We report element-specific X-ray spectroscopy results
on the structure
and bonding of Au24Pt, a thiolate-protected bimetallic
nanocluster. Platinum L3-edge extended X-ray absorption
fine-structure (EXAFS) data, in association with X-ray photoelectron
spectroscopy (XPS) compositional analysis, was used to identify the
location of the Pt dopant to be in the center of the icosahedron Au13 core. Comparison of Au24Pt with the structure
of Au25 by gold L3-edge EXAFS clearly shows
contraction of both metal–thiolate and metal–metal bond
distances, caused by Pt doping. The doping effect on the electronic
properties of Au24Pt was further evaluated by high-resolution
Au 4f core-level XPS and ab initio calculations, which elucidate the
importance of bimetallic (Pt–Au) bonding and bond contraction
effects on the properties of Pt-doped thiolate-gold nanoclusters.
The composites industry is increasingly using molecular dynamics (MD) simulations to inform its materials development decisions. As a result, there is growing awareness that simulated predictions require quantitative assessments of their quality in order to routinely provide reliable and actionable information. In the following, we develop a suite of uncertainty quantification (UQ) tools designed to assess simulation-based estimates of the glass transition temperature T g of polymer systems for aerospace applications. We consider contributions to this uncertainty arising from: (i) identification of asymptotic regimes in density versus temperature relations; (ii) fluctuations associated with limited time-averaging of dynamical noise; (iii) and finite-size effects associated with partial averaging over polymer-network configurations. We present a sequence of analyses by which we assess each of these contributions and quantify their net effect on estimates of T g. Importantly, these methods suggest more efficient workflows by indicating when multiple small simulations can be combined to yield estimates with uncertainties comparable to larger, more expensive simulations. We expect that related approaches will, in the future, be applicable to other physical quantities of interest as well as to a broader class of computational tools.
We demonstrate preparation and detection of an atom number distribution in a one-dimensional atomic lattice with the variance -14 dB below the Poissonian noise level. A mesoscopic ensemble containing a few thousand atoms is trapped in the evanescent field of a nanofiber. The atom number is measured through dual-color homodyne interferometry with a pW-power shot noise limited probe. Strong coupling of the evanescent probe guided by the nanofiber allows for a real-time measurement with a precision of ±8 atoms on an ensemble of some 10(3) atoms in a one-dimensional trap. The method is very well suited for generating collective atomic entangled or spin-squeezed states via a quantum nondemolition measurement as well as for tomography of exotic atomic states in a one-dimensional lattice.
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