Eric Sembrat's Test Bonanza

In this lecture, Georgia Tech Physics Professor Ed Conrad and Director of Research at the French National Center for Scientific Research (CNRS) Amina Taleb will give a feel for how modern research is conducted in the era of small materials and big machines, showing an example of an international materials research collaboration between Georgia Tech’s School of Physics and researchers at the Synchrotron SOLEIL near Paris.

The large-scale distribution of galaxies can be explained fairly simply by assuming i) all galaxies are hosted by halos and ii) a cosmological model. This simple framework, called the `halo-model', has been remarkably successful at reproducing the large-scale clustering of galaxies observed in various galaxy redshift surveys. However, none of these studies have truly tested the `halo-model' by carefully modeling the systematics. We present the results from a fully-numerical, accurate `halo-model' framework and show that the theory can not simultaneously reproduce the galaxy projected correlation function and the group multiplicity function in the SDSS main samples. In particular, the bright galaxy sample shows significant tension with theory. We discuss the implications of our findings, as well as how to constrain different aspects of galaxy formation by simultaneously fitting multiple statistics.

The extragalactic background light (EBL) that fills the Universe is mainly the result of star formation activity over cosmic time. Therefore, it contains fundamental information on galaxy evolution and cosmology. The detection of the EBL by direct methods is hampered by the strong foregrounds mainly from our own Solar System and Galaxy. Interestingly, there are other indirect methodologies that allows us its study such as observations from deep galaxy surveys and gamma-ray observations of distant sources. The latter methodology is based on the fact that very high energy photons traveling across cosmological distances interact by pair production with EBL photons producing an energy-dependent attenuation of the emitted flux. Knowledge of the EBL is thus fundamental in order to correctly interpret extragalactic observations from Cherenkov telescopes such as HAWC and the future CTA. A summary of our EBL knowledge and current and future lines of work will I will also discuss how the recent progress in the EBL understanding has allowed us to measure the expansion rate of the Universe using multiwavelength observations of blazars, which include Fermi and Cherenkov observations.

The quantum Hall effect (QHE) observed in two dimensional electron gas at low temperature and under a strong perpendicular magnetic field has revolutionized the resistance metrology since its discovery in 1980 by Klaus von Klitzing. It provides a representation of the ohm based on the Planck constant and the electron charge only. In 2005, graphene, a purely two dimensional arrangement of carbon atoms in a honeycomb lattice, where the charge carriers behave as Dirac fermions, has revealed a new flavor of the QHE. From the metrological point of view the QHE in graphene is very promising since it is much more robust than in conventional semiconductors. It could lead to a more convenient resistance standard operating at higher temperature and lower magnetic induction, which is an advantage for a broader dissemination of a precise standard for industrial end-users.

During this presentation I’ll first present the impact in the QHE regime of line defects such as wrinkles or grain boundaries, ubiquitous in graphene grown by chemical vapor deposition on metal. We will show that these line defects lead to a non conventional dissipation mechanism that jeopardize the quantum Hall effect accuracy, pointing to the use of wrinkle-free monocrystals for further metrological studies.

The second part of my presentation will be focused on monolayer graphene grown by chemical vapor deposition on silicon carbide. We compared in detail the Hall resistance of the graphene sample from 10 T to 19 T at 1.4 K with a GaAs/AlGaAs resistance standard with a discrepancy of (-2± 4)x1010. For the first time a graphene-based standard was able to operate not only in the same temperature and magnetic field conditions as the semiconductor-based standard, but in a magnetic range more than ten times larger. We have carefully studied the dissipation mechanisms taking place in this sample and measured precisely the value of the localization length in the QHE regime. It saturates interestingly at the charge carrier wavelength, opening interesting questions about the close link between Hall quantization and localization physics in graphene grown on SiC.

The Ebola epidemic in West Africa has spurred an international response. This response has been strongly influenced by epidemiological models that predicted a devastating rise in cases without large-scale changes in behavior and intervention. In this talk, I introduce the mathematical principles underlying predictions of the rate and scope of a disease epidemic. I then explain how such principles have been applied to forecasting Ebola virus disease (EVD) dynamics and identifying the type and scale of necessary control. One control mechanism involves influencing behavior and social norms to limit post-death transmission, e.g., during burial ceremonies of individuals who died from EVD. Post-death transmission for EVD has been recognized for over 10 years, yet its relative importance in the current epidemic remains uncertain. I conclude my talk with an analysis of ongoing challenges in estimating the relative importance of post-death transmission from early-stage epidemic data. I show why such estimation is hard and yet, nonetheless, why controlling post-death transmission is likely to have a substantial effect on short- and long-term epidemic outcomes.

Dehydration and lack of fresh water are key problems for vertebrates that have secondarily invaded marine environments. However, recent investigations of 9 species representing the principal clades of sea snakes indicate these reptiles do not drink seawater but will drink fresh water at variable thresholds of dehydration, which these snakes tolerate well. Marine reptiles were previously thought to remain in water balance without consuming fresh water owing to the ability of extrarenal salt glands to excrete excess salts obtained either from prey or from drinking sea water directly. Thus, species of marine snakes which dehydrate at sea and are dependent on environmental sources of fresh water represent a shift of paradigm from previous “textbook” literature. Recent studies also demonstrate that the abundance and diversity of sea snakes correlate with access to fresh water, and that global distributions and evolutionary origins are related to low and variable ocean salinity. Currently available data indicate that sea snakes have relatively high levels of total body water (around 80% of body mass), are comparatively resistant to dehydration, and have diverse thresholds for thirst.

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