1,236 research outputs found
The crossover between lasing and polariton condensation in optical microcavities
We study a model of a photon mode dipole-coupled to a medium of two-level
oscillators in a microcavity in the presence of dephasing processes introduced
by coupling to external baths. Decoherence processes can be classified as
pair-breaking or non-pair-breaking in analogy with magnetic or non-magnetic
impurities in superconductors. In the absence of dephasing, the ground state of
the model is a polariton condensate with a gap in the excitation spectrum.
Increase of the pair-breaking parameter reduces the gap, which becomes
zero at a critical value ; for large , the conventional
laser regime is obtained in a way that demonstrates its close analogy to a
gapless superconductor. In contrast, weak non-pair-breaking processes have no
qualitative effect on the condensate or the existence of a gap, although they
lead to inhomogeneous broadening of the excitations
On the dynamics of Extrasolar Planetary Systems under dissipation. Migration of planets
We study the dynamics of planetary systems with two planets moving in the
same plane, when frictional forces act on the two planets, in addition to the
gravitational forces. The model of the general three-body problem is used.
Different laws of friction are considered. The topology of the phase space is
essential in understanding the evolution of the system. The topology is
determined by the families of stable and unstable periodic orbits, both
symmetric and non symmetric. It is along the stable families, or close to them,
that the planets migrate when dissipative forces act. At the critical points
where the stability along the family changes, there is a bifurcation of a new
family of stable periodic orbits and the migration process changes route and
follows the new stable family up to large eccentricities or to a chaotic
region. We consider both resonant and non resonant planetary systems. The 2/1,
3/1 and 3/2 resonances are studied. The migration to larger or smaller
eccentricities depends on the particular law of friction. Also, in some cases
the semimajor axes increase and in other cases they are stabilized. For
particular laws of friction and for special values of the parameters of the
frictional forces, it is possible to have partially stationary solutions, where
the eccentricities and the semimajor axes are fixed.Comment: Accepted in Celestial Mechanics and Dynamical Astronom
Work Precarity and Gig Literacies in Online Freelancing
Many workers have been drawn to the gig economy by the promise of flexible, autonomous work, but scholars have highlighted how independent working arrangements also come with the drawbacks of precarity. Digital platforms appear to provide an alternative to certain aspects of precarity by helping workers find work consistently and securely. However, these platforms also introduce their own demands and constraints. Drawing on 20 interviews with online freelancers, 19 interviews with corresponding clients and a first-hand walkthrough of the Upwork platform, we identify critical literacies (what we call gig literacies), which are emerging around online freelancing. We find that gig workers must adapt their skills and work strategies in order to leverage platforms creatively and productively, and as a component of their ‘personal holding environment’. This involves not only using the resources provided by the platform effectively, but also negotiating or working around its imposed structures and control mechanisms
Change in Blood Pressure Variability Among Treated Elderly Hypertensive Patients and Its Association With Mortality
Growth, competition and cooperation in spatial population genetics
We study an individual based model describing competition in space between
two different alleles. Although the model is similar in spirit to classic
models of spatial population genetics such as the stepping stone model, here
however space is continuous and the total density of competing individuals
fluctuates due to demographic stochasticity. By means of analytics and
numerical simulations, we study the behavior of fixation probabilities,
fixation times, and heterozygosity, in a neutral setting and in cases where the
two species can compete or cooperate. By concluding with examples in which
individuals are transported by fluid flows, we argue that this model is a
natural choice to describe competition in marine environments.Comment: 29 pages, 14 figures; revised version including a section with
results in the presence of fluid flow
Feasibility of a healthcare system-based tetralogy of Fallot patient registry
Background Patient-reported outcomes and epidemiological studies in adults with tetralogy of Fallot are lacking. Recruitment and longitudinal follow-up investigation across institutions is particularly challenging. Objectives of this study were to assess the feasibility of recruiting adult patients with tetralogy of Fallot for a patient-reported outcomes study, describe challenges for recruitment, and create an interactive, online tetralogy of Fallot registry. Methods Adult patients living with tetralogy of Fallot, aged 18-58 years, at the University of North Carolina were identified using diagnosis code query. A survey was designed to collect demographics, symptoms, history, and birth mother information. Recruitment was attempted by phone (Part I, n=20) or by email (Part II, n=20). Data analysis included thematic grouping of recruitment challenges and descriptive statistics. Feasibility threshold was 75% for recruitment and for data fields completed per patient. Results In Part I, 60% (12/20) were successfully contacted and eight (40%) were enrolled. Demographics and birth mother information were obtained for all enrolled patients. In Part II, 70% (14/20) were successfully contacted; 30% (6/20) enrolled and completed all data fields linked to REDCap database; the median time for survey completion was 8 minutes. Half of the patients had cardiac operations/procedures performed at more than one hospital. Automatic electronic data entry from the online survey was uncomplicated. Conclusions Although recruitment (54%) fell below our feasibility threshold, enrolled individuals were willing to complete phone or online surveys. Incorrect contact information, privacy concerns, and patient-reported time constraints were challenges for recruitment. Creating an online survey and linked database is technically feasible and efficient for patient-reported outcomes research
Phase space reduction of the one-dimensional Fokker-Planck (Kramers) equation
A pointlike particle of finite mass m, moving in a one-dimensional viscous
environment and biased by a spatially dependent force, is considered. We
present a rigorous mapping of the Fokker-Planck equation, which determines
evolution of the particle density in phase space, onto the spatial coordinate
x. The result is the Smoluchowski equation, valid in the overdamped limit,
m->0, with a series of corrections expanded in powers of m. They are determined
unambiguously within the recurrence mapping procedure. The method and the
results are interpreted on the simplest model with no field and on the damped
harmonic oscillator.Comment: 13 pages, 1 figur
Are the magnetic fields of millisecond pulsars ~ 10^8 G?
