3,930 research outputs found
Ab initio derivation of multi-orbital extended Hubbard model for molecular crystals
From configuration interaction (CI) ab initio calculations, we derive an
effective two-orbital extended Hubbard model based on the gerade (g) and
ungerade (u) molecular orbitals (MOs) of the charge-transfer molecular
conductor (TTM-TTP)I_3 and the single-component molecular conductor
[Au(tmdt)_2]. First, by focusing on the isolated molecule, we determine the
parameters for the model Hamiltonian so as to reproduce the CI Hamiltonian
matrix. Next, we extend the analysis to two neighboring molecule pairs in the
crystal and we perform similar calculations to evaluate the inter-molecular
interactions. From the resulting tight-binding parameters, we analyze the band
structure to confirm that two bands overlap and mix in together, supporting the
multi-band feature. Furthermore, using a fragment decomposition, we derive the
effective model based on the fragment MOs and show that the staking TTM-TTP
molecules can be described by the zig-zag two-leg ladder with the
inter-molecular transfer integral being larger than the intra-fragment transfer
integral within the molecule. The inter-site interactions between the fragments
follow a Coulomb law, supporting the fragment decomposition strategy.Comment: 16 pages, 8 figures, published versio
Tunable Functionality and toxicity studies of Titanium Dioxide Nanotube Layers
In this work, we have developed economic process to elaborate scalable
titanium dioxide nanotube layers which show a tunable functionality. The
titanium dioxide nanotube layers was prepared by electrochemical anodization of
Ti foil in 0.4 wt% hydrofluoric acid solution. The nanotube layers structure
and morphology were characterized using x-ray diffraction and scanning electron
microscopy. The surface topography and wettability was studied according to the
anodization time. The sample synthesized while the current density reached a
local minimum displayed higher contact angle. Beyond this point, the contact
angles decrease with the anodization time. Photo-degradation of acid orange 7
in aqueous solution was used as a probe to assess the photo-catalytic activity
of titanium dioxide nanotube layers under UV irradiation. We obtained better
photocatalitic activity for the sample elaborate at higher current density.
Finally we use the Ciliated Protozoan T. pyriformis, an alternative cell model
used for in vitro toxicity studies, to predict the toxicity of titanium dioxide
nanotube layers in biological system. We did not observe any characteristic
effect in the presence of the titanium dioxide nanotube layers on two
physiological parameters related to this organism, non-specific esterases
activity and population growth rate
Time-approximation trade-offs for inapproximable problems
In this paper we focus on problems which do not admit a constant-factor approximation in polynomial time and explore how quickly their approximability improves as the allowed running time is gradually increased from polynomial to (sub-)exponential. We tackle a number of problems: For Min Independent Dominating Set, Max Induced Path, Forest and Tree, for any r(n), a simple, known scheme gives an approximation ratio of r in time roughly rn/r. We show that, for most values of r, if this running time could be significantly improved the ETH would fail. For Max Minimal Vertex Cover we give a nontrivial √r-approximation in time 2n/r. We match this with a similarly tight result. We also give a log r-approximation for Min ATSP in time 2n/r and an r-approximation for Max Grundy Coloring in time rn/r. Furthermore, we show that Min Set Cover exhibits a curious behavior in this superpolynomial setting: for any δ > 0 it admits an mδ-approximation, where m is the number of sets, in just quasi-polynomial time. We observe that if such ratios could be achieved in polynomial time, the ETH or the Projection Games Conjecture would fail. © Édouard Bonnet, Michael Lampis and Vangelis Th. Paschos; licensed under Creative Commons License CC-BY
Photometry of the SW Sex-type nova-like BH Lyncis in high state
Aims: We present a photometric study of the deeply eclipsing SW Sex-type
nova-like cataclysmic variable star BH Lyn
Methods: Time-resolved V-band CCD photometry was obtained for seven nights
between 1999 and 2004.
Results: We determined 11 new eclipse timings of BH Lyn and derived a refined
orbital ephemeris with an orbital period of 0.155875577(14) day. During the
observations, BH Lyn was in high-state with V~15.5 mag. The star presents ~1.5
mag deep eclipses with mean full-width at half-flux of 0.0683(+/-0.0054)P_orb.
The eclipse shape is highly variable, even changing form cycle to cycle. This
is most likely due to accretion disc surface brightness distribution
variations, most probably caused by strong flickering. Time-dependent accretion
disc self-occultation or variations of the hot spot(s) intensity are also
possible explanations. Negative superhumps with period of ~0.145 day are
detected in two long runs in 2000. A possible connection between SW Sex and
negative superhump phenomena through the presence of tilted accretion disc is
discussed, and a way to observationally test this is suggested
Yang-Lee Zeros of the Two- and Three-State Potts Model Defined on Feynman Diagrams
We present both analytic and numerical results on the position of the
partition function zeros on the complex magnetic field plane of the
(Ising) and states Potts model defined on Feynman diagrams
(thin random graphs). Our analytic results are based on the ideas of
destructive interference of coexisting phases and low temperature expansions.
For the case of the Ising model an argument based on a symmetry of the saddle
point equations leads us to a nonperturbative proof that the Yang-Lee zeros are
located on the unit circle, although no circle theorem is known in this case of
random graphs. For the states Potts model our perturbative results
indicate that the Yang-Lee zeros lie outside the unit circle. Both analytic
results are confirmed by finite lattice numerical calculations.Comment: 16 pages, 2 figures. Third version: the title was slightly changed.
