Polarization dependent photocurrents in thin films of the topological insulator Bi₂Se₃

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 67-68).Topological insulators are a new class of three-dimensional quantum materials whose interior or bulk is an insulator but whose surface is a conductor. Bi₂Se₃ is a prototypical topological insulator that physicists at MIT are manufacturing and studying. Various interesting properties of the topological insulator include the flow of pure spin currents and topological protection. Pure spin current, distinct from electric current, is a net flow of spin without a net flow of charge. Recent research at MIT has revealed that shining circularly polarized laser light on a topological insulator turns its surface's pure spin current into a spin-polarized electrical current. The band structure of the bulk of a topological insulator resembles that of an ordinary insulator; the conduction band and valence band are separated, with the Fermi level falling between them. However, for Bi₂Se₃, the conducting surface's dispersion relation can be modeled by a Dirac cone, which crosses the Fermi level. Electrons with opposing spins reside on opposite sides of the Dirac cone. Illuminating a topological insulator with either left or right circularly polarized light depopulates one side of the Dirac cone, leaving on the other side the desired spin-polarized electrical current. In the experiment performed for this thesis, thin films of Bi₂Se₃ were grown on substrates of sapphire via molecular beam epitaxy (MBE). Electrical devices on a micron scale were then fabricated on the thin film surface and used to measure surface currents. Steps of this experiment included characterizing the surface quality of a sapphire substrate using atomic force microscopy (AFM), making electrical devices with Bi₂Se₃ via the processes of optical lithography, ion milling, and electron beam metal deposition. Photocurrents across these electrical devices were induced by the manipulation of optics and lasers and measured using low noise electronics. Experimental results revealed that it was indeed possible to induce spin-polarized electrical currents on thin films of MBE grown Bi₂Se₃. The desired photocurrent was observed when the laser beam spot size was enlargened to illuminate the entirety of the Bi₂Se₃ device simultaneously. These results were not replicable when the laser was more tightly focused onto a smaller area. Scanning the focused laser beam across the Bi₂Se₃ confirmed that different photocurrents were being induced at different points; these results led us to the conclude that there was something inhomogenous about our device. The reason behind this device inhomogeneity is still under investigation.by Claudia Lau.S.B

Interactions between genes and environmental factors in asthma and atopy: new developments

Asthma and associated phenotypes are complex traits most probably caused by an interaction of multiple disease susceptibility genes and environmental factors. Major achievements have occurred in identifying chromosomal regions and polymorphisms in candidate genes linked to or associated with asthma, atopic dermatitis, IgE levels and response to asthma therapy. The aims of this review are to explain the methodology of genetic studies of multifactorial diseases, to summarize chromosomal regions and polymorphisms in candidate genes linked to or associated with asthma and associated traits, to list genetic alterations that may alter response to asthma therapy, and to outline genetic factors that may render individuals more susceptible to asthma and atopy due to environmental changes

The Literariness of Media Art

“Language can be this incredibly forceful material—there’s something about it where if you can strip away its history, get to the materiality of it, it can rip into you like claws” (Hill in Vischer 1995, 11). This arresting image by media artist Gary Hill evokes the nearly physical force of language to hold recipients in its grip. That power seems to lie in the material of language itself, which, with a certain rawness, may captivate or touch, pounce on, or even harm its addressee. Hill’s choice of words is revealing: ‘rip into’ suggests not only a metaphorical emotional pull but also the literal physicality of linguistic attack. It is no coincidence that the statement comes from a media artist, since media artworks often use language to produce a strong sensorial stimulus. Media artworks not only manipulate language as a material in itself, but they also manipulate the viewer’s perceptual channels. The guises and effects of language as artistic material are the topic of this book, The Literariness of Media Art

