164 research outputs found
Charged exctions in two-dimensional transition-metal dichalcogenides - semiclassical calculation of Berry-curvature effects
We theoretically study the role of the Berry curvature on neutral and charged
excitons in two-dimensional transition-metal dichalcogenides. The Berry
curvature arises due to a strong coupling between the conduction and valence
bands in these materials that can to great extent be described within the model
of massive Dirac fermions. The Berry curvature lifts the degeneracy of exciton
states with opposite angular momentum. Using an electronic interaction that
accounts for non-local screening effects, we find a Berry-curvature induced
splitting of meV between the 2 and 2 exciton states in
WS, consistent with experimental findings. Furthermore, we calculate the
trion binding energies in WS and WSe for a large variety of screening
lenghts and different dielectric constants for the environment. Our approach
indicates the prominent role played by the Berry curvature along with non-local
electronic interactions in the understanding of the energy spectra of neutral
and charged excitons in transition-metal dichalcogenides and in the the
interpretation of their optical properties.Comment: 11 pages, 3 figure
Hybridized quadrupole-dipole exciton effects in - Organic Heterostructure
In the present work we discuss resonant hybridization of the quadrupole
Wannier-Mott exciton (WE) in a quantum well with the Frenkel (FE)
dipole exciton in an adjacent layer of organic DCM2:CA:PA. The coupling between
excitons is due to interaction between the gradient of electric field induced
by DCM2 Frenkel exciton and the quadrupole moment of the transition in the
cuprous oxide. The specific choice of the organic allows us to use the
mechanism of 'solid state solvation' to dynamically tune the WE and FE into
resonance during time (comparable with the big life time of
the WE) of the 'slow' phase of the solvation. The quadrupole-dipole hybrid
utilizes the big oscillator strength of the FE along with the big lifetime of
the quadrupole exciton, unlike dipole-dipole hybrid exciton which utilizes big
oscillator strength of the FE and big radius of the dipole allowed WE. Due to
strong spatial dispersion and big mass of the quadrupole WE the hybridization
is not masked by the kinetic energy or the radiative broadening. The lower
branch of the hybrid dispersion exhibits a pronounced minimum and may be used
in applications. Also we investigate and report noticeable change in the
coupling due to a induced 'Stark effect' from the strong local electric field
of the FE. We investigated the fine energy structure of the quantum well
confined ortho and para excitons in cuprous oxide
Indium selenide: An insight into electronic band structure and surface excitations
We have investigated the electronic response of single crystals of indium selenide by means of angle-resolved photoemission spectroscopy, electron energy loss spectroscopy and density functional theory. The loss spectrum of indium selenide shows the direct free exciton at similar to 1.3 eV and several other peaks, which do not exhibit dispersion with the momentum. The joint analysis of the experimental band structure and the density of states indicates that spectral features in the loss function are strictly related to single-particle transitions. These excitations cannot be considered as fully coherent plasmons and they are damped even in the optical limit, i.e. for small momenta. The comparison of the calculated symmetry-projected density of states with electron energy loss spectra enables the assignment of the spectral features to transitions between specific electronic states. Furthermore, the effects of ambient gases on the band structure and on the loss function have been probed
Phytochemical, antimicrobial, antioxidant and antigenotoxic potentials of Cyperus rotundus extracts
AbstractThe aqueous, ethyl acetate, methanolic and Total Oligomer Flavonoids (TOF) enriched extracts, obtained from the aerial parts of Cyperus rotundus, were investigated for their contents in phenolic compounds. Antioxidative activity using the NBT/riboflavin assay system, antimicrobial activity against Gram positive and Gram negative bacterial reference strains as well as antigenotoxic activity tested with the SOS chromotest assay were also studied. Significant antibacterial activity against reference strains; Staphylococcus aureus, Enterococcus faecalis, Salmonella enteritidis and Salmonella typhimurium, was detected in the presence of ethyl acetate and TOF enriched extracts. In addition to their antimicrobial activity, the same extracts showed a significant ability to inhibit nitroblue tetrazolium reduction by the superoxide radical in a non enzymatic O2.− generating system, and were also able to reduce significantly the genotoxicity induced by nifuroxazide and Aflatoxin B1. The antioxidant, antimicrobial and antigenotoxic activities exhibited by C. rotundus depend on the chemical composition of the tested extracts
DREAM: III.A helium survey in exoplanets on the edge of the hot Neptune desert with GIANO-B@TNG
The population of close-in exoplanets features a desert of hot Neptunes whose
origin is uncertain. These planets may have lost their atmosphere, eroding into
mini-Neptunes and super-Earths. Direct observations of evaporating atmospheres
are essential to derive mass-loss estimates and constrain this scenario. The
metastable 1083.3nm HeI triplet represents a powerful diagnostic of atmospheric
evaporation since it traces the hot gas in extended exoplanet atmospheres, is
observable from the ground, and is weakly affected by interstellar medium
absorption. We conducted a uniform HeI transmission spectroscopy survey,
focusing on 9 planets located at the edges of the Neptunian desert, aiming to
gain insights into the role of photo-evaporation in its formation. We observed
one transit per planet using the high-resolution, near-infrared spectrograph
GIANO-B on the Telescopio Nazionale Galileo. We focused our analysis on the HeI
triplet by computing high-resolution transmission spectra. We then employed the
p-winds model to interpret the observed transmission spectra. We found no sign
of planetary absorption in the HeI triplet in any of the investigated targets.
