Universality in modelling non-equilibrium pattern formation in polariton condensates

The key to understanding the universal behaviour of systems driven away from equilibrium lies in the common description obtained when particular microscopic models are reduced to order parameter equations. Universal order parameter equations written for complex matter fields are widely used to describe systems as different as Bose-Einstein condensates of ultra cold atomic gases, thermal convection, nematic liquid crystals, lasers and other nonlinear systems. Exciton-polariton condensates recently realised in semiconductor microcavities are pattern forming systems that lie somewhere between equilibrium Bose-Einstein condensates and lasers. Because of the imperfect confinement of the photon component, exciton-polaritons have a finite lifetime, and have to be continuously re-populated. As photon confinement improves, the system more closely approximates an equilibrium system. In this chapter we review a number of universal equations which describe various regimes of the dynamics of exciton-polariton condensates: the Gross-Pitaevskii equation, which models weakly interacting equilibrium condensates, the complex Ginsburg-Landau equation---the universal equation that describes the behaviour of systems in the vicinity of a symmetry--breaking instability, and the complex Swift-Hohenberg equation that in comparison with the complex Ginsburg-Landau equation contains additional nonlocal terms responsible for spacial mode selection. All these equations can be derived asymptotically from a generic laser model given by Maxwell-Bloch equations. Such an universal framework allows the unified treatment of various systems and continuously cross from one system to another. We discuss the relevance of these equations, and their consequences for pattern formation.Comment: 19 pages; Chapter to appear in Springer&Verlag book on "Quantum Fluids: hot-topics and new trends" eds. A. Bramati and M. Modugn

Coherent Particle Transfer in an On-Demand Single-Electron Source

Coherent electron transfer from a localized state trapped in a quantum dot into a ballistic conductor, taking place in on-demand electron sources, in general may result in excitation of particle-hole pairs. We consider a simple model for these effects, involving a resonance level with time-dependent energy, and derive Floquet scattering matrix describing inelastic transitions of particles in the Fermi sea. We find that, as the resonance level is driven through the Fermi level, particle transfer may take place completely without particle-hole excitations for certain driving protocols. In particular, such noiseless transfer occurs when the level moves with constant rapidity, its energy changing linearly with time. A detection scheme for studying the coherence of particle transfer is proposed.Comment: 5 pages, 3 figures. Updated introduction, Fig. 1, and reference

Global surface-ocean pCO2 and sea–air CO2 flux variability from an observation-driven ocean mixed-layer scheme

A temporally and spatially resolved estimate of the global surface-ocean CO₂ partial pressure field and the sea–air CO₂ flux is presented, obtained by fitting a simple data-driven diagnostic model of ocean mixed-layer biogeochemistry to surface-ocean CO₂ partial pressure data from the SOCAT v1.5 database. Results include seasonal, interannual, and short-term (daily) variations. In most regions, estimated seasonality is well constrained from the data, and compares well to the widely used monthly climatology by Takahashi et al. (2009). Comparison to independent data tentatively supports the slightly higher seasonal variations in our estimates in some areas. We also fitted the diagnostic model to atmospheric CO₂ data. The results of this are less robust, but in those areas where atmospheric signals are not strongly influenced by land flux variability, their seasonality is nevertheless consistent with the results based on surface-ocean data. From a comparison with an independent seasonal climatology of surface-ocean nutrient concentration, the diagnostic model is shown to capture relevant surface-ocean biogeochemical processes reasonably well. Estimated interannual variations will be presented and discussed in a companion paper

Non-equilibrium quantum condensation in an incoherently pumped dissipative system

We study spontaneous quantum coherence in an out of equilibrium system, coupled to multiple baths describing pumping and decay. For a range of parameters describing coupling to, and occupation of the baths, a stable steady-state condensed solution exists. The presence of pumping and decay significantly modifies the spectra of phase fluctuations, leading to correlation functions that differ both from an isolated condensate and from a laser.Comment: 5 pages, 2 eps figure

Stochastic oscillations in models of epidemics on a network of cities

We carry out an analytic investigation of stochastic oscillations in a susceptible-infected-recovered model of disease spread on a network of $n$ cities. In the model a fraction $f_{jk}$ of individuals from city $k$ commute to city $j$, where they may infect, or be infected by, others. Starting from a continuous time Markov description of the model the deterministic equations, which are valid in the limit when the population of each city is infinite, are recovered. The stochastic fluctuations about the fixed point of these equations are derived by use of the van Kampen system-size expansion. The fixed point structure of the deterministic equations is remarkably simple: a unique non-trivial fixed point always exists and has the feature that the fraction of susceptible, infected and recovered individuals is the same for each city irrespective of its size. We find that the stochastic fluctuations have an analogously simple dynamics: all oscillations have a single frequency, equal to that found in the one city case. We interpret this phenomenon in terms of the properties of the spectrum of the matrix of the linear approximation of the deterministic equations at the fixed point.Comment: 13 pages, 7 figure

