We report multiple lines of evidence for a stochastic signal that is correlated among 67 pulsars from the 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. The correlations follow the Hellings–Downs pattern expected for a stochastic gravitational-wave background. The presence of such a gravitational-wave background with a power-law spectrum is favored over a model with only independent pulsar noises with a Bayes factor in excess of 1014, and this same model is favored over an uncorrelated common power-law spectrum model with Bayes factors of 200–1000, depending on spectral modeling choices. We have built a statistical background distribution for the latter Bayes factors using a method that removes interpulsar correlations from our data set, finding p = 10−3 (≈3σ) for the observed Bayes factors in the null no-correlation scenario. A frequentist test statistic built directly as a weighted sum of interpulsar correlations yields p = 5 × 10−5 to 1.9 × 10−4 (≈3.5σ–4σ). Assuming a fiducial f −2/3 characteristic strain spectrum, as appropriate for an ensemble of binary supermassive black hole inspirals, the strain amplitude is 2.4 − 0.6 + 0.7 × 10 − 15 (median + 90% credible interval) at a reference frequency of 1 yr−1. The inferred gravitational-wave background amplitude and spectrum are consistent with astrophysical expectations for a signal from a population of supermassive black hole binaries, although more exotic cosmological and astrophysical sources cannot be excluded. The observation of Hellings–Downs correlations points to the gravitational-wave origin of this signal.
Light-harvesting dyes in dye-sensitized solar cells (DSSCs) must be designed not only to effectively harvest visible light but also to maintain an adsorption geometry at the solvent/TiO2 interface that encourages electron injection. Electron injection is encouraged when the dye is adsorbed to the TiO2 surface such that the LUMO of the dye is spatially near the surface. Furthermore, deleterious recombination pathways between the surface and dye are suppressed if the HOMO of the dye is spatially well-separated from the surface. Thus, measuring the configuration of dyes at these interfaces is important for understanding why some dyes perform better than others as well as providing insight into designing more ideal dyes. We investigate the adsorption geometry of N3 dye on gold and TiO2 using heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Incorporating heterodyne detection into our VSFG experiment provides both enhanced SFG signal and phase sensitivity, which enables the measurement of the absolute orientation of molecules at interfaces. On gold, we find that N3 adsorbs to the surface by binding through one of its isothiocyanate ligands at a 36° tilt angle from the surface normal. The other isothiocyanate ligand exhibits a tilt angle of 82° and thus does not interact with the interface as strongly. Conversely, on TiO2, we find that N3 adsorbs to the surface through three carboxylic acid groups with both isothiocyanate ligands facing away from the surface at a 180° tilt angle from the surface normal. This adsorption geometry of N3 on the TiO2 is arranged such that its LUMO, which resides primarily on the bipyridine ligands, is positioned near the surface while the HOMO, which resides primarily on the isothiocyanate ligands, is oriented far away from the surface. This study presents the first HD-VSFG spectra of N3 on nanoparticulate TiO2 and on gold.
Perylene diimides (PDIs) are a family of molecules that have potential applications to organic photovoltaics. These systems typically aggregate cofacially due to π-stacking interactions between the aromatic perylene cores. In this study, the structure and characteristics of aggregated N, N'-bis(2,6-diisopropylphenyl)-3,4,9,10-perylenetetracarboxylic diimide (common name lumogen orange), a perylene diimide (PDI) with sterically bulky imide functional groups, were investigated using both experimental vibrational spectroscopy and molecular dynamics (MD) simulations. Samples of lumogen orange dispersed in chloroform exhibited complex aggregation behavior, as evidenced by the evolution of the FTIR spectrum over a period of several hours. While for many PDI systems with less bulky imide functional groups aggregation is dominated by π-stacking interactions between perylene cores, MD simulations of lumogen orange dimers indicated a second, more energetically favorable aggregate structure mediated by "edge-to-edge" interactions between PDI units. Two-dimensional infrared spectroscopy together with orientational statistics obtained from MD simulations were employed to identify and rationalize aggregation-induced coupling between vibrational modes.
Efficient, accurate, and adaptable implicit solvent models remain a significant challenge in the field of molecular simulation. A recent implicit solvent model, IS-SPA, based on approximating the mean solvent force using the superposition approximation, provides a platform to achieve these goals. IS-SPA was originally developed to handle nonpolar solutes in a polar solvent and did not accurately capture polar solvation. Here, we demonstrate that IS-SPA can accurately capture polar solvation by incorporating solvent orientation and accounting for the contributions from long ranged electrostatics. Solvent orientation is approximated as that of an ideal dipole aligned in a mean electrostatic field and an analytic form of the long ranged electrostatics is derived. Parameters for the model are calculated from explicit solvent simulations of an isolated atom or molecule and include atom-based solvent densities, mean electric field functions, radially symmetric averaged Lennard-Jones forces, and multipoles of the explicit solvent model. Using these parameters, IS-SPA accounts for asymmetry of charge solvation and reproduces the explicit solvent potential of mean force of dimerization of two oppositely charged Lennard-Jones spheres in chloroform with high fidelity. Additionally, the model more accurately captures the effect of explicit solvent on the monomer and dimer configurations of alanine dipeptide in chloroform than a generalized Born or constant density dielectric model. The current version of the algorithm is expected to outperform explicit solvent simulations for aggregation of small peptides at concentrations below 150 mM, well above the typical experimental concentrations for these materials.
We show that variations of the E~ reflectance peak in Hg-based superlattices can be used to probe low-temperature interdiffusion by monitoring the shift of the E1 peak with time over extended periods. Little evidence of interdiffusion was detected for a number of HgTe/CdTe and HgCdTe/CdTe superlattices stored at room temperature for approximately two years. Two HgTe/CdTe superlattices and one HgCdTe/CdTe superlattice were subsequently annealed in a dry nitrogen atmosphere at 100~ for approximately six months, and then at 150~ for 24 days. During these intervals, the superlattices were periodically removed from the anneal for reflectance measurements to assess the extent of the interdiffusion. Comparison of these results with calculations of superlattice bandgaps and interdiffusion profiles has led to an evaluation of the low temperature interdiffusion coefficients. These extend previous results to lower temperatures and confirm that the degradation of Hg-based superlattices devices due to thermal interdiffusion under normal processing, storage, and operating conditions should not be an issue of concern.
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