Excitonic effects in optical absorption and electron-energy loss spectra of hexagonal boron nitride

A new interpretation of the optical and energy-loss spectra of hexagonal boron nitride is provided based on first-principle calculations. We show that both spectra cannot be explained by independent-particle transitions but are strongly dominated by excitonic effects. The lowest direct and indirect gaps are much larger than previously reported. The direct gap amounts to 6.8 eV. The first absorption peak at 6.1 eV is due to an exciton with a binding energy of 0.7 eV. We show that this strongly bound Frenkel exciton is also responsible for the low frequency shoulder of the pi plasmon in the energy-loss function. Implications for nanotube studies are discussed.Comment: 5 pages, 5 figure

Excitons in boron nitride nanotubes: dimensionality effects

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed "optical gap" constancy for different tubes and bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].Comment: 5 pages, 2 figure

Intimate partner violence, HIV and sexually transmitted infections in fishing, trading and agrarian communities in Rakai, Uganda.

BackgroundIntimate partner violence (IPV), HIV and sexually transmitted infections (STI) can contribute to disparities in population health, depending on the individual, social and environmental factors characterizing a setting. To better understand the place-based determinants and patterns of these key interrelated public health problems in Uganda, we compared risk factors for IPV, HIV and STI in fishing, trading and agrarian communities in Rakai, Uganda by gender.MethodThis study used cross-sectional data collected from 14,464 sexually active men (n = 6531) and women (n = 7933) as part of the Rakai Community Cohort Study, a population-based open cohort study of men and women aged 15-49 years. We used multilevel modified poisson regression models, which incorporated random intercepts for community and households. Factors associated with IPV, HIV and STI were assessed separately for men and women in fishing, trading and agrarian communities.ResultsA larger proportion of participants in the fishing communities than those in trading and agrarian communities were HIV positive, engaged in HIV risk behaviors, had STI symptoms and reported perpetration of or victimization by IPV. Female gender was a shared correlate of IPV, HIV and STI in the fishing communities. Engagement in multiple sexual relationships or partner's engagement in multiple relationships were shared correlates of IPV, and HIV in agrarian communities and IPV and STI in trading communities.ConclusionPrograms should target factors at multiple levels to reduce risk for syndemic conditions of HIV, STI and IPV in Rakai, Uganda particularly among men and women in fishing communities

Sociolinguistic competence and varietal repertoires in a second language: A study on addressee‐dependent varietal behavior using virtual reality

The present study takes a variationist perspective to explore the varietal repertoires of adult learners of German as a second language (L2), that is, their variable use of standard German, Austro‐Bavarian dialect, and mixture varieties. Forty L2 learners completed a virtual reality task involving interactions with dialect‐speaking and standard‐German‐speaking interlocutors. Using Bayesian multilevel modeling, the goal was to explore differential outcomes in the acquisition of sociolinguistic competence by determining whether participants adjusted their varietal behavior to match that of the interlocutor (i.e., varietal convergence). The results show that there were no interindividual addressee‐dependent convergence tendencies. A holistic person‐centered analysis of individual learners’ intraspeaker variation revealed that only select L2 learners adjusted their usage patterns but did not entirely invert their usage of dialect and standard language as a function of the variety of the interlocutor. Introspective qualitative data speak to potential drivers behind the differential development of L2 (multi)varietal repertoires

Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach

arXiv:cond-mat/0308126We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy.This work was supported by the European Community Research Training Networks NANOPHASE (HPRN-CT-2000-00167), COMELCAN (HPRN-CT-2000-00128), and M-DNA (IST-2001-38051), by the French Ministère de la Recherche through the project NABOCO (Programme No.MPH 724 D), by the SpanishMCyT (MAT2001- 0946), and by the University of the Basque Country (9/UPV 00206.215- 13639/2001). The computer time was granted by IDRIS (Project No. 544), DIPC, and CEPBA (Barcelona).Peer Reviewe

The influence of occupational stress factors on the nicotine dependence: a cross sectional study

Objective This study analyses the association between occupational stress factors and nicotine dependence. Our hypothesis is that occupational stress factors increase nicotine dependence. Methods Data were taken from the Cologne Smoking Study, a case-control study that examines which genetic/psychosocial factors lead to a higher risk for smokers to suffer from cardiac infarction, lung cancer and/or to become addicted to nicotine. Our sample consisted of N = 197 currently smoking and employed participants. Nicotine dependence was measured using the Fagerström Test for Nicotine Dependence (FTND). The extent of the stress factors experienced at work was assessed using the Effort-Reward Imbalance scale (ERI). Logistic regression was used for the statistical analysis. Results Contrary to our hypothesis, the results show that occupational stress factors are actually associated with lower levels of nicotine dependence (N = 197; adjusted OR = 0.439; p = .059). Conclusions One possible explanation for the study's findings is that the participants have a heavy workload and can only smoke in their spare time. Another reason may be workplace smoking bans. Furthermore, the Fagerström Test for Nicotine Dependence is unable to examine nicotine dependence during working hours

Time-Dependent Screening Explains the Ultrafast Excitonic Signal Rise in 2D Semiconductors

We calculate the time evolution of the transient reflection signal in an MoS2 monolayer on a SiO2/Si substrate using first-principles out-of-equilibrium real-time methods. Our simulations provide a simple and intuitive physical picture for the delayed, yet ultrafast, evolution of the signal whose rise time depends on the excess energy of the pump laser: at laser energies above the A- and B-exciton, the pump pulse excites electrons and holes far away from the K valleys in the first Brillouin zone. Electron–phonon and hole–phonon scattering lead to a gradual relaxation of the carriers toward small Active Excitonic Regions around K, enhancing the dielectric screening. The accompanying time-dependent band gap renormalization dominates over Pauli blocking and the excitonic binding energy renormalization. This explains the delayed buildup of the transient reflection signal of the probe pulse, in excellent agreement with recent experimental data. Our results show that the observed delay is not a unique signature of an exciton formation process but rather caused by coordinated carrier dynamics and its influence on the screening