958 research outputs found
Ab initio study of a mechanically gated molecule: From weak to strong correlation
The electronic spectrum of a chemically contacted molecule in the junction of
a scanning tunneling microscope can be modified by tip retraction. We analyze
this effect by a combination of density functional, many-body perturbation and
numerical renormalization group theory, taking into account both the
non-locality and the dynamics of electronic correlation. Our findings, in
particular the evolution from a broad quasiparticle resonance below to a narrow
Kondo resonance at the Fermi energy, correspond to the experimental
observations.Comment: 4 pages, 3 figure
Imaging Pauli repulsion in scanning tunneling microscopy
A scanning tunneling microscope (STM) has been equipped with a nanoscale
force sensor and signal transducer composed of a single D2 molecule that is
confined in the STM junction. The uncalibrated sensor is used to obtain
ultra-high geometric image resolution of a complex organic molecule adsorbed on
a noble metal surface. By means of conductance-distance spectroscopy and
corresponding density functional calculations the mechanism of the
sensor/transducer is identified. It probes the short-range Pauli repulsion and
converts this signal into variations of the junction conductance.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
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
The Inactivation of Catalytically Active Thermal Polyanhydro-alpha-amino Acids. /inactivation of Thermal Polyamino Acids/
Inactivation of catalytically active thermal polyanhydro-alpha-amino acids by heating in buffered solutio
Prior-based Coregistration and Cosegmentation
We propose a modular and scalable framework for dense coregistration and
cosegmentation with two key characteristics: first, we substitute ground truth
data with the semantic map output of a classifier; second, we combine this
output with population deformable registration to improve both alignment and
segmentation. Our approach deforms all volumes towards consensus, taking into
account image similarities and label consistency. Our pipeline can incorporate
any classifier and similarity metric. Results on two datasets, containing
annotations of challenging brain structures, demonstrate the potential of our
method.Comment: The first two authors contributed equall
The Catalytic Activity of Thermal Polyanhydro-alpha-amino Acids for the Hydrolysis of Rho-nitrophenyl Acetate /catalysis by Thermal Polyamino Acids/
Catalytic activity of thermal polyanhydro-alpha- amino acids for hydrolysis of rho-nitrophenyl acetate - catalysis by thermal polyamino acid
Two-color resonant four-wave mixing: a tool for double resonance spectroscopy
Two-color resonant four-wave mixing (RFWM) shows great promise in a variety of double-resonance applications in molecular spectroscopy and chemical dynamics. One such application is stimulated emission pumping (SEP), which is a powerful method of characterizing ground-state potential energy surfaces in regions of chemical interest. The authors use time-independent, diagrammatic perturbation theory to identify the resonant terms in the third-order nonlinear susceptibility for each possible scheme by which two-color RFWM can be used for double-resonance spectroscopy. After a spherical tensor analysis they arrive at a signal expression for two-color RFWM that separates the molecular properties from purely laboratory-frame factors. In addition, the spectral response for tuning the DUMP laser in RFWM-SEP is found to be a simple Lorentzian in free-jet experiments. The authors demonstrate the utility of RFWM-SEP and test their theoretical predictions in experiments on jet-cooled transient molecules. In experiments on C{sub 3} they compare the two possible RFWM-SEP processes and show that one is particularly well-suited to the common situation in which the PUMP transition is strong but the DUMP transitions are weak. They obtain RFWM-SEP spectra of the formyl radical, HCO, that probe quasibound vibrational resonances lying above the low threshold for dissociation to H+CO. Varying the polarization of the input beams or PUMP rotational branch produces dramatic effects, in the relative intensities of rotational lines in the RFWM-SEP spectra of HCO; these effects are well-described by their theoretical analysis. Finally, RFWM-SEP spectra of HCO resonances that are homogeneously broadened by dissociation confirm the predicted lineshape and give widths that are in good agreement with those determined via unsaturated fluorescence depletion SEP
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
Benefits of repeated book readings in children with SLI
In this pilot study, we ask whether repeated storybook reading is also beneficial for word learning in children diagnosed with specific language impairment (SLI). We compared 3-year-old German learning children diagnosed with SLI to typically developing children matched on age and socioeconomic status (SES). One week later, children with SLI retained the target words from the stories just as well as their peers, although they did perform significantly worse on immediate recall
Optical absorption spectra of finite systems from a conserving Bethe-Salpeter equation approach
We present a method for computing optical absorption spectra by means of a
Bethe-Salpeter equation approach, which is based on a conserving linear
response calculation for electron-hole coherences in the presence of an
external electromagnetic field. This procedure allows, in principle, for the
determination of the electron-hole correlation function self-consistently with
the corresponding single-particle Green function. We analyze the general
approach for a "one-shot" calculation of the photoabsorption cross section of
finite systems, and discuss the importance of scattering and dephasing
contributions in this approach. We apply the method to the closed-shell
clusters Na_4, Na^+_9 and Na^+_(21), treating one active electron per Na atom.Comment: 9 pages, 3 figure
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