6,159 research outputs found
Statistics of low energy excitations for the directed polymer in a random medium ()
We consider a directed polymer of length in a random medium of space
dimension . The statistics of low energy excitations as a function of
their size is numerically evaluated. These excitations can be divided into
bulk and boundary excitations, with respective densities
and . We find that both densities follow the scaling
behavior , where is the exponent governing the
energy fluctuations at zero temperature (with the well-known exact value
in one dimension). In the limit , both scaling
functions and behave as , leading to the droplet power law
in the regime . Beyond their common singularity near , the two scaling functions
are very different : whereas decays
monotonically for , the function first decays for
, then grows for , and finally presents a power law
singularity near . The density
of excitations of length accordingly decays as
where
. We obtain , and , suggesting the possible relation
.Comment: 15 pages, 25 figure
Tentative detection of ethylene glycol toward W51/e2 and G34.3+0.2
How complex organic - and potentially prebiotic - molecules are formed in
regions of low- and high-mass star-formation remains a central question in
astrochemistry. In particular, with just a few sources studied in detail, it is
unclear what role environment plays in complex molecule formation. In this
light, a comparison of relative abundances of related species between sources
might be useful to explain observed differences. We seek to measure the
relative abundance between three important complex organic molecules, ethylene
glycol ((CHOH)), glycolaldehyde (CHOHCHO) and methyl formate
(HCOOCH), toward high-mass protostars and thereby provide additional
constraints on their formation pathways. We use IRAM 30-m single dish
observations of the three species toward two high-mass star-forming regions -
W51/e2 and G34.3+0.2 - and report a tentative detection of (CH2OH)2 toward both
sources. Assuming that (CHOH), CHOHCHO and HCOOCH spatially
coexist, relative abundance ratios, HCOOCH/(CHOH), of 31 and 35 are
derived for G34.3+0.2 and W51/e2, respectively. CHOHCHO is not detected,
but the data provide lower limits to the HCOOCH/CHOHCHO abundance
ratios of 193 for G34.3+0.2 and 550 for W51/e2. A comparison of these
results to measurements from various sources in the literature indicates that
the source luminosities may be correlated with the HCOOCH/(CHOH)
and HCOOCH/CHOHCHO ratios. This apparent correlation may be a
consequence of the relative timescales each source spend at different
temperatures-ranges in their evolution. Furthermore, we obtain lower limits to
the ratio of (CHOH)/CH2OHCHO for G34.3+0.2 (6) and W51/e2
(16). This result confirms that a high (CHOH)/CHOHCHO
abundance ratio is not a specific property of comets, as previously speculated.Comment: Accepted for publication by A&
The Dalhousie Dentistry Story: A Case for Proportionality, Professionalism, and the Promotion of Moral Character
La tribune de l'Ă©diteur / Editor's Soapbo
Probing the tails of the ground state energy distribution for the directed polymer in a random medium of dimension via a Monte-Carlo procedure in the disorder
In order to probe with high precision the tails of the ground-state energy
distribution of disordered spin systems, K\"orner, Katzgraber and Hartmann
\cite{Ko_Ka_Ha} have recently proposed an importance-sampling Monte-Carlo
Markov chain in the disorder. In this paper, we combine their Monte-Carlo
procedure in the disorder with exact transfer matrix calculations in each
sample to measure the negative tail of ground state energy distribution
for the directed polymer in a random medium of dimension .
In , we check the validity of the algorithm by a direct comparison with
the exact result, namely the Tracy-Widom distribution. In dimensions and
, we measure the negative tail up to ten standard deviations, which
correspond to probabilities of order . Our results are
in agreement with Zhang's argument, stating that the negative tail exponent
of the asymptotic behavior
as is directly related to the fluctuation exponent
(which governs the fluctuations
of the ground state energy for polymers of length ) via the simple
formula . Along the paper, we comment on the
similarities and differences with spin-glasses.Comment: 13 pages, 16 figure
Effect of pine bark and compost on the biological denitrification process of non-hazardous landfill leachate: Focus on the microbiology
In an attempt to optimize the cost-efficiency of landfill leachate treatment by biological denitrification process, our study focused on finding low-cost alternatives to traditional expensive chemicals such as composted garden refuse and pine bark, which are both available in large amount in South African landfill sites. The overall objective was to assess the behaviour of the bacterial community in relation to each substrate while treating high strength landfill leachates. Denitrification processes in fixed bed reactors were simulated at laboratory scale using anaerobic batch tests with immature compost and pine bark. High strength leachate was simulated using a solution of water and nitrate at a concentration of 500 mg lâ1. Results suggest that pine bark released large amounts of phenolic compounds and hydroxylated benzene rings, which both can delay the acclimatization time and inhibit the biological denitrification (only 30% efficiency). Furthermore, presence of potential pathogens like Enterobacter and Pantoea agglomerans prevents the applicability of the pine bark in full-scale operations. On the other hand, lightly composted garden refuse (CGR) offered an adequate substrate for the formation of a biofilm necessary to complete the denitrification process (total nitrate removal observed within 7 days). CGR further contributed to a rapid establishment of an active consortium of denitrifiers including Acinetobacter, Rhizobium, Thermomonas, Rheinheimera, Phaeospirillum and Flavobacterium. Clearly the original composition, nature, carbon to nitrogen ratio (C/N) and degree of maturity and stability of the substrates play a key role in the denitrification process, impacting directly on the development of the bacterial population and, therefore, on the long-term removal efficiency
Sedimentation of active colloidal suspensions
In this paper, we investigate experimentally the non-equilibrium steady state
of an active colloidal suspension under gravity field. The active particles are
made of chemically powered colloids, showing self propulsion in the presence of
an added fuel, here hydrogen peroxide. The active suspension is studied in a
dedicated microfluidic device, made of permeable gel microstructures. Both the
microdynamics of individual colloids and the global stationary state of the
suspension under gravity - density profiles, number fluctuations - are measured
with optical microscopy. This allows to connect the sedimentation length to the
individual self-propelled dynamics, suggesting that in the present dilute
regime the active colloids behave as 'hot' particles. Our work is a first step
in the experimental exploration of the out-of-equilibrium properties of
artificial active systems.Comment: 4 pages, 4 figure
Ion specificity and anomalous electrokinetic effects in hydrophobic nanochannels
We demonstrate with computer simulations that anomalous electrokinetic
effects, such as ion specificity and non-zero zeta potentials for uncharged
surfaces, are generic features of electro-osmotic flow in hydrophobic channels.
This behavior is due to the stronger attraction of larger ions to the
``vapour--liquid-like'' interface induced by a hydrophobic surface. An
analytical model involving a modified Poisson--Boltzmann description for the
ion density distributions is proposed, which allows the anomalous flow profiles
to be predicted quantitatively. This description incorporates as a crucial
component an ion-size-dependent hydrophobic solvation energy. These results
provide an effective framework for predicting specific ion effects, with
important implications for the modeling of biological problems
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