Auger Recombination in Semiconductor Quantum Wells

The principal mechanisms of Auger recombination of nonequilibrium carriers in semiconductor heterostructures with quantum wells are investigated. It is shown for the first time that there exist three fundamentally different Auger recombination mechanisms of (i) thresholdless, (ii) quasi-threshold, and (iii) threshold types. The rate of the thresholdless Auger process depends on temperature only slightly. The rate of the quasi-threshold Auger process depends on temperature exponentially. However, its threshold energy essentially varies with quantum well width and is close to zero for narrow quantum wells. It is shown that the thresholdless and the quasi-threshold Auger processes dominate in narrow quantum wells, while the threshold and the quasi-threshold processes prevail in wide quantum wells. The limiting case of a three-dimensional (3D)Auger process is reached for infinitely wide quantum wells. The critical quantum well width is found at which the quasi-threshold and threshold Auger processes merge into a single 3D Auger process. Also studied is phonon-assisted Auger recombination in quantum wells. It is shown that for narrow quantum wells the act of phonon emission becomes resonant, which in turn increases substantially the coefficient of phonon-assisted Auger recombination. Conditions are found under which the direct Auger process dominates over the phonon-assisted Auger recombination at various temperatures and quantum well widths.Comment: 38 pages, 7 figure

Highly conductive Sb-doped layers in strained Si

The ability to create stable, highly conductive ultrashallow doped regions is a key requirement for future silicon-based devices. It is shown that biaxial tensile strain reduces the sheet resistance of highly doped n-type layers created by Sb or As implantation. The improvement is stronger with Sb, leading to a reversal in the relative doping efficiency of these n-type impurities. For Sb, the primary effect is a strong enhancement of activation as a function of tensile strain. At low processing temperatures, 0.7% strain more than doubles Sb activation, while enabling the formation of stable, ~10-nm-deep junctions. This makes Sb an interesting alternative to As for ultrashallow junctions in strain-engineered complementary metal-oxide-semiconductor device

Charge and Orbital Ordering in Pr_{0.5} Ca_{0.5} MnO_3 Studied by ^{17}O NMR

The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for the first time by ^{17}O NMR. This local probe is sensitive to spin, charge and orbital correlations. Two transitions exist in this system: the charge and orbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition at T_N = 170 K. Both are clearly seen in the NMR spectra measured in a magnetic field of 7T. Above T_{CO} there exists only one NMR line with a large isotropic shift, whose temperature dependence is in accordance with the presence of ferromagnetic (FM) correlations. This line splits into two parts below T_{CO}, which are attributed to different types of oxygen in the charge/orbital ordered state. The interplay of FM and AF spin correlations of Mn ions in the charge ordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the hole hopping motion that is slowed down with decreasing temperature. The developing fine structure of the spectra evidences, that there still exist charge-disordered regions at T_{CO} > T > T_N and that the static (t > 10^{-6}s) orbital order is established only on approaching T_N. The CE-type magnetic correlations develop gradually below T_{CO}, so that at first the AF correlations between checkerboard ab-layers appear, and only at lower temperature - CE correlations within the ab-planes

'You can take a horse to water but you can't make it drink': Exploring children's engagement and resistance in family therapy

The final publication is available at Springer via http://dx.doi.org/10.1007/s10591-012-9220-8Children’s engagement and disengagement, adherence and non-adherence, compliance and non-compliance in healthcare have important implications for services. In family therapy mere attendance to the appointments is no guarantee of engaging in the treatment process and as children are not the main initiators of attendance engaging them through the process can be a complex activity for professionals. Through a conversation analysis of naturally occurring family therapy sessions we explore the main discursive strategies that children employ in this context to passively and actively disengage from the therapeutic process and investigate how the therapists manage and attend to this. We note that children competently remove themselves from therapy through passive resistance, active disengagement, and by expressing their autonomy. Analysis reveals that siblings of the constructed ‘problem’ child are given greater liberty in involvement. We conclude by demonstrating how therapists manage the delicate endeavour of including all family members in the process and how engagement and re-engagement are essential for meeting goals and discuss broader implications for healthcare and other settings where children may disengage

Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models

We present a detailed study of the effect of local chemical ordering on the structural, electronic, and dynamical properties of amorphous gallium arsenide. Using the recently-proposed ``activation-relaxation technique'' and empirical potentials, we have constructed two 216-atom tetrahedral continuous random networks with different topological properties, which were further relaxed using tight-binding molecular dynamics. The first network corresponds to the traditional, amorphous, Polk-type, network, randomly decorated with Ga and As atoms. The second is an amorphous structure with a minimum of wrong (homopolar) bonds, and therefore a minimum of odd-membered atomic rings, and thus corresponds to the Connell-Temkin model. By comparing the structural, electronic, and dynamical properties of these two models, we show that the Connell-Temkin network is energetically favored over Polk, but that most properties are little affected by the differences in topology. We conclude that most indirect experimental evidence for the presence (or absence) of wrong bonds is much weaker than previously believed and that only direct structural measurements, i.e., of such quantities as partial radial distribution functions, can provide quantitative information on these defects in a-GaAs.Comment: 10 pages, 7 ps figures with eps

Personal identity (de)formation among lifestyle travellers: A double-edged sword?