To be published in Physical Review
Death rates from malaria epidemics, Burundi and Ethiopia.
Death rates exceeded emergency thresholds at 4 sites during epidemics of Plasmodium falciparum malaria in Burundi (2000-2001) and in Ethiopia (2003-2004). Deaths likely from malaria ranged from 1,000 to 8,900, depending on site, and accounted for 52% to 78% of total deaths. Earlier detection of malaria and better case management are needed
DNA Methyltransferase 3 (MET3) is regulated by Polycomb group complex during Arabidopsis endosperm development
Complex epigenetic changes occur during plant reproduction. These regulations ensure the proper transmission of epigenetic information as well as allowing for zygotic totipotency. In Arabidopsis, the main DNA methyltransferase is called MET1 and is responsible for methylating cytosine in the CG context. The Arabidopsis genome encodes for three additional reproduction-specific homologs of MET1, namely MET2a, MET2b and MET3. In this paper, we show that the DNA methyltransferase MET3 is expressed in the seed endosperm and its expression is later restricted to the chalazal endosperm. MET3 is biallelically expressed in the endosperm but displays a paternal expression bias. We found that MET3 expression is regulated by the Polycomb complex proteins FIE and MSI1. Seed development is not impaired in met3 mutant, and we could not observe significant transcriptional changes in met3 mutant. MET3 might regulates gene expression in a Polycomb mutant background suggesting a further complexification of the interplay between H3K27me3 and DNA methylation in the seed endosperm
Does One Size Fit All? Drug Resistance and Standard Treatments: Results of Six Tuberculosis Programmes in Former Soviet Countries.
SETTING: After the collapse of the Soviet Union, countries in the region faced a dramatic increase in tuberculosis cases and the emergence of drug resistance. OBJECTIVE: To discuss the relevance of the DOTS strategy in settings with a high prevalence of drug resistance. DESIGN: Retrospective analysis of one-year treatment outcomes of short-course chemotherapy (SCC) and results of drug susceptibility testing (DST) surveys of six programmes located in the former Soviet Union: Kemerovo prison, Russia; Abkhasia, Georgia; Nagorno-Karabagh, Azerbaijan; Karakalpakstan, Uzbekistan; Dashoguz Velayat, Turkmenistan; and South Kazakhstan Oblast, Kazakhstan. Results are reported for new and previously treated smear-positive patients. RESULTS: Treatment outcomes of 3090 patients and DST results of 1383 patients were collected. Treatment success rates ranged between 87% and 61%, in Nagorno-Karabagh and Kemerovo, respectively, and failure rates between 7% and 23%. Any drug resistance ranged between 66% and 31% in the same programmes. MDR rates ranged between 28% in Karakalpakstan and Kemerovo prison and 4% in Nagorno-Karabagh. CONCLUSION: These results show the limits of SCC in settings with a high prevalence of drug resistance. They demonstrate that adapting treatment according to resistance patterns, access to reliable culture, DST and good quality second-line drugs are necessary
What drives the active involvement in business angel groups? The role of angels' decision-making style, investment-specific human capital and motivations
This paper sheds light over the operations and internal structure of business angel groups (BAGs), a leading actor inside the informal venture capital industry, due to its capability to build cognitive resources and shared competencies that are eventually provided to funded ventures alongside equity capital. We develop a framework based on the role of business angels' decision-making style, human capital and motivation as major determinants of their active involvement in the many different activities performed by angel groups, either investment related activities or group management activities. Our empirical analysis relies on a novel survey-based dataset containing qualitative and quantitative information provided by the members of two large and rather homogeneous business angel groups located in France and in Italy. Results show that business angels with a control-oriented decision-making style tend to be more actively involved in key angel group activities. Human capital built through investment experience, retirement status, as well as initial motivation to join an angel group are also significant drivers of angel involvement in several key BAG activities
Reaching micro-arcsecond astrometry with long baseline optical interferometry; application to the GRAVITY instrument
A basic principle of long baseline interferometry is that an optical path
difference (OPD) directly translates into an astrometric measurement. In the
simplest case, the OPD is equal to the scalar product between the vector
linking the two telescopes and the normalized vector pointing toward the star.
However, a too simple interpretation of this scalar product leads to seemingly
conflicting results, called here "the baseline paradox". For micro-arcsecond
accuracy astrometry, we have to model in full the metrology measurement. It
involves a complex system subject to many optical effects: from pure baseline
errors to static, quasi-static and high order optical aberrations. The goal of
this paper is to present the strategy used by the "General Relativity Analysis
via VLT InTerferometrY" instrument (GRAVITY) to minimize the biases introduced
by these defects. It is possible to give an analytical formula on how the
baselines and tip-tilt errors affect the astrometric measurement. This formula
depends on the limit-points of three type of baselines: the wide-angle
baseline, the narrow-angle baseline, and the imaging baseline. We also,
numerically, include non-common path higher-order aberrations, whose amplitude
were measured during technical time at the Very Large Telescope Interferometer.
We end by simulating the influence of high-order common-path aberrations due to
atmospheric residuals calculated from a Monte-Carlo simulation tool for
Adaptive optics systems. The result of this work is an error budget of the
biases caused by the multiple optical imperfections, including optical
dispersion. We show that the beam stabilization through both focal and pupil
tracking is crucial to the GRAVITY system. Assuming the instrument pupil is
stabilized at a 4 cm level on M1, and a field tracking below 0.2, we
show that GRAVITY will be able to reach its objective of 10as accuracy.Comment: 14 pages. Accepted by A&
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