Dynamic sound attenuation at hypersonic frequencies in silica glass

In order to clarify the origin of the dominant processes responsible for the acoustic attenuation of phonons, which is a much debatted topic, we present Bril louin scattering experiments in various silica glasses of different OH impurities content. A large temperature range, from 5 to 1500 K is investigated, up to the glass transition temperature. Comparison of the hypersonic wave attenuation in various samples allows to identify two different processes. The first one induce s a low temperature peak related to relaxational processes; it is strongly sensitive to the extrinsic defects. The second, dominant in the hig h temperature range, is weakly dependent on the impurities and can be ascribed to anharmonic interactions

Interaction of quasilocal harmonic modes and boson peak in glasses

The direct proportionality relation between the boson peak maximum in glasses, $\omega_b$, and the Ioffe-Regel crossover frequency for phonons, $\omega_d$, is established. For several investigated materials $\omega_b = (1.5\pm 0.1)\omega_d$. At the frequency $\omega_d$ the mean free path of the phonons $l$ becomes equal to their wavelength because of strong resonant scattering on quasilocal harmonic oscillators. Above this frequency phonons cease to exist. We prove that the established correlation between $\omega_b$ and $\omega_d$ holds in the general case and is a direct consequence of bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur

Sensing of Fatty Acids for Octanoylation of Ghrelin Involves a Gustatory G-Protein

Ghrelin is an important regulator of energy--and glucose homeostasis. The octanoylation at Ser(3) is essential for ghrelin's biological effects but the mechanisms involved in the octanoylation are unknown. We investigated whether the gustatory G-protein, α-gustducin, and the free fatty acid receptors GPR40 and GPR120 are involved in the fatty acid sensing mechanisms of the ghrelin cell.Wild-type (WT) and α-gustducin knockout (gust(-/-)) mice were fed a glyceryl trioctanoate-enriched diet (OD) during 2 weeks. Ghrelin levels and gastric emptying were determined. Co-localization between GPR40, GPR120 and ghrelin or α-gustducin/α-transducin was investigated by immunofluorescence staining. The role of GPR120 in the effect of medium and long chain fatty acids on the release of ghrelin was studied in the ghrelinoma cell line, MGN3-1. The effect of the GPR40 agonist, MEDICA16, and the GPR120 agonist, grifolic acid, on ghrelin release was studied both in vitro and in vivo.Feeding an OD specifically increased octanoyl ghrelin levels in the stomach of WT mice but not of gust(-/-) mice. Gastric emptying was accelerated in WT but not in gust(-/-) mice. GPR40 was colocalized with desoctanoyl but not with octanoyl ghrelin, α-gustducin or α-transducin positive cells in the stomach. GPR120 only colocalized with ghrelin in the duodenum. Addition of octanoic acid or α-linolenic acid to MGN3-1 cells increased and decreased octanoyl ghrelin levels, respectively. Both effects could not be blocked by GPR120 siRNA. MEDICA16 and grifolic acid did not affect ghrelin secretion in vitro but oral administration of grifolic acid increased plasma ghrelin levels.This study provides the first evidence that α-gustducin is involved in the octanoylation of ghrelin and shows that the ghrelin cell can sense long- and medium-chain fatty acids directly. GPR120 but not GPR40 may play a role in the lipid sensing cascade of the ghrelin cell

Timing of host feeding drives rhythms in parasite replication

Circadian rhythms enable organisms to synchronise the processes underpinning survival and reproduction to anticipate daily changes in the external environment. Recent work shows that daily (circadian) rhythms also enable parasites to maximise fitness in the context of ecological interactions with their hosts. Because parasite rhythms matter for their fitness, understanding how they are regulated could lead to innovative ways to reduce the severity and spread of diseases. Here, we examine how host circadian rhythms influence rhythms in the asexual replication of malaria parasites. Asexual replication is responsible for the severity of malaria and fuels transmission of the disease, yet, how parasite rhythms are driven remains a mystery. We perturbed feeding rhythms of hosts by 12 hours (i.e. diurnal feeding in nocturnal mice) to desynchronise the hosts' peripheral oscillators from the central, light-entrained oscillator in the brain and their rhythmic outputs. We demonstrate that the rhythms of rodent malaria parasites in day-fed hosts become inverted relative to the rhythms of parasites in night-fed hosts. Our results reveal that the hosts' peripheral rhythms (associated with the timing of feeding and metabolism), but not rhythms driven by the central, light-entrained circadian oscillator in the brain, determine the timing (phase) of parasite rhythms. Further investigation reveals that parasite rhythms correlate closely with blood glucose rhythms. In addition, we show that parasite rhythms resynchronise to the altered host feeding rhythms when food availability is shifted, which is not mediated through rhythms in the host immune system. Our observations suggest that parasites actively control their developmental rhythms. Finally, counter to expectation, the severity of disease symptoms expressed by hosts was not affected by desynchronisation of their central and peripheral rhythms. Our study at the intersection of disease ecology and chronobiology opens up a new arena for studying host-parasite-vector coevolution and has broad implications for applied bioscience

