University of Fribourg

About: University of Fribourg is a education organization based out in Fribourg, Freiburg, Switzerland. It is known for research contribution in the topics: Population & Context (language use). The organization has 6040 authors who have published 14975 publications receiving 542500 citations. The organization is also known as: UNIFR & Universität Freiburg.

Strongly bound excitons in anatase TiO2 single crystals and nanoparticles.

Abstract: Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO2, revealing the existence of strongly bound excitons in single crystals and nanoparticles.

Temporal effects in the growth of networks.

Abstract: We show that to explain the growth of the citation network by preferential attachment (PA), one has to accept that individual nodes exhibit heterogeneous fitness values that decay with time. While previous PA-based models assumed either heterogeneity or decay in isolation, we propose a simple analytically treatable model that combines these two factors. Depending on the input assumptions, the resulting degree distribution shows an exponential, log-normal or power-law decay, which makes the model an apt candidate for modeling a wide range of real systems.

Hydrogen adsorption on sp 2 -bonded carbon: Influence of the local curvature

Abstract: The interaction of atomic hydrogen and low-energy hydrogen ions with ${\mathrm{sp}}^{2}$-bonded carbon is investigated on the surfaces of ${\mathrm{C}}_{60}$ multilayer films, single-walled carbon nanotubes, and graphite (0001). These three materials have been chosen to represent ${\mathrm{sp}}^{2}$-bonded carbon networks with different local curvatures and closed surfaces (i.e. no dangling bonds). Chemisorption of hydrogen on these surfaces reduces emission from photoemission features associated with the \ensuremath{\pi} electrons and leads to a lowering of the work function up to 1.3 eV. It is found that the energy barrier for hydrogen adsorption decreases with increasing local curvature of the carbon surface. Whereas in the case of ${\mathrm{C}}_{60}$ and single-walled carbon nanotubes, hydrogen adsorption can be achieved by exposure to atomic hydrogen, the hydrogen adsorption on graphite (0001) requires ${H}^{+}$ ions of low kinetic energy (\ensuremath{\sim}1 eV). On all three materials, the adsorption energy barrier is found to increase with coverage. Accordingly, hydrogen chemisorption saturates at coverages that depend on the local curvature of the sample and the form of hydrogen (i.e., atomic or ionic) used for the treatment.

Robust Circadian Rhythmicity of Per1 and Per2 Mutant Mice in Constant Light, and Dynamics of Per1 and Per2 Gene Expression under Long and Short Photoperiods

Abstract: The Per1 and Per2 genes are components of the mammalian circadian clock. Mutations in these genes alter phase resetting in response to a nocturnal light pulse, and Per2 mutant mice are known to become arrhythmic in constant darkness. We show that under constant light conditions, Per2 mutant mice exhibit robust activity rhythms as well as body temperature rhythms with a period length that is less than 24 h. In Per1 mutants, the period length of both activity and body temperature rhythms is longer than 24 h in constant light. Per1 mutants prolong their period length (tao) when illuminance is increased, whereas Per2 mutants shorten their endogenous period. Additionally, the authors show that the circadian pattern of Per1 and Per2 gene expression in mice is modified under different photoperiods and that there is a mutual influence of these genes on their timing of expression. We propose that, in mice, the phase relationship between Per1 and Per2 gene expression might be critical for transducing day length information to the organism. Per1 could be part of a morning oscillator tracking dawn, and Per2 could be part of an evening oscillator tracking dusk.

Trust in medical organizations predicts pandemic (H1N1) 2009 vaccination behavior and perceived efficacy of protection measures in the Swiss public

Abstract: Following the recent avian influenza and pandemic (H1N1) 2009 outbreaks, public trust in medical and political authorities is emerging as a new predictor of compliance with officially recommended protection measures. In a two-wave longitudinal survey of adults in French-speaking Switzerland, trust in medical organizations longitudinally predicted actual vaccination status 6 months later, during the pandemic (H1N1) 2009 vaccination campaign. No other variables explained significant amounts of variance. Trust in medical organizations also predicted perceived efficacy of officially recommended protection measures (getting vaccinated, washing hands, wearing a mask, sneezing into the elbow), as did beliefs about health issues (perceived vulnerability to disease, threat perceptions). These findings show that in the case of emerging infectious diseases, actual behavior and perceived efficacy of protection measures may have different antecedents. Moreover, they suggest that public trust is a crucial determinant of vaccination behavior and underscore the practical importance of managing trust in disease prevention campaigns.