2,725 research outputs found
Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering
The in-plane effective mass of quantum well states in thin Pb films on a Bi
reconstructed Si(111) surface is studied by angle-resolved photoemission
spectroscopy. It is found that this effective mass is a factor of three lower
than the unusually high values reported for Pb films grown on a Pb
reconstructed Si(111) surface. Through a quantitative low-energy electron
diffraction analysis the change in effective mass as a function of coverage and
for the different interfaces is linked to a change of around 2% in the in-plane
lattice constant. To corroborate this correlation, density functional theory
calculations were performed on freestanding Pb slabs with different in-plane
lattice constants. These calculations show an anomalous dependence of the
effective mass on the lattice constant including a change of sign for values
close to the lattice constant of Si(111). This unexpected relation is due to a
combination of reduced orbital overlap of the 6p_z states and altered
hybridization between the 6p_z and 6p_xy derived quantum well states.
Furthermore it is shown by core level spectroscopy that the Pb films are
structurally and temporally stable at temperatures below 100 K.Comment: 7 pages, 6 figure
Holograms of Conformal Chern-Simons Gravity
We show that conformal Chern-Simons gravity in three dimensions has various
holographic descriptions. They depend on the boundary conditions on the
conformal equivalence class and the Weyl factor, even when the former is
restricted to asymptotic Anti-deSitter behavior. For constant or fixed Weyl
factor our results agree with a suitable scaling limit of topologically massive
gravity results. For varying Weyl factor we find an enhancement of the
asymptotic symmetry group, the details of which depend on certain choices. We
focus on a particular example where an affine u(1) algebra related to
holomorphic Weyl rescalings shifts one of the central charges by 1. The Weyl
factor then behaves as a free chiral boson in the dual conformal field theory.Comment: 5
Existence of a āHotā Atom Mechanism for the Dissociation of O<sub>2</sub> on Pt(111)
The dissociation of O2 on a Pt(111) surface was studied by variable temperature scanning tunneling microscopy at 150ā106 K.āThe two oxygen atoms created by the dissociation appear in pairs, with average distances of two lattice constants. Since thermal random walk sets in only at around 200 K, with a diffusion barrier of 0.43 eV and a preexponential factor of 10ā6.3cm2sā1, the distribution of distances at around 160 K evidences nonthermal processes during the dissociation. It is concluded that transient ballistic motion exists, where the short range traveled is in agreement with recent molecular dynamics studies
Can Viruses be Modified to Achieve Sustained Gene Transfer
It is very easy to replace a faulty gene in an immunocompromised mouse. First, one takes a well-characterized virus, such as an adenovirus or an adeno-associated virus, and incorporates the correct version of the faulty gene together with some regulatory sequences into the genome. Then, one transduces the recombinant genome into helper cells, which will add the viral capsid. At last, one injects the resulting viral vector into the sick mouse, and the mouse is cured. It is not that easy in an immunocompetent mouse, let alone in a human, as over the eons the immune system evolved to eliminate viruses regardless if they penetrate as dangerous pathogens or are injected by a well-meaning gene therapist. Here we offer our perspective on the potential of how viral vectors achieve sustained gene transfer in the face of a hostile immune system
The quality of advice provided by pharmacists to patients taking direct oral anticoagulants: A mystery shopper study
Pharmacists report being less confident in their knowledge of direct acting oral anticoagulants (DOACs) than of vitamin K antagonists, which may influence their ability to detect and manage complications arising from DOAC use. In a mystery shopper study, patient agents were sent into community pharmacies with symptom or product-related requests related to common complications that might arise during treatment with oral anticoagulants, with each visit being assessed for the preferred outcome. Only 10/41 (24.4%) visits resulted in the preferred outcome. A complete history-taking process, obtaining a medical history, patient characteristics and pharmacist involvement were strong predictors of the preferred outcome being achieved. The preferred outcome was not consistently achieved without pharmacist involvement. The potential for strategies that support comprehensive pharmacist involvement in over-the-counter requests should be considered to ensure the provision of optimal care to patients taking high-risk medications such as DOACs
Enhanced reactivity of adsorbed oxygen on Pd(111) induced by compression of the oxygen layer
The reaction between O atoms and CO molecules on Pd(111) was investigated by scanning tunneling microscopy (STM). CO was dosed on the (2x2)O-covered surface at temperatures between 100 and 190 K, and the structure changes were monitored by STM. CO adsorption causes compression of the (2x2)O overlayer into islands of the (2x1)O structure, followed by reaction of the O atoms to give CO2. The (2x2)O overlayer does not react with CO at temperatures up to 180 K, whereas the (2x1)O phase reacts at temperatures as low as 136 K. The analysis of the reaction kinetics reveals an activation energy for the O+CO reaction of 0.4 eV and a reaction order of 1 with respect to the O coverage