This article explores the personal identity work of lifestyle travellers – individuals for whom extended leisure travel is a preferred lifestyle that they return to repeatedly. Qualitative findings from in-depth semi-structured interviews with lifestyle travellers in northern India and southern Thailand are interpreted in light of theories on identity formation in late modernity that position identity as problematic. It is suggested that extended leisure travel can provide exposure to varied cultural praxes that may contribute to a sense of social saturation. Whilst a minority of the respondents embraced a saturation of personal identity in the subjective formation of a cosmopolitan cultural identity, several of the respondents were paradoxically left with more identity questions than answers as the result of their travels

Dynamical principles in neuroscience

Dynamical modeling of neural systems and brain functions has a history of success over the last half century. This includes, for example, the explanation and prediction of some features of neural rhythmic behaviors. Many interesting dynamical models of learning and memory based on physiological experiments have been suggested over the last two decades. Dynamical models even of consciousness now exist. Usually these models and results are based on traditional approaches and paradigms of nonlinear dynamics including dynamical chaos. Neural systems are, however, an unusual subject for nonlinear dynamics for several reasons: (i) Even the simplest neural network, with only a few neurons and synaptic connections, has an enormous number of variables and control parameters. These make neural systems adaptive and flexible, and are critical to their biological function. (ii) In contrast to traditional physical systems described by well-known basic principles, first principles governing the dynamics of neural systems are unknown. (iii) Many different neural systems exhibit similar dynamics despite having different architectures and different levels of complexity. (iv) The network architecture and connection strengths are usually not known in detail and therefore the dynamical analysis must, in some sense, be probabilistic. (v) Since nervous systems are able to organize behavior based on sensory inputs, the dynamical modeling of these systems has to explain the transformation of temporal information into combinatorial or combinatorial-temporal codes, and vice versa, for memory and recognition. In this review these problems are discussed in the context of addressing the stimulating questions: What can neuroscience learn from nonlinear dynamics, and what can nonlinear dynamics learn from neuroscience?This work was supported by NSF Grant No. NSF/EIA-0130708, and Grant No. PHY 0414174; NIH Grant No. 1 R01 NS50945 and Grant No. NS40110; MEC BFI2003-07276, and Fundación BBVA

Therapist telephone-delivered CBT and web-based CBT compared with treatment as usual in refractory irritable bowel syndrome: the ACTIB three-arm RCT

Background: Irritable bowel syndrome (IBS) affects 10–22% of people in the UK. Abdominal pain, bloating and altered bowel habits affect quality of life and can lead to time off work. Current treatment relies on a positive diagnosis, reassurance, lifestyle advice and drug therapies, but many people suffer ongoing symptoms. Cognitive–behavioural therapy (CBT) is recommended in guidelines for patients with ongoing symptoms but its availability is limited. Objectives: To determine the clinical effectiveness and cost-effectiveness of therapist telephone-delivered CBT (TCBT) and web-based CBT (WCBT) with minimal therapist support compared with treatment as usual (TAU) in refractory IBS. Design: This was a three-arm randomised controlled trial. Setting: This trial took place in UK primary and secondary care. Participants: Adults with refractory IBS (clinically significant symptoms for 12 months despite first-line therapies) were recruited from 74 general practices and three gastroenterology centres from May 2014 to March 2016. Interventions: TCBT – patient CBT self-management manual, six 60-minute telephone sessions over 9 weeks and two 60-minute booster sessions at 4 and 8 months (8 hours’ therapist time). WCBT – interactive, tailored web-based CBT, three 30-minute telephone sessions over 9 weeks and two 30-minute boosters at 4 and 8 months (2.5 hours’ therapist time). Main outcome measures: Primary outcomes – IBS symptom severity score (IBS SSS) and Work and Social Adjustment Scale (WSAS) at 12 months. Cost-effectiveness [quality-adjusted life-years (QALYs) and health-care costs]. Results: In total, 558 out of 1452 patients (38.4%) screened for eligibility were recruited – 186 were randomised to TCBT, 185 were randomised to WCBT and 187 were randomised to TAU. The mean baseline Irritable Bowel Syndrome Symptom Severity Score (IBS SSS) was 265.0. An intention-to-treat analysis with multiple imputation was carried out at 12 months; IBS SSS were 61.6 points lower in the TCBT arm [95% confidence interval (CI) 89.5 to 33.8; p < 0.001] and 35.2 points lower in the WCBT arm (95% CI 57.8 to 12.6; p = 0.002) than in the TAU arm (IBS SSS of 205.6). The mean WSAS score at 12 months was 10.8 in the TAU arm, 3.5 points lower in the TCBT arm (95% CI 5.1 to 1.9; p < 0.001) and 3.0 points lower in the WCBT arm (95% CI 4.6 to 1.3; p = 0.001). For the secondary outcomes, the Subject’s Global Assessment showed an improvement in symptoms at 12 months (responders) in 84.8% of the TCBT arm compared with 41.7% of the TAU arm [odds ratio (OR) 6.1, 95% CI 2.5 to 15.0; p < 0.001] and 75.0% of the WCBT arm (OR 3.6, 95% CI 2.0 to 6.3; p < 0.001). Patient enablement was 78.3% (responders) for TCBT, 23.5% for TAU (OR 9.3, 95% CI 4.5 to 19.3; p < 0.001) and 54.8% for WCBT (OR 3.5, 95% CI 2.0 to 5.9; p < 0.001). Adverse events were similar between the trial arms. The incremental cost-effectiveness ratio (ICER) (QALY) for TCBT versus TAU was £22,284 and for WCBT versus TAU was £7724. Cost-effectiveness reduced after imputation for missing values. Qualitative findings highlighted that, in the CBT arms, there was increased capacity to cope with symptoms, negative emotions and challenges of daily life. Therapist input was important in supporting WCBT. Conclusions: In this large, rigorously conducted RCT, both CBT arms showed significant improvements in IBS outcomes compared with TAU. WCBT had lower costs per QALY than TCBT. Sustained improvements in IBS symptoms are possible at an acceptable cost. Suggested future research work is longer-term follow-up and research to translate these findings into usual clinical practice. Future work: Longer-term follow-up and research to translate these findings into usual clinical practice is needed