Ultrasonic Attenuation Measurements in Neutron-Irradiated Silicon

At low temperatures, amorphous and partly disordered solids exhibit properties which are different from those in crystals. These anomalies can phenomenologically be described by low-energy excitations which are characterized by a broad distribution of energy and relaxation times : the so-called tunneling states (TS). It was first believed that these TS can only occur in typical glass-forming amorphous solids with a low average coordination of the individual atoms, and also in partly disordered, low-coordinated solids such as neutron-irradiated SiO2, TS have been observed. In order to explore the possibility of the presence of TS in topological more constrained disordered solids, ultrasonic attenuation measurements were started on the fourfold coordinated silicon which was irradiated with fast neutrons to create large regions of lattice disorder. In this paper we will present and discuss the first results of these measurements and we will compare them with results of similar measurements on unirradiated, single-crystalline silicon

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LOW TEMPERATURE ULTRASONIC ATTENUATION IN NEUTRON-IRRADIATED QUARTZ VERSUS THE INDUCED MASS-DENSITY CHANGES

Ces mesures sont une extension d'une étude précédente de l'atténuation ultrasonore dans le quartz irradié aux neutrons vers des températures plus basses (0.3 K). Les mesures ont été faites dans la gamme de fréquence 300 - 650 MHz et de doses 1 x 1018 à 26 x 1018 n/cm2 (E ≥ 0.3 MeV). Le comportement ultrasonore est typique pour l'existence des systèmes à deux niveaux : une loi en T3 aux plus basses températures, qui graduellement change vers une atténuation indépendente de T. La densité a été mesurée et une comparaison entre le paramètre typique [MATH] γ2l des systèmes à deux niveaux et la variation relative de densité, effectuée par l'irradiation aux neutrons a été faite. Pour des doses assez faibles, considérées ici, une relation linéaire a été observée.Previous ultrasonic attenuation measurements in neutron-irradiated quartz are extended to lower temperatures (0.3 K). The measurements were performed for different frequencies (300 - 650 Mhz) and doses (1 x 1018 to 26 x 1018 n/cm2 ; E ≥ 0.3 MeV). The ultrasonic behaviour is typical for the existence of two-level systems (TLS) : a T3-law at the lowest temperatures which gradually levels off to a temperature independent attenuation. Mass density mgasurements were done and a comparison was made between the typical parameters [MATH]γ2l of the TLS and the relative mass density change induced by the irradiation. For these rather low neutron doses, a linear relation is observed

X-RAY INVESTIGATION OF ELECTRON-IRRADIATED QUARTZ

L'étude aux rayons de monocristaux de quartz irradiés aux électrons (dose 1,8 x 1020e/cm2) sur lesquels des mesures de conductivité thermique ont été faites auparavant n'a pas montré, dans des limites d'erreur expérimentale, de diffusion provenant des régions amorphes, ni de changement de paramètre de réseau ou de diffusion près des pics de BRAGG. Contrairement aux mesures précédentes sur les monocristaux de quartz irradiés aux neutrons, les résultats concernant la conductivité thermique ne peuvent être expliqués par des régions amorphes déformant le réseau.An X-ray study on electron-irradiated quartz single crystals (dose 1.8 x 1020 e/cm2) on which thermal conductivity measurements had been made before showed within experimental error no scattering of amorphous regions, no lattice parameter change and no diffuse scattering intensity close to Bragg peaks. In contrast to similar earlier measurements on neutron-irradiated quartz single crystals, the results of thermal conductivity cannot be explained by amorphous regions distorting the lattice

THERMAL PHONON TRANSPORT STUDY OF THE DEFECTS CREATED IN QUARTZ SINGLE CRYSTALS BY DIFFERENT IRRADIATIONS (γ RAYS, ELECTRONS, NEUTRONS)

Thermal conductivity measurements have been carried out between 80 mK and 30 K in quartz after γ, electron and neutron irradiation at different doses. All the samples were natural quartz of the same origin, in order to rule out the effects of different sample state. Electron and neutrons cause an additional phonon scattering which, below 1K, can be explained by the presence of tunneling states similar as in amorphous solids but which is very different above IK, indicating the different origin of the glassy "plateau". γ-irradiation does not induce a detectable number of tunneling states