Interaction-dependent enhancement of the localisation length for two interacting particles in a one-dimensional random potential

We present calculations of the localisation length, $\lambda_{2}$, for two interacting particles (TIP) in a one-dimensional random potential, presenting its dependence on disorder, interaction strength $U$ and system size. $\lambda_{2}(U)$ is computed by a decimation method from the decay of the Green function along the diagonal of finite samples. Infinite sample size estimates $\xi_{2}(U)$ are obtained by finite-size scaling. For U=0 we reproduce approximately the well-known dependence of the one-particle localisation length on disorder while for finite $U$, we find that $\xi_{2}(U) \sim \xi_2(0)^{\beta(U)}$ with $\beta(U)$ varying between $\beta(0)=1$ and $\beta(1) \approx 1.5$. We test the validity of various other proposed fit functions and also study the problem of TIP in two different random potentials corresponding to interacting electron-hole pairs. As a check of our method and data, we also reproduce well-known results for the two-dimensional Anderson model without interaction.Comment: 34 RevTeX 3.0 pages with 16 figures include

Resonant tunnelling in semiconductor heterostructures

This thesis examines the electrical transport properties of a series of n-type GaAs/(AIGa)As double barrier resonant tunnelling devices with well widths between 50 angstrem and 2400 angstrem . The current-voltage characteristics show peak-to-valley ratios as high as 25:1 and as many as seventy resonances, with clear evidence of quantum interference effects at room temperature. The application of a high magnetic field parallel to the current flow produces magnetooscillations in the transport properties which allow the sheet charge density in the accumulation layer to be determined. The energy level in the well over a wide range of bias is obtained from analysis of thermal activation of resonant tunnelling. The contributions of elastic scattering and LO phonon emission to the valley current are investigated spectroscopically with a magnetic field and two phonon modes of the (AIGa)As barrier are observed. The buildup of space charge in the quantum well at resonance leads to intrinsic bistability in the current and differential capacitance of an asymmetric structure. Magnetoquantum oscillations due to a degenerate electron gas in the well are used to measure this charge buildup and demonstrate that the tunnelling process is truly sequential. The bistability is dramatically enhanced at high magnetic fields when the lowest energy Landau level of the well can accommodate a high electron density. In a strongly asymmetric sample, a new kind of bistability is observed where the off-resonant current exceeds the resonant current due to enhancement of charge buildup by intersubband scattering. The modulation of the scattering rate by a magnetic field produces periodic oscillations in the width of the bistability. In a magnetic field applied perpendicular to the current, the resonances are broadened as a consequence of the conservation of canonical momentum. The transition from electric to magnetic quantisation in wide wells is investigated and tunnelling into interfacial Landau levels is observed. The angular dependence of the resonances is used to probe conduction band anisotropy. In a tilted field, a completely new type of magneto-oscillations is observed

Resonant tunnelling in semiconductor heterostructures

This thesis examines the electrical transport properties of a series of n-type GaAs/(AIGa)As double barrier resonant tunnelling devices with well widths between 50 angstrem and 2400 angstrem . The current-voltage characteristics show peak-to-valley ratios as high as 25:1 and as many as seventy resonances, with clear evidence of quantum interference effects at room temperature. The application of a high magnetic field parallel to the current flow produces magnetooscillations in the transport properties which allow the sheet charge density in the accumulation layer to be determined. The energy level in the well over a wide range of bias is obtained from analysis of thermal activation of resonant tunnelling. The contributions of elastic scattering and LO phonon emission to the valley current are investigated spectroscopically with a magnetic field and two phonon modes of the (AIGa)As barrier are observed. The buildup of space charge in the quantum well at resonance leads to intrinsic bistability in the current and differential capacitance of an asymmetric structure. Magnetoquantum oscillations due to a degenerate electron gas in the well are used to measure this charge buildup and demonstrate that the tunnelling process is truly sequential. The bistability is dramatically enhanced at high magnetic fields when the lowest energy Landau level of the well can accommodate a high electron density. In a strongly asymmetric sample, a new kind of bistability is observed where the off-resonant current exceeds the resonant current due to enhancement of charge buildup by intersubband scattering. The modulation of the scattering rate by a magnetic field produces periodic oscillations in the width of the bistability. In a magnetic field applied perpendicular to the current, the resonances are broadened as a consequence of the conservation of canonical momentum. The transition from electric to magnetic quantisation in wide wells is investigated and tunnelling into interfacial Landau levels is observed. The angular dependence of the resonances is used to probe conduction band anisotropy. In a tilted field, a completely new type of magneto-oscillations is observed

"‘Inert’ Ingredients Are Understudied, Potentially Dangerous to Bees and Deserve More Research Attention

