Infinite order excitonic Bloch equations for asymmetric nanostructures

We present a new exciton-based formalism for calculating the coherent response of asymmetric semiconductor multiple quantum well structures to ultra-short optical pulses valid to infinite order in the optical field and including the self-generated intraband fields. We use these equations to calculate and explain the oscillations with time delay of peaks in the spectrally-resolved degenerate four wave mixing signals from biased semiconductor superlattices, obtaining good agreement with experiment

Photon correlations in positron annihilation

The two-photon positron annihilation density matrix is found to separate into a diagonal center of energy factor implying maximally entangled momenta, and a relative factor describing decay. For unknown positron injection time, the distribution of the difference in photon arrival times is a double exponential at the para-Ps decay rate, consistent with experiment (V. D. Irby, Meas. Sci. Technol. 15, 1799 (2004)).Comment: Accepted for publication in Phys. Rev.

An investigation of anisotropic magnetic properties of bare barth iron boron alloys

In 1983 Sumitomo in Japan and General Motors in the USA both announced the production of new 'super magnets' based on a rare earth iron boron mixture. The Sumitomo magnets are based on an intermetallic compound with a composition Nd(_2)Fe(_14)B. This thesis describes an Investigation towards understanding the reason why permanent magnets made from this material have such favourable magnetic properties. A description is given of a new type of torque magnetometer designed to operate within the bore of a superconducting solenoid to make measurements at a field high enough for this type of material. The construction and testing of the instrument are described. Results are presented of uniaxial magnetocrystalllne anisotropy values calculated from torque measurements and also of magnetisation measurements for R(_2)Fe(_14)B, R=Nd,Ho,Gd and Dy from 4.2K to room temperature. The basal plane anisotropy calculated from torque measurements for Gd(_2)Fe(_14)B from l00K to 300K is also reported. These anisotropy results are related qualitatively to the crystal structure and electronic structure of the rare earth ions. Observations of regions of uniform magnetization, domains, within the material are also described, and the movement of the boundaries between such domains, domain walls, under varying magnetic fields. These movements’ characterise the material and help In understanding the processes Involved in making a magnet resistant to demagnetisation. From the observation of equilibrium domain wall patterns domain wall energies and domain wall thicknesses have been calculated

Photon position measure

The positive operator valued measure (POVM) for a photon counting array detector is derived and found to equal photon flux density integrated over pixel area and measurement time. Since photon flux density equals number density multiplied by the speed of light, this justifies theoretically the observation that a photon counting array provides a coarse grained measurement of photon position. The POVM obtained here can be written as a set of projectors onto a basis of localized states, consistent with the description of photon position in a recent quantum imaging proposal [M. Tsang, Phys. Rev. Lett. \textbf{102}, 253601 (2009)]. The wave function that describes a photon counting experiment is the projection of the photon state vector onto this localized basis. Collapse is to the electromagnetic vacuum and not to a localized state, thus violating the text book rules of quantum mechanics but compatible with the theory of generalized observables and the nonlocalizability of an incoming photon

How adolescents who cut themselves differ from those who take overdoses

The aims of this study were to identify in what ways adolescents who cut themselves differ from those who take overdoses, and to investigate the role of contagion in these behaviours. Data from an anonymous self-report questionnaire survey of 6,020 adolescents in 41 schools were analysed. Comparison of 220 adolescents who reported self-cutting in the previous year with 86 who had taken overdoses in the previous year as the sole method of deliberate self-harm (DSH) showed that far more of those who cut themselves had friends who had also engaged in DSH in the same period (OR 2.84, 95% CI 1.5–5.3, P < 0.001), and fewer had sought help from friends before cutting (OR 0.5, 95% CI 0.3–0.9, P < 0.02). Self-cutting usually involved less premeditation. Analyses at both the individual and school level showed that the association between engaging in DSH and exposure to DSH amongst peers was largely confined to girls who cut themselves. There are important differences between adolescents who cut themselves and those who take overdoses. Contagion may be an important factor in DSH by adolescents, especially in girls who cut themselves. These findings are relevant to the design of prevention and treatment programmes

News coverage of suicidal behaviour in the United Kingdom and the Republic of Ireland

Background: Media reporting of suicide has been associated with imitative acts. Internationally, this has led to the development of guidelines to promote responsible reporting of suicide. Aims: To examine the nature and quality of news coverage of suicidal behaviour in the United Kingdom (UK) and Republic of Ireland (ROI). Method: UK and ROI press clippings relating to suicide over 12 months (N=8101) were coded for content and assessed for quality against existing guidelines. We examined variability in relation to key characteristics (e.g. type of publication) and compared newspaper portrayal of suicide against official statistics. Results: Reports were biased towards young, female and relatively unusual suicides (including those involving a celebrity, more than one individual and violent methods). Almost a third of reports had inappropriate headlines, but only a minority was of poor overall quality, and editors appear to be responsive to feedback. There was considerable variability in the quality of reports for different suicide methods. Limitations: This work cannot account for the impact of reporting on suicidal behaviour. The speed of change in media trends also limits its conclusions. Conclusion: Our findings underscore the need for sustained efforts to promote responsible reporting of suicide