It is generally assumed that the magnetic fields of millisecond pulsars
(MSPs) are G. We argue that this may not be true and the fields
may be appreciably greater. We present six evidences for this: (1) The G field estimate is based on magnetic dipole emission losses which is
shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are
claimed to have G on the basis of a Rayleygh-Taylor instability
accretion argument. We show that the accretion argument is questionable and the
upper limit G may be much higher; (3) Low magnetic field neutron
stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting
indicating a much higher magnetic field; (5) The data that predict G for MSPs also predict ages on the order of, and greater than, ten
billion years, which is much greater than normal pulsars. If the predicted ages
are wrong, most likely the predicted G fields of MSPs are wrong;
(6) When magnetic fields are measured directly with cyclotron lines in X-ray
binaries, fields G are indicated. Other scenarios should be
investigated. One such scenario is the following. Over 85% of MSPs are
confirmed members of a binary. It is possible that all MSPs are in large
separation binaries having magnetic fields G with their magnetic
dipole emission being balanced by low level accretion from their companions.Comment: 16 pages, accept for publication in Astrophysics and Space Scienc
Proteomic and transcriptomic changes in hibernating grizzly bears reveal metabolic and signaling pathways that protect against muscle atrophy
Muscle atrophy is a physiological response to disuse and malnutrition, but hibernating bears are largely resistant to this phenomenon. Unlike other mammals, they efficiently reabsorb amino acids from urine, periodically activate muscle contraction, and their adipocytes differentially responds to insulin. The contribution of myocytes to the reduced atrophy remains largely unknown. Here we show how metabolism and atrophy signaling are regulated in skeletal muscle of hibernating grizzly bear. Metabolic modeling of proteomic changes suggests an autonomous increase of non-essential amino acids (NEAA) in muscle and treatment of differentiated myoblasts with NEAA is sufficient to induce hypertrophy. Our comparison of gene expression in hibernation versus muscle atrophy identified several genes differentially regulated during hibernation, including Pdk4 and Serpinf1. Their trophic effects extend to myoblasts from non-hibernating species (including C. elegans), as documented by a knockdown approach. Together, these changes reflect evolutionary favored adaptations that, once translated to the clinics, could help improve atrophy treatment
Sex and virulence in Escherichia coli: an evolutionary perspective
Pathogenic Escherichia coli cause over 160 million cases of dysentery and one million deaths per year, whereas non-pathogenic E. coli constitute part of the normal intestinal flora of healthy mammals and birds. The evolutionary pathways underlying this dichotomy in bacterial lifestyle were investigated by multilocus sequence typing of a global collection of isolates. Specific pathogen types [enterohaemorrhagic E. coli, enteropathogenic E. coli, enteroinvasive E. coli, K1 and Shigella] have arisen independently and repeatedly in several lineages, whereas other lineages contain only few pathogens. Rates of evolution have accelerated in pathogenic lineages, culminating in highly virulent organisms whose genomic contents are altered frequently by increased rates of homologous recombination; thus, the evolution of virulence is linked to bacterial sex. This long-term pattern of evolution was observed in genes distributed throughout the genome, and thereby is the likely result of episodic selection for strains that can escape the host immune response
- …