Agrochemical formulations are composed of two broad groups of chemicals: active ingredients, which confer pest control action, and ‘inert’ ingredients, which facilitate the action of the active ingredient. Most research into the effects of agrochemicals focusses on the effects of active ingredients. This reflects the assumption that ‘inert’ ingredients are non-toxic. A review of relevant research shows that for bees, this assumption is without empirical foundation. After conducting a systematic literature search, we found just 19 studies that tested the effects of ‘inert’ ingredients on bee health. In these studies, ‘inert’ ingredients were found to cause mortality in bees through multiple exposure routes, act synergistically with other stressors and cause colony level effects. This lack of research is compounded by a lack of diversity in study organism used. We argue that ‘inert’ ingredients have distinct, and poorly understood, ecological persistency profiles and toxicities, making research into their individual effects necessary. We highlight the lack of mitigation in place to protect bees from ‘inert’ ingredients and argue that research efforts should be redistributed to address the knowledge gap identified here. If so-called ‘inert’ ingredients are, in fact, detrimental to bee health, their potential role in widespread bee declines needs urgent assessment

No evidence for negative impacts of acute sulfoxaflor exposure on bee olfactory conditioning or working memory

Systemic insecticides such as neonicotinoids and sulfoximines can be present in the nectar and pollen of treated crops, through which foraging bees can become acutely exposed. Research has shown that acute, field realistic dosages of neonicotinoids can negatively influence bee learning and memory, with potential consequences for bee behaviour. As legislative reassessment of neonicotinoid use occurs globally, there is an urgent need to understand the potential risk of other systemic insecticides. Sulfoxaflor, the first branded sulfoximine-based insecticide, has the same mode of action as neonicotinoids, and may potentially replace them over large geographical ranges. Here we assessed the impact of acute sulfoxaflor exposure on performance in two paradigms that have previously been used to illustrate negative impacts of neonicotinoid pesticides on bee learning and memory. We assayed whether acute sulfoxaflor exposure influences (a) olfactory conditioning performance in both bumblebees (Bombus terrestris) and honeybees (Apis mellifera), using a proboscis extension reflex assay, and (b) working memory performance of bumblebees, using a radial-arm maze. We found no evidence to suggest that sulfoxaflor influenced performance in either paradigm. Our results suggest that despite a shared mode of action between sulfoxaflor and neonicotinoid-based insecticides, widely-documented effects of neonicotinoids on bee cognition may not be observed with sulfoxaflor, at least at acute exposure regimes

Participation as Post-Fordist Politics: Demos, New Labour, and Science Policy

In recent years, British science policy has seen a significant shift ‘from deficit to dialogue’ in conceptualizing the relationship between science and the public. Academics in the interdisciplinary field of Science and Technology Studies (STS) have been influential as advocates of the new public engagement agenda. However, this participatory agenda has deeper roots in the political ideology of the Third Way. A framing of participation as a politics suited to post-Fordist conditions was put forward in the magazine Marxism Today in the late 1980s, developed in the Demos thinktank in the 1990s, and influenced policy of the New Labour government. The encouragement of public participation and deliberation in relation to science and technology has been part of a broader implementation of participatory mechanisms under New Labour. This participatory program has been explicitly oriented toward producing forms of social consciousness and activity seen as essential to a viable knowledge economy and consumer society. STS arguments for public engagement in science have gained influence insofar as they have intersected with the Third Way politics of post-Fordism

Money, (Co)Production and Power in Digital

This article discusses the contribution of critical political economy approaches to digital journalism studies and argues that these offer important correctives to celebratory perspectives. The first part offers a review and critique of influential claims arising from self-styled new studies of convergence culture, media and creative industries. The second part discusses the contribution of critical political economy in examining digital journalism and responding to celebrant claims. The final part reflects on problems of restrictive normativity and other limitations within media political economy perspectives and considers ways in which challenges might be addressed by more synthesising approaches. The paper proposes developing radical pluralist, media systems and comparative analysis, and advocates drawing on strengths in both political economy and culturalist traditions to map and evaluate practices across all sectors of digital journalism

Resource bricolage and growth of product and market scope in social enterprises

This research aims to understand how resource bricolage strategy plays a role in the growth of social enterprises in terms of their product and market. Based on interviews with nine social enterprises, our exploratory finding suggests that social enterprises often employ both internal and network resources in the process of making do. We further explore the relationship between the form of resource utilisation and the nature and scope of activities that the social enterprises embark upon, and find that only those relying on both internal and network bricolage are able to expand into new markets utilising newly developed products. We also find that social enterprises relying on only internal resources can reach the same point through incremental improvisation, by first moving towards either product extension or market expansion, before then embarking on the other. This research contributes to the social entrepreneurship literature by enhancing our understanding of the relationship between resource bricolage strategy and growth of social enterprises through product/ market scope in a penurious environment. The findings of this research also have implications for social enterprise managers and policy makers in utilising their resources and responding to environmental opportunities and challenges