464 research outputs found
Design and synthesis of aromatic molecules for probing electric-fields at the nanoscale
We propose using halogenated organic dyes as nanoprobes for electric field
and show their greatly enhanced Stark coefficients using density functional
theory (DFT) calculations. We analyse halogenated variants of three molecules
that have been of interest for cryogenic single molecule spectroscopy,
perylene, terrylene, and dibenzoterrylene, with the zero-phonon optical
transitions at blue, red, and near infrared. Out of all the combinations of
halides and binding sites that are calculated, we have found that fluorination
of the optimum binding site induces a dipole difference between ground and
excited states larger than 0.5 D for all three molecules with the highest value
of 0.69 D for fluoroperylene. We also report on synthesis of 3-fluoroterrylene
and bulk spectroscopy of this compound in liquid and solid organic
environments.Comment: Article presented in Faraday Discussions on September 201
Computing the Mertens and Meissel-Mertens constants for sums over arithmetic progressions
We give explicit numerical values with 100 decimal digits for the Mertens
constant involved in the asymptotic formula for and, as a by-product, for the Meissel-Mertens constant
defined as , for , ...,
and .Comment: 12 pages, 6 table
Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory
We propose a model for magnetic noise based on spin-flips (not
electron-trapping) of paramagnetic dangling-bonds at the
amorphous-semiconductor/oxide interface. A wide distribution of spin-flip times
is derived from the single-phonon cross-relaxation mechanism for a
dangling-bond interacting with the tunneling two-level systems of the amorphous
interface. The temperature and frequency dependence is sensitive to three
energy scales: The dangling-bond spin Zeeman energy delta, as well as the
minimum (E_min) and maximum (E_max) values for the energy splittings of the
tunneling two-level systems. We compare and fit our model parameters to a
recent experiment probing spin coherence of antimony donors implanted in
nuclear-spin-free silicon [T. Schenkel {\it et al.}, Appl. Phys. Lett. 88,
112101 (2006)], and conclude that a dangling-bond area density of the order of
10^{14}cm^{-2} is consistent with the data. This enables the prediction of
single spin qubit coherence times as a function of the distance from the
interface and the dangling-bond area density in a real device structure. We
apply our theory to calculations of magnetic flux noise affecting SQUID devices
due to their Si/SiO_2 substrate. Our explicit estimates of flux noise in SQUIDs
lead to a noise spectral density of the order of 10^{-12}Phi_{0}^{2} {Hz}^{-1}
at f=1Hz. This value might explain the origin of flux noise in some SQUID
devices. Finally, we consider the suppression of these effects using surface
passivation with hydrogen, and the residual nuclear-spin noise resulting from a
perfect silicon-hydride surface.Comment: Final published versio
Biodégradation anaérobie de l'acide crotonique par une biomasse bactérienne spécialisée dans la dégradation de l'acide butyrique
La connaissance, actuellement trĂšs limitĂ©e, du mĂ©tabolisme des bactĂ©ries acĂ©togĂšnes intervenant dans la biodĂ©gradation anaĂ©robie de l'acide butyrique et d'un de ses sous-produits, l'acide crotonique, est Ă l'origine de cette Ă©tude.AprĂšs avoir mis au point un rĂ©acteur anaĂ©robie Ă biomasse fixĂ©e, cette derniĂšre a, dans un premier temps, Ă©tĂ© adaptĂ©e Ă la biodĂ©gradation exclusive du butyrate. La dĂ©gradation du crotonate a ensuite Ă©tĂ© Ă©tudiĂ©e, selon diffĂ©rents protocoles expĂ©rimentaux (pulses de crotonate en alimentation continue avec du butyrate puis alimentation continue avec du crotonate). Des injections de crotonate ont Ă©galement Ă©tĂ© effectuĂ©es en circuit fermĂ©, avec une biomasse adaptĂ©e dans un premier temps Ă la dĂ©gradation d'un mĂ©lange d'AGV, le rĂ©acteur Ă©tant ensuite alimentĂ© avec du propionate puis du butyrate seuls.Contrairement Ă ce que laissait penser la bibliographie, il a Ă©tĂ© constatĂ© que les bactĂ©ries adaptĂ©es Ă la dĂ©gradation exclusive du butyrate sons trĂšs rapidement Ă mĂȘme de dĂ©grader le crotonate.Les rĂ©sultats obtenus permettent d'approcher les spĂ©cificitĂ©s bactĂ©riennes, la voie catabolique suivie par le crotonate, son mode de rĂ©gulation enzymatique et les Ă©quilibres qui la gouvernent. C'est ainsi qu'il est possible de proposer un modĂšle explicatif relativement simple du mĂ©canisme de biodĂ©gradation du crotonate.Volatile Fatty Acids (VFAs) are intermediate metabolites formed in the anaerobic biodegradation of organic matter. They are commonly found in sewage, municipal sanitary landfill leachate and effluents from agricultural and food-processing industries. A good knowledge of the microorganisms involved in VFA biodegradation is necessary to operate satisfactory biotreatment of