Polariton condensation with localised excitons and propagating photons

We estimate the condensation temperature for microcavity polaritons, allowing for their internal structure. We consider polaritons formed from localised excitons in a planar microcavity, using a generalised Dicke model. At low densities, we find a condensation temperature T_c \propto \rho, as expected for a gas of structureless polaritons. However, as T_c becomes of the order of the Rabi splitting, the structure of the polaritons becomes relevant, and the condensation temperature is that of a B.C.S.-like mean field theory. We also calculate the excitation spectrum, which is related to observable quantities such as the luminescence and absorption spectra.Comment: 5 pages, 4 figures, Corrected typos, replaced figure

Chronic constipation in the elderly: an unusual presentation of colonic dysmotility in an elderly patient.

Introduction. Chronic constipation is common in the elderly, and often no underlying pathology is found. Primary colonic dysmotility has been described in children but is rare in the elderly. Case report. We present an 82-year-old female with long standing constipation presenting acutely with large bowel obstruction. Laparotomy and Hartman's procedure was performed, and a grossly distended sigmoid colon was resected. Histology revealed a primary myopathic process. Conclusion. Primary colonic myopathy should be considered in elderly patients presenting with large bowel obstruction and a long preceding history of constipation, particularly when previous endoscopic examinations were normal

Angular distribution of photoluminescence as a probe of Bose Condensation of trapped excitons

Recent experiments on two-dimensional exciton systems have shown the excitons collect in shallow in-plane traps. We find that Bose condensation in a trap results in a dramatic change of the exciton photoluminescence (PL) angular distribution. The long-range coherence of the condensed state gives rise to a sharply focussed peak of radiation in the direction normal to the plane. By comparing the PL profile with and without Bose Condensation we provide a simple diagnostic for the existence of a Bose condensate. The PL peak has strong temperature dependence due to the thermal order parameter phase fluctuations across the system. The angular PL distribution can also be used for imaging vortices in the trapped condensate. Vortex phase spatial variation leads to destructive interference of PL radiation in certain directions, creating nodes in the PL distribution that imprint the vortex configuration.Comment: 4 pages, 3 figure

Me, Myself, and Future Generations: The Role of Affinity and Effectiveness in the Creation of Consumer Environmental Stewardship (CENS)

Policymakers, consumer advocate groups, and researchers agree that consumers need to increase their proenvironmental behaviors if a decent standard of living is to be ensured for future generations. Despite high levels of environmental concern, consumers still refrain from large-scale adoption of proenvironmental behaviors. Social marketers agree that a change in attitudes is not enough to stimulate the necessary behavioral change and are looking for ways to help consumers overcome the costs (e.g., price premiums, inconvenience) that are often associated with proenvironmental behaviors. Currently, consumers often see proenvironmental behavior as a trade-off between short-term personal benefits and longer term collective benefits. The authors contribute to the social marketing literature on proenvironmental behavior by introducing the concept of Consumer Environmental Stewardship (CENS), which centers on the use of intrinsic motivation to stimulate a personal sense of responsibility for the environment. The findings, based on a survey and three experiments, show that the stimulation of consumers’ affinity with future generations (AFGs) and perceived consumer effectiveness (PCE) can help to promote CENS, which in turn raises proenvironmental behaviors. However, this research also shows that increasing levels of AFGs can backfire and result in lower levels of CENS, if consumers experience low levels of PCE

The impact of adverse childhood experiences: A portfolio of work incorporating an empirical study on adverse childhood experiences in learning disability professionals.

This study draws on the Adverse Childhood Experiences (ACE) research and Jung’s concept of the wounded healer (1945). The current research aims to extend these concepts to understand the prevalence of adverse childhood experiences in healthcare professionals, along with if this influenced their career choice and experiences within the workplace. Specifically, healthcare professionals working in learning disability services were chosen for this study to extend the research begun by Keesler (2016; 2018). This current study takes a new direction by using a mixed methods approach to investigate the prevalence of adverse childhood experiences in professionals and their experiences working in learning disability services. The structured ACE questionnaire was administered online to professionals working in learning disability services. The score of healthcare professionals was compared to the ACE prevalence of the general population using statistics from Hughes et al. (2020) study. A significant difference between 0 ACE (U=114594, p=.000) and 4+ ACE was found between samples (U=249606, p=.000). A subset of professionals opted in to follow up semi-structured interviews to understand their experiences, analysed using reflexive Thematic Analysis. Major themes from interviews were experiences of help, communication in learning disability services and the journey of services and clients. The study provides evidence for the concept of the wounded healer attracting people to helping professions. Previous adversity interacted with the theme of help, and specific experiences working in learning disability services were explored in the remaining themes. The study provides valuable preliminary information, which could be applied to developing staff support, with the potential to improve staff wellbeing and client care