Prevention of suicidal behaviour in prisons: an overview of initiatives based on a systematic review of research on near-lethal suicide attempts

Background: Worldwide, prisoners are at high risk of suicide. Research on near-lethal suicide attempts can provide important insights into risk and protective factors, and inform suicide prevention initiatives in prison. Aims: To synthesize findings of research on near-lethal attempts in prisons, and consider their implications for suicide prevention policies and practice, in the context of other research in custody and other settings. Method: We searched two bibliographic indexes for studies in any language on near-lethal and severe self-harm in prisoners, supplemented by targeted searches over the period 2000–2014. We extracted information on risk factors descriptively. Data were not meta-analyzed owing to heterogeneity of samples and methods. Results: We identified eight studies reporting associations between prisoner near-lethal attempts and specific factors. The latter included historical, prison-related, and clinical factors, including psychiatric morbidity and comorbidity, trauma, social isolation, and bullying. These factors were also identified as important in prisoners' own accounts of what may have contributed to their attempts (presented in four studies). Conclusion: Factors associated with prisoners' severe suicide attempts include a range of potentially modifiable clinical, psychosocial, and environmental factors. We make recommendations to address these factors in order to improve detection, management, and prevention of suicide risk in prisoners

Media coverage and speculation about the impact of the COVID-19 pandemic on suicide: A content analysis of UK news

Objectives: Since the start of the COVID-19 pandemic, there has been much concern and speculation about rises in suicide rates, despite evidence that suicides did not in fact increase in the first year of the pandemic in most countries with real-time suicide data. This public narrative is potentially harmful, as well as misleading, and is likely to be perpetuated by sensational news coverage. Method: Using a bespoke database, we analysed the quality and content of print and online UK news (including opinion pieces) on the impact of COVID-19 on suicidality, based on adherence to international recommendations. Chi-square tests were conducted to examine variability in relation to key characteristics (e.g., type of publication) and to four ‘restriction phases’ (based on UK government official lockdown measures) over the first 14 months of the pandemic. Results: We identified 372 stories about COVID-19 and suicidality in online and print news between the first UK lockdown (March 2020) and May 2021 (when restrictions were significantly eased in the UK). Throughout this period, over a third of articles (39.2%) and headlines (41.4%) claimed or predicted a rise in suicide, often attributed to feelings of entrapment and poor mental health (especially amongst young people), and fueled by expert commentary and speculation. Almost a third of reports were rated as being of negative quality (116, 31.2%), and at least half included no signposting to help and support. However, reporting improved in phases of less stringent COVID-19 restrictions and over time, with later articles and headlines including fewer negative statements and predictions about rises in suicides, and greater reliance on academic evidence. Conclusions: As the longer-term consequences of the pandemic develop, and other national and global events unfold, it is increasingly important that the media, and the wider community of experts shaping its narratives, strive for a positive and evidence-informed approach to news coverage of suicide

Have news reports on suicide and attempted suicide during the COVID-19 pandemic adhered to guidance on safer reporting? A UK-wide content analysis study

Background: Associations between sensational news coverage of suicide and increases in suicidal behaviour have been well documented. Amidst growing concern over the impact of the COVID-19 pandemic on suicide rates, it is especially important that news coverage adheres to recommended standards. Methods: We analysed the quality and content of print and online UK news reports of possible COVID-19 related suicides and suicide attempts in the first four months of the pandemic (N=285). Results: Most reports made explicit links between suicidal behaviour and the COVID-19 pandemic in the headline (65.5%), based on statements by family, friends or acquaintances of the deceased (60%). The impact of the pandemic on suicidal behaviour was most often attributed to feelings of isolation (27.4%), poor mental health (14.7%) and entrapment due to government-imposed restrictions (14.4%). Although rarely of poor overall quality, reporting was biased towards young people, frontline staff and relatively unusual suicides and, to varying degrees, failed to meet recommended standards (e.g. 41.1% did not signpost readers to sources of support). Limitations: This analysis cannot account for the impact of reporting on suicide. Conclusions: Careful attention must be paid to the quality and content of reports, especially as longer-term consequences of the pandemic develop

Effective Hamiltonian for Excitons with Spin Degrees of Freedom

Starting from the conventional electron-hole Hamiltonian ${\cal H}_{eh}$, we derive an effective Hamiltonian $\tilde{\cal H}_{1s}$ for $1s$ excitons with spin degrees of freedom. The Hamiltonian describes optical processes close to the exciton resonance for the case of weak excitation. We show that straightforward bosonization of ${\cal H}_{eh}$ does not give the correct form of $\tilde{\cal H}_{1s}$, which we obtain by a projection onto the subspace spanned by the $1s$ excitons. The resulting relaxation and renormalization terms generate an interaction between excitons with opposite spin. Moreover, exciton-exciton repulsive interaction is greatly reduced by the renormalization. The agreement of the present theory with the experiment supports the validity of the description of a fermionic system by bosonic fields in two dimensions.Comment: 12 pages, no figures, RevTe