Showing papers by "Boston College published in 2008"

Perfect metamaterial absorber.

Abstract: We present the design for an absorbing metamaterial (MM) with near unity absorbance A(omega). Our structure consists of two MM resonators that couple separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer. We fabricate, characterize, and analyze a MM absorber with a slightly lower predicted A(omega) of 96%. Unlike conventional absorbers, our MM consists solely of metallic elements. The substrate can therefore be optimized for other parameters of interest. We experimentally demonstrate a peak A(omega) greater than 88% at 11.5 GHz.

High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys

Abstract: The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100°C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250°C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86°, 106°, and 119°C with hot-side temperatures set at 50°, 100°, and 150°C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.

A metamaterial absorber for the terahertz regime: Design, fabrication and characterization

Abstract: We present a metamaterial that acts as a strongly resonant absorber at terahertz frequencies. Our design consists of a bilayer unit cell which allows for maximization of the absorption through independent tuning of the electrical permittivity and magnetic permeability. An experimental absorptivity of 70% at 1.3 terahertz is demonstrated. We utilize only a single unit cell in the propagation direction, thus achieving an absorption coefficient α=2000 cm-1. These metamaterials are promising candidates as absorbing elements for thermally based THz imaging, due to their relatively low volume, low density, and narrow band response.

Enhanced thermoelectric figure-of-merit in nanostructured p-type silicon germanium bulk alloys.

Abstract: A dimensionless thermoelectric figure-of-merit (ZT) of 0.95 in p-type nanostructured bulk silicon germanium (SiGe) alloys is achieved, which is about 90% higher than what is currently used in space flight missions, and 50% higher than the reported record in p-type SiGe alloys. These nanostructured bulk materials were made by using a direct current-induced hot press of mechanically alloyed nanopowders that were initially synthesized by ball milling of commercial grade Si and Ge chunks with boron powder. The enhancement of ZT is due to a large reduction of thermal conductivity caused by the increased phonon scattering at the grain boundaries of the nanostructures combined with an increased power factor at high temperatures.

Experimental demonstration of frequency-agile terahertz metamaterials

Abstract: Metamaterials exhibit numerous novel effects1,2,3,4,5 and operate over a large portion of the electromagnetic spectrum6,7,8,9,10. Metamaterial devices based on these effects include gradient-index lenses11,12, modulators for terahertz radiation13,14,15 and compact waveguides16. The resonant nature of metamaterials results in frequency dispersion and narrow bandwidth operation where the centre frequency is fixed by the geometry and dimensions of the elements comprising the metamaterial composite. The creation of frequency-agile metamaterials would extend the spectral range over which devices function and, further, enable the manufacture of new devices such as dynamically tunable notch filters. Here, we demonstrate such frequency-agile metamaterials operating in the far-infrared by incorporating semiconductors in critical regions of metallic split-ring resonators. For this first-generation device, external optical control results in tuning of the metamaterial resonance frequency by ∼20%. Our approach is integrable with current semiconductor technologies and can be implemented in other regions of the electromagnetic spectrum. Metamaterials that possess frequency tunability enable new device functions. By external optical control through the incorporation of semiconductors in metallic split-ring resonators, the researchers provide an elegant solution to frequency-agile terahertz metamaterials.

Highly flexible wide angle of incidence terahertz metamaterial absorber: Design, fabrication, and characterization

Abstract: We present the design, fabrication, and characterization of a metamaterial absorber which is resonant at terahertz frequencies. We experimentally demonstrate an absorptivity of 0.97 at 1.6 THz. Importantly, our absorber is only $16\text{ }\ensuremath{\mu}\text{m}$ thick, resulting in a highly flexible material that, further, operates over a wide range of angles of incidence for both transverse electric and transverse magnetic radiation.

Observation of Fermi-surface–dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2

Abstract: We have performed a high-resolution angle-resolved photoelectron spectroscopy study on the newly discovered superconductor Ba0.6K0.4Fe2As2 (Tc=37 K). We have observed two superconducting gaps with different values: a large gap (Δ~12 meV) on the two small hole-like and electron-like Fermi surface (FS) sheets, and a small gap (~6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (Tc), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Δ/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (π, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.

Active Terahertz Metamaterial Devices

Abstract: We demonstrate THz metamaterials exhibiting either amplitude control, via carrier injection or depletion in the active semiconductor substrate or frequency control, via photoexcitation of carriers into active semiconducting materials incorporated into the sub-wavelength metamaterial structure.

Bulk production of a new form of sp(2) carbon: crystalline graphene nanoribbons.