Adsorbate-adsorbate interactions from statistical analysis of STM images: N/Ru(0001)
Atomic nitrogen on Ru(0001) was prepared by dissociative chemisorption of N2 and studied by scanning tunneling microscopy (STM) at 300 K. Nitrogen occupies the hcp threefold hollow site and is imaged as a depression with a diameter of about 5 Ć
. Interactions between the adsorbed nitrogen atoms were obtained by statistical analysis of STM images, by extraction of the two-dimensional pair distribution function from the arrangement of the N atoms. Since the nearest-neighbor separations could be identified with atomic precision, the pair distribution function g and hence the potential of mean force Veff were obtained as a function of the discrete neighbor sites j up to the tenth nearest neighbor. A comparison with Monte Carlo calculations for balls with a hard-sphere potential provides information about the pair potential Vpair(j): The nearest-neighbor site is strongly repulsive, the second-neighbor site is weakly repulsive, and the third-neighbor site is weakly attractive. These findings rationalize the absence of island formation and of a well-ordered 2Ć2 phase for the N/Ru(0001) system: At temperatures ā„300 K the attractive interaction on the third-neighbor site is too weak, while at lower temperatures the diffusion barrier of 0.9 eV represents a kinetic obstacle. The fact that the range of the interaction is identical to the diameter of the N-atom features in the STM topographs is taken as evidence that the interaction is caused by substrate-mediated electronic forces
Diffusion and Atomic Hopping of N Atoms on Ru(0001) Studied by Scanning Tunneling Microscopy
The dynamic behavior of N atoms adsorbed on a Ru(0001) surface was studied by scanning tunneling microscopy. N atoms formed by dissociation of NO molecules show an initial sharp concentration profile at atomic steps. Its decay was followed as a function of time, providing a quasicontinuum diffusion constant; the activation energy is 0.94 eV and the prefactor is 2Ć10ā2cm2sā1. The diffusion constant was determined also at equilibrium, from statistical jumps of individual N atoms in a uniform overlayer, and is found to be identical to the Fickian value
Estimation of synthetic accessibility score of drug-like molecules based on molecular complexity and fragment contributions
<p>Abstract</p> <p>Background</p> <p>A method to estimate ease of synthesis (synthetic accessibility) of drug-like molecules is needed in many areas of the drug discovery process. The development and validation of such a method that is able to characterize molecule synthetic accessibility as a score between 1 (easy to make) and 10 (very difficult to make) is described in this article.</p> <p>Results</p> <p>The method for estimation of the synthetic accessibility score (SAscore) described here is based on a combination of fragment contributions and a complexity penalty. Fragment contributions have been calculated based on the analysis of one million representative molecules from PubChem and therefore one can say that they capture historical synthetic knowledge stored in this database. The molecular complexity score takes into account the presence of non-standard structural features, such as large rings, non-standard ring fusions, stereocomplexity and molecule size. The method has been validated by comparing calculated SAscores with ease of synthesis as estimated by experienced medicinal chemists for a set of 40 molecules. The agreement between calculated and manually estimated synthetic accessibility is very good with <it>r</it><sup>2 </sup>= 0.89.</p> <p>Conclusion</p> <p>A novel method to estimate synthetic accessibility of molecules has been developed. This method uses historical synthetic knowledge obtained by analyzing information from millions of already synthesized chemicals and considers also molecule complexity. The method is sufficiently fast and provides results consistent with estimation of ease of synthesis by experienced medicinal chemists. The calculated SAscore may be used to support various drug discovery processes where a large number of molecules needs to be ranked based on their synthetic accessibility, for example when purchasing samples for screening, selecting hits from high-throughput screening for follow-up, or ranking molecules generated by various <it>de novo </it>design approaches.</p
Aluminum arsenide cleaved-edge overgrown quantum wires
We report conductance measurements in quantum wires made of aluminum
arsenide, a heavy-mass, multi-valley one-dimensional (1D) system. Zero-bias
conductance steps are observed as the electron density in the wire is lowered,
with additional steps observable upon applying a finite dc bias. We attribute
these steps to depopulation of successive 1D subbands. The quantum conductance
is substantially reduced with respect to the anticipated value for a spin- and
valley-degenerate 1D system. This reduction is consistent with
disorder-induced, intra-wire backscattering which suppresses the transmission
of 1D modes. Calculations are presented to demonstrate the role of strain in
the 1D states of this cleaved-edge structure.Comment: Submitted to Applied Physics Letter
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