those effluents.The objective of the present study is to better understand the metabolism of the anaerobic bacteria responsible for the degradation of butyric acid and one of its metabolites (crotonic acid), which is still poorly known.Syntrophomonaswolfei is one of the few butyrate-degrading acetogenic bacteria that bas been documented. First studios have shown that this microorganism is not capable of degrading crotonic acid (MCINERNEY et al., 1979, 1981). This is surprising since crotonyl-Coenzyme A, in its activated form, is an intermediate metabolite of n-butyrate Ă-oxidation, which is the most common mechanism of butyrate biodegradation. In addition, Ă-oxidatlon of crotonate is thermodynamically possible, even under standard conditions.These observations are al the origin of the present study, which investigates the anaerobic biodegradation of crotonate. Other Investigators have followed a similar approach and isolated S. wolfei in pure culture on crotonate.The degradation of crotonate was studied in a bench-scale up-flow anaerobic filter of twenty liters, operated in the dark, at 35 °C.A first set of experiments was carried out with a biomass exclusively adapted to the biodegradation of butyrate. Heat-expansed vermiculite was used as a packing medium. Various experimental protocols were successive followed. First, pulses of crotonate were injected into the reactor under conditions of continuous feeding with butyrate, and then, the reactor was continuously fed with crotonate. The objective was to determine whether a bacterial population exclusively adapted to butyrate biodegradation would be capable of degrading crotonate.It was found that crotonate was actually biodegraded in the reactor. Woth the first protocol, when pulses of crotonate were injected into the reactor, crotonate was totally removed in 55 hours (fig. 3). Butyrate and acetate concentrations increased as crotonate was degraded, but no significant increase in biogas production was observed. On the other hand, under the same conditions, it was found that iso-butyrate was not degraded, which is consistent with other published data (MCINERNEY et al., 1979, 1981 ; STIEB and SCHINK, 1985,1989).With the second protocol (continuous feeding with crotonate at 5.2 gg/l), crotonate was totally biodegraded in 48 hours after a 24 hours lag period. This biodegradation resulted in the accumulation of acetate and, in a lower extend, butyrate (fig.4).Following this stage, the reactor was fed with a higher crotonate concentration (12 g/l), and it was observed that crotonate was totally degraded in 20 hours, without any lag period (fig. 5).These results showed that butyrate-degrading bacteria were capable of degrading crotonate effectively after a short period of adaptation.Further experiments were conducted with a biomass previously adapted to the degradation of a mixture of VFAs (acetate, propionate, iso-butyrate, butyrate and caproate). Berl saddles were used as a support for bacterial growth. The reactor was operated in a recirculated batch mode and spiked with crotonate. Finally, the reactor was successively fed for four weeks with propionate and for two weeks with butyrate, before being spiked with crotonate.In all these experiments, crotonate biodegradation was observed, but, in contrast to the results obtained with the âvermiculite reactorâ, no butyrate accumulation occured (fig.6).These results show that a bacterial population adapted to the degradation of a mixture of VFAs or to the degradation of individual VFAs such as propionate and n-butyrate, is capable of degrading crotonate.Based on the present study and on literature data, the following mechanism can be proposed for the biodegradation of crotonate (fig.7). The first stage is the activation of crotonate into crotonyl-Coenzyme A by an acetyl-CoA/crotonyl-CoA transferase, as recently isolated from S. wolfei (BEATY and MCINERNEY, 1987). When present at low concentrations, crotonate is probably directly degraded into acetate, as shown by the results obtained with the âselles de Berl reactorâ, in which no intermediate metabolite has been detected. At higher concentrations, enzymatic sites may be saturated and an equilibrium be established with butyrate, which is then released into the medium. This has been shown by the accumulation of butyrate under conditions of continuous feeding with crotonate. In addition, another intermediate metabolite has been formed, which has not been identified in the present study. This product is most probably poly-Ă-hydroxy-butyrate, which has been found in S.wolfei (MCINERNEY et al, 1979) although if is not very common in chemiotrophic bacteria
Impeded Growth of Magnetic Flux Bubbles in the Intermediate State Pattern of Type I Superconductors
Normal state bubble patterns in Type I superconducting Indium and Lead slabs
are studied by the high resolution magneto-optical imaging technique. The size
