391 research outputs found
Spontaneous rotating vortex rings in a parametrically driven polariton fluid
We present the theoretical prediction of spontaneous rotating vortex rings in
a parametrically driven quantum fluid of polaritons -- coherent superpositions
of coupled quantum well excitons and microcavity photons. These rings arise not
only in the absence of any rotating drive, but also in the absence of a
trapping potential, in a model known to map quantitatively to experiments. We
begin by proposing a novel parametric pumping scheme for polaritons, with
circular symmetry and radial currents, and characterize the resulting
nonequilibrium condensate. We show that the system is unstable to spontaneous
breaking of circular symmetry via a modulational instability, following which a
vortex ring with large net angular momentum emerges, rotating in one of two
topologically distinct states. Such rings are robust and carry distinctive
experimental signatures, and so they could find applications in the new
generation of polaritonic devices.Comment: 6 pages, 4 figure
Supercooling and fragile glassiness in a dipolar kagome Ising magnet
We study equilibration and ordering in the classical dipolar kagome Ising
antiferromagnet, which we show behaves as a disorder-free fragile spin glass.
By identifying an appropriate order parameter, we demonstrate a transition to
the ordered state proposed by Chioar et al. [Phys. Rev. B 93, 214410 (2016)]
with a 12-site unit cell that breaks time-reversal and sublattice symmetries,
and further provide evidence that the nature of the transition is first order.
Upon approaching the transition, the spin dynamics slow dramatically. The
system readily falls out of equilibrium, overshooting the transition and
entering a supercooled liquid regime. Using extensive Monte Carlo simulations,
we show that the system exhibits super-Arrhenius behaviour above the ordering
transition. The relaxation time diverges according to a Vogel-Fulcher form at a
finite `glass transition' temperature in the supercooled regime. Such
behaviour, characteristic of fragile glasses, is particularly remarkable as the
model is free of quenched disorder, does not straightforwardly conform to the
avoided criticality paradigm, and is simple and eminently realisable in
engineered nanomagnetic arrays
Emergent Coulombic criticality and Kibble-Zurek scaling in a topological magnet
When a classical system is driven through a continuous phase transition, its
nonequilibrium response is universal and exhibits Kibble-Zurek scaling. We
explore this dynamical scaling in the novel context of a three-dimensional
topological magnet with fractionalized excitations, namely the liquid-gas
transition of the emergent mobile magnetic monopoles in dipolar spin ice. Using
field-mixing and finite-size scaling techniques, we place the critical point of
the liquid-gas line in the three-dimensional Ising universality class. We then
demonstrate Kibble-Zurek scaling for sweeps of the magnetic field through the
critical point. Unusually slow microscopic time scales in spin ice offer a
unique opportunity to detect this universal nonequilibrium physics in current
experimental setups.This work was supported in part by Engineering and Physical Sciences Research Council (EPSRC) Grant No. EP/G049394/1 (C.C.), the Helmholtz Virtual Institute “New States of Matter and Their Excitations,” and the EPSRC NetworkPlus on “Emergence and Physics far from Equilibrium.” Research at Perimeter Institute is supported by the Government of Canada through Industry Canada and by the Province of Ontario through the Ministry of Economic Development and Innovation. The calculations were performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/) and the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk/, for which access was provided by the ARCHER Driving Test scheme). The authors are grateful to A. Sandvik for useful discussions and to S. L. Sondhi for advice and collaboration on several pieces of related work. J.O.H. is grateful to the EPSRC for funding, and to Michael Rutter for computing support.This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevB.92.07514
Structural and Functional Analysis of Multi-Interface Domains
10.1371/journal.pone.0050821PLoS ONE71
Cooling of Quark Stars in the Color Superconductive Phase: Effect of Photons from Glueball decay
The cooling history of a quark star in the color superconductive phase is
investigated. Here we specifically focus on the 2-flavour color (2SC) phase
where novel process of photon generation via glueball (GLB) decay have been
already investigated (Ouyed & Sannino 2001). The picture we present here can in
principle be generalized to quark stars entering a superconductive phase where
similar photon generation mechanisms are at play. As much as 10^{45}-10^{47}
erg of energy is provided by the GLB decay in the 2SC phase. The generated
photons slowly diffuse out of the quark star keeping it hot and radiating as a
black-body (with possibly a Wien spectrum in gamma-rays) for millions of years.