We thus provided 3sigma upper-limit estimations on the thermosphere absorption,
temperature, and mass loss, and combined them with past measurements to search
for correlations with parameters thought to be drivers in the formation of the
HeI triplet. Our results strengthen the importance of performing homogeneous
surveys and analyses to bring clarification in the HeI detection and hence in
the Neptunian desert origin. Our findings corroborate the literature
expectations that the HeI absorption signal correlates with the stellar mass
and the received XUV flux. However, these trends seem to disappear in terms of
mass-loss rates; further studies are essential to shed light on this aspect and
to understand better the photo-evaporation process.Comment: 23 pages, 13 figures, accepted for publication in A&A, after language
editin
Vibrational Enhancement of the Effective Donor - Acceptor Coupling
The paper deals with a simple three sites model for charge transfer phenomena
in an one-dimensional donor (D) - bridge (B) - acceptor (A) system coupled with
vibrational dynamics of the B site. It is found that in a certain range of
parameters the vibrational coupling leads to an enhancement of the effective
donor - acceptor electronic coupling as a result of the formation of the
polaron on the B site. This enhancement of the charge transfer efficiency is
maximum at the resonance, where the effective energy of the fluctuating B site
coincides with the donor (acceptor) energy.Comment: 5 pages, 3 figure
Dairy Consumption and the Incidence of Hyperglycemia and the Metabolic Syndrome: Results from a French prospective study, Data from the Epidemiological Study on the Insulin Resistance Syndrome (DESIR)
International audienceOBJECTIVE: In the French Data from the Epidemiological Study on the Insulin Resistance Syndrome (DESIR) cohort, cross-sectional analyses have shown that a higher consumption of dairy products and calcium are associated with a lower prevalence of the metabolic syndrome (MetS). We assess the influence of dairy products on 9-year incident MetS and on impaired fasting glycemia and/or type 2 diabetes (IFG/T2D). RESEARCH DESIGN AND METHODS: Men and women who completed a food frequency questionnaire at baseline and after 3 years were studied (n = 3,435). Logistic regression models were used to study associations between the average year 0 and year 3 consumption of milk and dairy products, cheese, dietary calcium density, and incident MetS and IFG/T2D after adjusting for 1) sex, age, alcohol, smoking, physical activity, fat intake and 2) additionally for BMI. Associations between dairy products and continuous variables were studied by repeated-measures ANCOVA, using the same covariates. RESULTS: Dairy products other than cheese, and dietary calcium density, were inversely associated with incident MetS and IFG/T2D; cheese was negatively associated with incident MetS. All three parameters were associated with lower diastolic blood pressure, and with a lower BMI gain. Higher cheese intake and calcium density were associated with a lower increase in waist circumference and lower triglyceride levels. Calcium density was also associated with a lower systolic blood pressure and a lower 9-year increase in plasma triglyceride levels. CONCLUSIONS: A higher consumption of dairy products and calcium was associated with a lower 9-year incidence of MetS and IFG/T2D in a large cohort drawn from the general population
Phosphonomethyl Oligonucleotides as Backbone-Modified Artificial Genetic Polymers
Although several synthetic or xenobiotic nucleic acids (XNAs) have been shown to be viable genetic materials in vitro, major hurdles remain for their in vivo applications, particularly orthogonality. The availability of XNAs that do not interact with natural nucleic acids and are not affected by natural DNA processing enzymes, as well as specialized XNA processing enzymes that do not interact with natural nucleic acids, is essential. Here, we report 3′–2′ phosphonomethyl-threosyl nucleic acid (tPhoNA) as a novel XNA genetic material and a prime candidate for in vivo XNA applications. We established routes for the chemical synthesis of phosphonate nucleic acids and phosphorylated monomeric building blocks, and we demonstrated that DNA duplexes were destabilized upon replacement with tPhoNA. We engineered a novel tPhoNA synthetase enzyme and, with a previously reported XNA reverse transcriptase, demonstrated that tPhoNA is a viable genetic material (with an aggregate error rate of approximately 17 × 10–3 per base) compatible with the isolation of functional XNAs. In vivo experiments to test tPhoNA orthogonality showed that the E. coli cellular machinery had only very limited potential to access genetic information in tPhoNA. Our work is the first report of a synthetic genetic material modified in both sugar and phosphate backbone moieties and represents a significant advance in biorthogonality toward the introduction of XNA systems in vivo
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