Abstract: We report the use of chemical vapor deposition (CVD) for the bulk production (grams per day) of long, thin, and highly crystalline graphene ribbons (<20−30 μm in length) exhibiting widths of 20−300 nm and small thicknesses (2−40 layers). These layers usually exhibit perfect ABAB... stacking as in graphite crystals. The structure of the ribbons has been carefully characterized by several techniques and the electronic transport and gas adsorption properties have been measured. With this material available to researchers, it should be possible to develop new applications and physicochemical phenomena associated with layered graphene.

Building a New Institutional Infrastructure for Corporate Responsibility

Abstract: Executive Overview This paper describes an emerging institutional infrastructure around corporate responsibility that has resulted in the evolution of initiatives such as the Global Reporting Initiative, the social investment movement, and related efforts that place more emphasis on corporate responsibility, accountability, transparency, and sustainability. Using a framework that roughly classifies initiatives into state/government, market/economic, and civil society categories, the paper illustrates the rapid evolution of new infrastructure that is pressuring companies to be more responsible.

Solvable model for chimera states of coupled oscillators

Abstract: Networks of identical, symmetrically coupled oscillators can spontaneously split into synchronized and desynchronized subpopulations. Such chimera states were discovered in 2002, but are not well understood theoretically. Here we obtain the first exact results about the stability, dynamics, and bifurcations of chimera states by analyzing a minimal model consisting of two interacting populations of oscillators. Along with a completely synchronous state, the system displays stable chimeras, breathing chimeras, and saddle-node, Hopf, and homoclinic bifurcations of chimeras.

Enhanced thermoelectric figure-of-merit in p-type nanostructured bismuth antimony tellurium alloys made from elemental chunks.

Abstract: By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 °C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 °C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.

When consumers and brands talk: Storytelling theory and research in psychology and marketing

Abstract: Storytelling is pervasive through life. Much information is stored, indexed, and retrieved in the form of stories. Although lectures tend to put people to sleep, stories move them to action. People relate to each other in terms of stories—and products and brands often play both central and peripheral roles in their stories. To aid storytelling research in consumer psychology, this article develops a narrative theory that describes how consumers use brands as props or anthropomorphic actors in stories they report about themselves and others. Such drama enactments enable these storytellers to experience powerful myths that reflect psychological archetypes. The article includes findings from case study research that probes propositions of the theory. Implications for consumer psychology and marketing practice follow the discussion of the findings. © 2008 Wiley Periodicals, Inc.

The role of work in psychological health and well-being: a conceptual, historical, and public policy perspective.

Abstract: The primary theme of this article, which serves as the introductory contribution of a special section of the American Psychologist, is that work plays a central role in the development, expression, and maintenance of psychological health. The argument underlying this assumption is articulated at the outset of the article in conjunction with a historical review of vocational psychology and industrial/organizational psychology. The article follows with an overview of contemporary vocational psychology and a presentation of the psychology-of-working perspective, which has emerged from critiques of vocational psychology and from multicultural, feminist, and expanded epistemological analyses of psychological explorations of working. Three illustrative lines of inquiry in which research has affected the potential for informing public policy are presented. These three lines of scholarship (role of work in recovery from mental illness; occupational health psychology; and working, racism, and psychological health) are reviewed briefly to furnish exemplars of how the psychological study of working can inform public policy.

Whole-genome sequencing and variant discovery in C. elegans.

Abstract: Massively parallel sequencing instruments enable rapid and inexpensive DNA sequence data production. Because these instruments are new, their data require characterization with respect to accuracy and utility. To address this, we sequenced a Caernohabditis elegans N2 Bristol strain isolate using the Solexa Sequence Analyzer, and compared the reads to the reference genome to characterize the data and to evaluate coverage and representation. Massively parallel sequencing facilitates strain-to-reference comparison for genome-wide sequence variant discovery. Owing to the short-read-length sequences produced, we developed a revised approach to determine the regions of the genome to which short reads could be uniquely mapped. We then aligned Solexa reads from C. elegans strain CB4858 to the reference, and screened for single-nucleotide polymorphisms (SNPs) and small indels. This study demonstrates the utility of massively parallel short read sequencing for whole genome resequencing and for accurate discovery of genome-wide polymorphisms.

Cash Holdings and Corporate Diversification

Abstract: This paper studies the relation between corporate liquidity and diversification. The key finding is that multi-division firms hold significantly less cash than standalone firms because they are diversified in their investment opportunities. Lower cross-divisional correlations in investment opportunity and higher correlations between investment opportunity and cash flow correspond to lower cash holdings, even after controlling for cash-flow volatility. The effects are strongest in financially constrained firms and in well-governed firms, and correspond to efficient fund transfers from low- to high-productivity divisions. Taken together, these results bring forth an efficient link between diversification in investment opportunity and corporate liquidity.

Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid

Abstract: This study investigates the thermal conductivity and viscosity of copper nanoparticles in ethylene glycol. The nanofluid was prepared by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. Volume loadings of up to 2% were prepared. The measured increase in thermal conductivity was twice the value predicted by the Maxwell effective medium theory. The increase in viscosity was about four times of that predicted by the Einstein law of viscosity. Analytical calculations suggest that this nanofluid would not be beneficial as a coolant in heat exchangers without changing the tube diameter. However, increasing the tube diameter to exploit the increased thermal conductivity of the nanofluid can lead to better thermal performance.

Sleep Preferentially Enhances Memory for Emotional Components of Scenes

Abstract: Central aspects of emotional experiences are often well remembered at the expense of background details. Previous studies of such memory trade-offs have focused on memory after brief delays, but little is known about how these components of emotional memories change over time. We investigated the evolution of memory for negative scenes across 30 min, 12 daytime hours spent awake, and 12 nighttime hours including sleep. After 30 min, negative objects were well remembered at the expense of information about their backgrounds. Time spent awake led to forgetting of the entire negative scene, with memories of objects and their backgrounds decaying at similar rates. Sleep, in contrast, led to a preservation of memories of negative objects, but not their backgrounds, a result suggesting that the two components undergo differential processing during sleep. Memory for a negative scene develops differentially across time delays containing sleep and wake, with sleep selectively consolidating those aspects of memory that are of greatest value to the organism.

Review of McKinsey report: How the world’s best performing school systems come out on top

Abstract: These are heady times in the world of international survey assessments. The Organization for Economic Cooperation and Development (OECD), which sponsors the Program in International Student Assessment (PISA) and the International Adult Literacy Strategy (IALS), and the International Education Agency (IEA), which sponsors the Trends In Mathematics and Science Study (TIMSS) and the Progress in International Reading and Literacy Study (PIRLS), have achieved substantial, if indirect, influence on education policy in many nations. Over the last two decades, these four assessments have expanded in both scope and coverage. Each successive administration has involved a larger number of jurisdictions and garnered greater coverage in the media when scores are announced. More significantly, in many of these jurisdictions policy makers attend to the outcomes (particularly the so-called league tables) and new governmental policies are enacted as a result. Of course, the interest in education stems largely from an appreciation of the role of human capital development in economic growth (Hanushek et al. 2008). At the same time, education researchers have taken advantage of the treasure trove of data to conduct sophisticated analyses: some focus on the relationships among student performance, student background characteristics and school contexts; others look for connections between student outcomes and classroom practices. More often than not, an important goal is to identify those conditions that account for the success of the ‘‘highflyers,’’ those jurisdictions that top the charts. To this ever-growing corpus, McKinsey and Company has contributed the present report, a study of 25 jurisdictions selected on the basis of their performance on PISA 2003, TIMSS or the National Assessment of Educational Progress (NAEP), a bi-annual assessment carried out in the United States. Most were either top scoring or displayed clear improvement trajectories over a number of administrations. A few countries in the Middle East and Latin America were included for comparison purposes, although they are not much mentioned in the text. The authors conducted an extensive literature review and a large number of interviews. In the end, they reached a rather unsurprising conclusion;

Learning‐goals‐driven design model: Developing curriculum materials that align with national standards and incorporate project‐based pedagogy

Abstract: Reform efforts in science education emphasize the importance of rigorous treatment of science standards and use of innovative pedagogical approaches to make science more meaningful and successful. In this paper, we present a learning-goals-driven design model for developing curriculum materials, which combines national standards and a project-based pedagogical approach. We describe our design model in the context of the Investigating and Questioning our World through Science and Technology (IQWST) project, which is developing a three-year coordinated series of middle grades science curriculum materials. From using this model in the development and enactment of the curriculum, we identified three important characteristics: unpacking national science standards, developing a learning performances approach to specifying learning goals, and aligning learning goals, instructional activities, and assessments. Using a case study of an IQWST unit from initial development through two cycles of enactment, we describe how these three characteristics help guide curriculum design, identify design issues in curriculum enactments, and guide the development of design solutions. The iterative use of the learning-goals-driven design model coupled with the analysis of multiple data sources informed our revision of the curriculum materials, resulting in substantial student learning gains for the targeted science content and scientific inquiry learning goals. © 2007 Wiley Periodicals, Inc. Sci Ed92:1–32, 2008

Defining and conceptualizing workplace flexibility

Abstract: We first compare and contrast current conceptualizations of workplace flexibility in order to arrive at a definition in harmony with its contemporary use: ‘the ability of workers to make choices influencing when, where, and for how long they engage in work-related tasks’. Next, we situate workplace flexibility in its theoretical contexts to provide researchers with a clearer understanding of what workplace flexibility is, what its mechanisms of operation are, and why it may be related to other concepts. Finally, we present a conceptual framework of antecedents and consequences of workplace flexibility, including illustrative examples of several basic associations from the peer-reviewed literature.