We discuss hot radio-quiet isolated neutron stars in our picture (such as RX
J185635-3754 and RX J0720.4-3125) and argue that their nearly blackbody spectra
(with a few broad features) and their remarkably tiny hydrogen atmosphere are
indications that these might be quark stars in the color superconductive phase
where some sort of photon generation mechanism (reminiscent of the GLB decay)
has taken place. Fits to observed data of cooling compact stars favor models
with superconductive gaps of Delta_2SC = 15-35 MeV and densities
rho_2SC=(2.5-3.0)rho_N (rho_N being the nuclear matter saturation density) for
quark matter in the 2SC phase. If correct, our model combined with more
observations of isolated compact stars could provide vital information to
studies of quark matter and its exotic phases.Comment: 7 journal pages, 4 figures, accepted for publication in MNRAS (more
discussions on photon cooling versus neutrino cooling before and after
pairing of quarks
Pricing European Options with a Log Student's t-Distribution: a Gosset Formula
The distribution of the returns for a stock are not well described by a
normal probability density function (pdf). Student's t-distributions, which
have fat tails, are known to fit the distributions of the returns. We present
pricing of European call or put options using a log Student's t-distribution,
which we call a Gosset approach in honour of W.S. Gosset, the author behind the
nom de plume Student. The approach that we present can be used to price
European options using other distributions and yields the Black-Scholes formula
for returns described by a normal pdf.Comment: 12 journal pages, 9 figures and 3 tables (Submitted to Physica A
Processfolio: uniting Academic Literacies and Critical Emancipatory Action Research for practitioner-led inquiry into EAP writing assessment
This paper reports on the design and implementation of an alternative form of writing assessment on a UK English for Academic Purposes (EAP) presessional course. The assessment, termed processfolio, was a response to research inquiry into how writing assessment in a local context negated student agency and inculcated disempowering models of teaching and learning academic writing. The project merged an Academic Literacies approach to writing (Lea and Street, 1998) with a Critical Emancipatory Action Research (Carr and Kemmis, 1986) framework and a Critical Realist(Bhaskar, 1989) perspective. Data collected from the folios and interviews with students and teachers on their experiences of the processfolio found that a small scale intervention has potential for agency to be exercised within the highly constrained context of a UK EAP pre-sessional. New directions in research are proposed which can engage students and teachers to work for change in UK EAP assessment within their internal and external constraints
The Impact of a SIG on Assessment Literacy
A major aim of professional associations is to provide opportunities for professionals to interact with others, share ideas and develop in their chosen profession. Professional associations exist to provide specialized networking and development opportunities to a specific profession, group of individuals or field of study. To promote and support specialized research and communication, smaller subgroups within an association are often chartered or developed. These subgroups are typically known as Special Interest Groups. According to Jacob et al. (2013), association members join SIGs because they want to go deeper into a specialized content area and they enjoy networking with others who ‘speak the same language.’ The TESOL Arabia Testing, Assessment and Evaluation SIG (TAE SIG) has focused their professional development activities on an important trend in the field, that of language assessment literacy (LAL). Language assessment literacy has been a critical topic in English language teaching since the late 1990s. Unfortunately, this is mainly due to the fact that so many English language teachers are not assessment literate. In other words, many English language teachers lack the knowledge and skills to write effective language tests, evaluate the effectiveness of their tests, and use their test results in meaningful ways. The purpose of this chapter is to critically examine the status of LAL in the Middle East and North Africa (MENA) region and report on activities that the TAE SIG has implemented to increase LAL
Factors in AIDS Dementia Complex Trial Design: Results and Lessons from the Abacavir Trial
OBJECTIVES: To determine the efficacy of adding abacavir (Ziagen, ABC) to optimal stable background antiretroviral therapy (SBG) to AIDS dementia complex (ADC) patients and address trial design. DESIGN: Phase III randomized, double-blind placebo-controlled trial. SETTING: Tertiary outpatient clinics. PARTICIPANTS: ADC patients on SBG for ≥8 wk. INTERVENTIONS: Participants were randomized to ABC or matched placebo for 12 wk. OUTCOME MEASURES: The primary outcome measure was the change in the summary neuropsychological Z score (NPZ). Secondary measures were HIV RNA and the immune activation markers β-2 microglobulin, soluble tumor necrosis factor (TNF) receptor 2, and quinolinic acid. RESULTS: 105 participants were enrolled. The median change in NPZ at week 12 was +0.76 for the ABC + SBG and +0.63 for the SBG groups (p = 0.735). The lack of efficacy was unlikely related to possible limited antiviral efficacy of ABC: at week 12 more ABC than placebo participants had plasma HIV RNA ≤400 copies/mL (p = 0.002). There were, however, other factors. Two thirds of patients were subsequently found to have had baseline resistance to ABC. Second, there was an unanticipated beneficial effect of SBG that extended beyond 8 wk to 5 mo, thereby rendering some of the patients at baseline unstable. Third, there was an unexpectedly large variability in neuropsychological performance that underpowered the study. Fourth, there was a relative lack of activity of ADC: 56% of all patients had baseline cerebrospinal fluid (CSF) HIV-1 RNA <100 copies/mL and 83% had CSF β-2 microglobulin <3 nmol/L at baseline. CONCLUSIONS: The addition of ABC to SBG for ADC patients was not efficacious, possibly because of the inefficacy of ABC per se, baseline drug resistance, prolonged benefit from existing therapy, difficulties with sample size calculations, and lack of disease activity. Assessment of these trial design factors is critical in the design of future ADC trials
- …