299 research outputs found
Electrical transport through a mechanically gated molecular wire
A surface-adsorbed molecule is contacted with the tip of a scanning tunneling
microscope (STM) at a pre-defined atom. On tip retraction, the molecule is
peeled off the surface. During this experiment, a two-dimensional differential
conductance map is measured on the plane spanned by the bias voltage and the
tip-surface distance. The conductance map demonstrates that tip retraction
leads to mechanical gating of the molecular wire in the STM junction. The
experiments are compared with a detailed ab initio simulation. We find that
density functional theory (DFT) in the local density approximation (LDA)
describes the tip-molecule contact formation and the geometry of the molecular
junction throughout the peeling process with predictive power. However, a
DFT-LDA-based transport simulation following the non-equilibrium Green's
functions (NEGF) formalism fails to describe the behavior of the differential
conductance as found in experiment. Further analysis reveals that this failure
is due to the mean-field description of electron correlation in the local
density approximation. The results presented here are expected to be of general
validity and show that, for a wide range of common wire configurations,
simulations which go beyond the mean-field level are required to accurately
describe current conduction through molecules. Finally, the results of the
present study illustrate that well-controlled experiments and concurrent ab
initio transport simulations that systematically sample a large configuration
space of molecule-electrode couplings allow the unambiguous identification of
correlation signatures in experiment.Comment: 31 pages, 10 figure
Phase diagram of silicon from atomistic simulations
In this letter we present a calculation of the temperature-pressure phase
diagram of Si in a range of pressures covering from -5 to 20 GPa and
temperatures up to the melting point. The phase boundaries and triple points
between the diamond, liquid, -Sn and clathrate phases are
reported. We have employed efficient simulation techniques to calculate free
energies and to numerically integrate the Clausius-Clapeyron equation, combined
with a tight binding model capable of an accuracy comparable to that of
first-principles methods. The resulting phase diagram agrees well with the
available experimental data.Comment: 5 pages, 1 figure, accepted in PR
Risk and protective factors for meningococcal disease in adolescents: matched cohort study
Objective: To examine biological and social risk factors for
meningococcal disease in adolescents.
Design: Prospective, population based, matched cohort study
with controls matched for age and sex in 1:1 matching.
Controls were sought from the general practitioner.
Setting: Six contiguous regions of England, which represent
some 65% of the country’s population.
Participants: 15-19 year olds with meningococcal disease
recruited at hospital admission in six regions (representing 65%
of the population of England) from January 1999 to June 2000,
and their matched controls.
Methods: Blood samples and pernasal and throat swabs were
taken from case patients at admission to hospital and from
cases and matched controls at interview. Data on potential risk
factors were gathered by confidential interview. Data were
analysed by using univariate and multivariate conditional
logistic regression.
Results: 144 case control pairs were recruited (74 male (51%);
median age 17.6). 114 cases (79%) were confirmed
microbiologically. Significant independent risk factors for
meningococcal disease were history of preceding illness
(matched odds ratio 2.9, 95% confidence interval 1.4 to 5.9),
intimate kissing with multiple partners (3.7, 1.7 to 8.1), being a
university student (3.4, 1.2 to 10) and preterm birth (3.7, 1.0 to
13.5). Religious observance (0.09, 0.02 to 0.6) and
meningococcal vaccination (0.12, 0.04 to 0.4) were associated
with protection.
Conclusions: Activities and events increasing risk for
meningococcal disease in adolescence are different from in
childhood. Students are at higher risk. Altering personal
behaviours could moderate the risk. However, the development
of further effective meningococcal vaccines remains a key
public health priority
Dynamical bi-stability of single-molecule junctions: A combined experimental/theoretical study of PTCDA on Ag(111)
The dynamics of a molecular junction consisting of a PTCDA molecule between
the tip of a scanning tunneling microscope and a Ag(111) surface have been
investigated experimentally and theoretically. Repeated switching of a PTCDA
molecule between two conductance states is studied by low-temperature scanning
tunneling microscopy for the first time, and is found to be dependent on the
tip-substrate distance and the applied bias. Using a minimal model Hamiltonian
approach combined with density-functional calculations, the switching is shown
to be related to the scattering of electrons tunneling through the junction,
which progressively excite the relevant chemical bond. Depending on the
direction in which the molecule switches, different molecular orbitals are
shown to dominate the transport and thus the vibrational heating process. This
in turn can dramatically affect the switching rate, leading to non-monotonic
behavior with respect to bias under certain conditions. In this work, rather
than simply assuming a constant density of states as in previous works, it was
modeled by Lorentzians. This allows for the successful description of this
non-monotonic behavior of the switching rate, thus demonstrating the importance
of modeling the density of states realistically.Comment: 20 pages, 6 figures, 1 tabl
Heavy traffic, low mortality - tram tracks as terrestrial habitat of newts
Amphibian mortality caused by rail traffic has not attracted much attention in comparison to road mortality. Density of railways in landscape, as well as traffic intensity, is usually much lower than in case of roads. As a consequence, their overall effect on amphibian populations is tacitly assumed to be less negative. To test whether very intensive rail traffic can cause substantial mortality in population of a small amphibian, we investigated a Smooth newt Lissotriton vulgaris population located in the city of Poznań, W Poland, where tram tracks border isolated breeding ponds. We performed controls during the peak of autumn migratory activity along the tracks. Less than 1% of all individuals found during the survey were killed by rail traffic. Observed mortality was very low despite large number of individuals present on the track and intensive tram traffic. As negative effects of traffic are low, rail or tram embankments can provide an important terrestrial habitat for small European newts
High-energy electron measurements with thin Si detectors
A technique for measuring high-energy electrons using Si detectors of various
thicknesses that are much smaller than the range of the examined electrons is
presented. The advantages of the method are discussed on the basis of
electron-positron pair creation recently studied in deuteron-deuteron fusion
reactions at very low energies. Careful Geant 4 Monte Carlo simulations enabled
the identification of the main spectral contributions of emitted electrons and
positrons resulting from the energy loss mechanisms and scattering processes
within the target, detector and their holders. Significant changes in the
intensity of the detected electrons, depending on the detector thickness and
the thicknesses of absorption foils placed in the front of the detector could
be observed. The corresponding correction factors have been calculated and can
be used for different applications in basic and applied research
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