of bubbles is found to be almost independent of the long-range interaction
between the normal state domains. Under bubble diameter and slab thickness
proper scaling, the results gather onto a single master curve. On this basis,
in the framework of the "current-loop" model [R.E. Goldstein, D.P. Jackson and
A.T. Dorsey, Phys. Rev. Lett. 76, 3818 (1996)], we calculate the equilibrium
diameter of an isolated bubble resulting from the competition between the
Biot-and-Savart interaction of the Meissner current encircling the bubble and
the superconductor-normal interface energy. A good quantitative agreement with
the master curve is found over two decades of the magnetic Bond number. The
isolation of each bubble in the superconducting matrix and the existence of a
positive interface energy are shown to preclude any continuous size variation
of the bubbles after their formation, contrary to the prediction of mean-field
models.Comment: \'{e}quipe Nanostructures Quantique
The expansion of 300 CTG repeats in myotonic dystrophy transgenic mice does not induce sensory or motor neuropathy
Summary: Although many studies have been carried out to verify the involvement of the peripheral nervous system (PNS) in dystrophia myotonica (DM1) patients, the results remain controversial. The generation of DM1 transgenic mice displaying the human DM1 phenotype provides a useful tool to investigate the type and incidence of structural abnormalities in the PNS. In the present study, the morphological and morphometric analysis of semi-thin sections of sciatic and sural nerves, lumbar dorsal root ganglia (DRG) and lumbar spinal cords revealed that in DM1 transgenic mice carrying 300 CTG repeats, there is no change in the number and diameter of myelinated axons compared to wild type. Only a non-significant reduction in the percentage of thin myelinated axons was detected in electron micrographs of ultra-thin sciatic nerve sections. Analysis of the number of neurons did not reveal a loss in number of either sensory neurons in the lumbar DRG or motor neurons in the lumbar spinal cord in these DM1 mice. Furthermore, in hind limb muscle sections, stained with a neurofilament antibody and α-bungarotoxin, the intramuscular axon arborization appeared normal in DM1 mice and undistinguishable from that in wild-type mice. Moreover, in DM1 mice, there was no irregularity in the structure or an increase in the endplate area. Also statistical analysis did not show an increase in endplate density or in the concentration of acetylcholine receptors. Altogether, these results suggest that 300 CTG repeats are not sufficient to induce axonopathy, demyelination or neuronopathies in this transgenic mouse mode
Nucleation and Collapse of the Superconducting Phase in Type-I Superconducting Films
The phase transition between the intermediate and normal states in type-I
superconducting films is investigated using magneto-optical imaging. Magnetic
hysteresis with different transition fields for collapse and nucleation of
superconducting domains is found. This is accompanied by topological hysteresis
characterized by the collapse of circular domains and the appearance of
lamellar domains. Magnetic hysteresis is shown to arise from supercooled and
superheated states. Domain-shape instability resulting from long-range magnetic
interaction accounts well for topological hysteresis. Connection with similar
effects in systems with long-range magnetic interactions is emphasized
Scale invariant correlations and the distribution of prime numbers
Negative correlations in the distribution of prime numbers are found to
display a scale invariance. This occurs in conjunction with a nonstationary
behavior. We compare the prime number series to a type of fractional Brownian
motion which incorporates both the scale invariance and the nonstationary
behavior. Interesting discrepancies remain. The scale invariance also appears
to imply the Riemann hypothesis and we study the use of the former as a test of
the latter.Comment: 13 pages, 8 figures, version to appear in J. Phys.
RAPIC project: toward a new generation of inexpensive heat exchanger-reactors for process intensification
Process intensification (PI) in chemical production is a major concern of chemical
manufacturers. This alternative technology involves transposing syntheses into continuous plug
flow reactors with process intensification, leading to a multifunctional heat exchanger-reactor.
In this context, the RAPIC R&D project aims to develop an innovative low-cost component (in
the 10 kg/hour range). This project deals with the design from the local to the global scale and
with testing, from elementary mock-ups to pilot scale. The present paper gives a detailed
description of this research project and presents the main results on specification and definition
of the reaction channel and the first simple mock-ups
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