Social Network, Cognitive Function, and Dementia Incidence Among Elderly Women

Abstract: Objectives. We examined whether social networks had a protective association with incidence of dementia among elderly women.Methods. We prospectively studied 2249 members of a health maintenance organization who were 78 years or older, were classified as free of dementia in 2001, and had completed at least 1 follow-up interview in 2002 through 2005. We used the Telephone Interview for Cognitive Status–modified, the Telephone Dementia Questionnaire, and medical record review to assess cognitive status. We used the Lubben Social Network Scale–6 to assess social network. We estimated hazard ratios for incident dementia with Cox proportional hazards models, adjusting for age at entry, education, hormone use, cognitive status scores, and health conditions.Results. We identified 268 incident cases of dementia during follow-up. Compared with women with smaller social networks, the adjusted hazard ratio for incident dementia in women with larger social networks was 0.74 (95% confidence interval=0.57, 0.97).Conclu...

Hedonic and Instrumental Motives in Anger Regulation

Abstract: What motivates individuals to regulate their emotions? One answer, which has been highlighted in emotion-regulation research, is that individuals are motivated by short-term hedonic goals (e.g., the motivation to feel pleasure). Another answer, however, is that individuals are motivated by instrumental goals (e.g., the motivation to perform certain behaviors). We suggest that both answers have merit. To demonstrate the role instrumental goals may play in emotion regulation, we pitted short-term hedonic motives and instrumental motives against each other, by testing whether individuals were motivated to experience a potentially useful, albeit unpleasant, emotion. We found that (a) individuals preferred activities that would increase their level of anger (but not their level of excitement) when they were anticipating confrontational, but not nonconfrontational, tasks and that (b) anger improved performance in a confrontational, but not a nonconfrontational, task. These findings support a functional view of emotion regulation, and demonstrate that in certain contexts, individuals may choose to experience emotions that are instrumental, despite short-term hedonic costs.

Art Infusion: The Influence of Visual Art on the Perception and Evaluation of Consumer Products

Abstract: In this research, the authors investigate the phenomenon of “art infusion,” in which the presence of visual art has a favorable influence on the evaluation of consumer products through a content-independent spillover of luxury perceptions. In three studies, the authors demonstrate the art infusion phenomenon in both real-world and controlled environments using a variety of stimuli in the contexts of packaging, advertising, and product design.

Constitutively Active Akt Induces Enhanced Myelination in the CNS

Abstract: The serine/threonine kinase Akt regulates multiple cellular functions. The current studies identify a new role for Akt in CNS myelination. In earlier studies on cultured oligodendrocytes, we showed that neuregulin signals through phosphatidylinositol-3'-OH kinase and Akt to enhance survival of oligodendrocytes. However, when transgenic animals were generated that overexpressed constitutively active Akt in oligodendrocytes and their progenitor cells, no enhanced survival of oligodendrocytes or progenitors was found. No alteration in the proliferation or death of progenitors was noted. In contrast, the major impact of Akt overexpression in oligodendrocytes was enhanced myelination. Most interestingly, oligodendrocytes in these mice continued actively myelinating throughout life. Thus, expression of constitutively active Akt in oligodendrocytes and their progenitor cells generated no more oligodendrocytes, but dramatically more myelin. The increased myelination continued as these mice aged, resulting in enlarged optic nerves and white matter areas. In older animals with enlarged white matter areas, the density of oligodendrocytes was reduced, but because of the increased area, the total number of oligodendrocytes remained comparable with wild-type controls. Interestingly, in these animals, overexpression of Akt in Schwann cells did not impact myelination. Thus, in vivo, constitutively active Akt enhances CNS myelination but not PNS myelination and has no impact developmentally on oligodendrocyte number. Understanding the unique aspects of Akt signal transduction in oligodendrocytes that lead to myelination rather than uncontrolled proliferation of oligodendrocyte progenitor cells may have important implications for understanding remyelination in the adult nervous system.

Stata tip 63: Modeling proportions

Abstract: You may often want to model a response variable that appears as a proportion or fraction: the share of consumers’ spending on food, the fraction of the vote for a candidate, or the fraction of days when air pollution is above acceptable levels in a city. To handle these data properly, you must take account of the bounded nature of the response. Just as a linear probability model on unit record data can generate predictions outside the unit interval, using a proportion in a linear regression model will generally yield nonsensical predictions for extreme values of the regressors. One way to handle this for response variables’ values strictly within the unit interval is the logit transformation y = 1