerical simulations of large
ensembles of point vortices confirm the universal expansion dynamics, and
further reveal the emergence of a frustrated lattice structure marked by strong
correlations. We present experimental results in a quasi-two-dimensional
Bose-Einstein condensate that are in excellent agreement with the vortex fluid
theory predictions, demonstrating that the signatures of vortex fluid theory
can be observed with as few as vortices. Our theoretical, numerical,
and experimental results establish the validity of the vortex fluid theory for
superfluid systems.Comment: V1: 6 pages, 3 figures in main text. 5 pages, 5 figures in
supplemental material. V2: Updated in response to reviewer comments: Improved
introduction and discussion, additional simulation data provided in
supplemental material
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