Motivation and needs of informal physics practitioners

Physicists engage with the public to varying degrees at different stages of their careers. However, their public engagement covers many activities, events, and audiences, making their motivations and professional development needs not well understood. As part of ongoing efforts to build and support community in the informal physics space, we conducted interviews with physicists with a range of different experiences in public engagement. We use personas methodology and self-determination theory to articulate their public engagement motivation, challenges, and needs. We present our set of three personas: the physicist who engages in informal physics for self-reflection, the physicist who wants to spark interest and understanding in physics, and the physicist who wants to provide diverse role models to younger students and inspire them to pursue a STEM career. Needs covered a range of resources including science communication training, community building among informal physics practitioners, and mechanisms to recognize, elevate and value informal physics. By bringing user-centered design methodology to a new topical area of physics education research, we expand our understanding of motivations and needs of practitioners in physics public engagement. Therefore, departments, organizations and institutions could draw upon the personas developed to consider the ways to better support physicists in their respective environment.Comment: 9 pages; preliminary portions of some of this analysis appeared in PERC 2022; submitted to PhysRevPE

The role of I-FABP as a biomarker of intestinal barrier dysfunction driven by gut microbiota changes in obesity

Financial support from Fundacao para Ciencia e Tecnologia (PTDC/AGR-TEC/2227/2012 and SFRH/BD/93073/2013) is gratefully acknowledged.Background: Intestinal fatty-acid binding protein (I-FABP) is expressed in epithelial cells of the mucosal layer of the small intestine tissue. When intestinal mucosal damage occurs, I-FABP is released into the circulation and its plasma concentration increases. In the context of obesity, the gut barrier integrity can be disrupted by dietary fat while intestinal permeability increases. Objective: To investigate whether intestinal fatty acid binding protein (I-FABP) is a suitable plasma marker of intestinal injury and inflammation in obesity. Methods: Twelve male Wistar rats were randomly divided into two groups of six animals each: standard (St) and high-fat (HF) diet fed groups for 12 weeks. Results: HF fed animals developed obesity, insulin resistance and seemed to present increased plasma levels of proinflammatory cytokines (MCP-1 and IL1 beta). The gut microbiota composition of these animals was also altered, with lower number of copies of Bacteroidetes, Prevotella spp. and Lactobacillus spp., in comparison with those from St diet group. Fecal lipopolysaccharide (LPS) concentrations tended to be increased in HF fed animals. Intestinal expression of TLR4 seemed to be also increased in HF fed animals suggesting that HF diet-induced dysbiosis may be behind the systemic inflammation observed. However, in contrast to other intestinal inflammatory diseases, plasma I-FABP levels were decreased in HF fed rats whereas I-FABP expression in jejunum tended to be increased. Conclusions: HF diet-induced obesity is characterized by dysbiosis, insulin resistance and systemic inflammation. In this context, plasmatic I-FABP should not be used as a marker of the intestinal barrier dysfunction and the low-grade chronic inflammatory status.publishersversionpublishe

Intention to Hire Older Workers: A Predictive Model

Malaysia is currently facing a serious labour shortage as a result of declining employment participation rate from its ageing population. Nevertheless, the utilisation of skilful and experienced older workers requires the willingness of business organizations to hire them. Therefore, this paper examines managers’ intention to hire older workers by using an extended model of the Theory of Planned Behaviour (TPB). Past experience is an additional variable used beside attitude, subjective norm and perceived behavioural control in the TPB. A quantitative method of research is adopted through 55 well-prepared questionnaires. The model and hypotheses are tested using partial least square - structural equation modelling analysis. The result of the survey involved 468 hiring managers from 11 industries and 9 states in Malaysia. The research model accounted for a moderate portion of the variance in future hiring intention (R2=0.392). The findings suggest that the predictive component of attitude, subjective norm, and perceived behavioural control are related to the future hiring intention whereas the past experience predictor found no relation. This study's result makes a valuable theoretical contribution as it positioned at age discrimination on the employment of older workers, and serves to future research with reasonable modification on the areas. This study has practical implication at two-levels: bring awareness to business organizations on ageing workforce and preparatory actions on good workplace practices to eliminate inequalities in the age-diversified workforce