Structure and stability of Co(n)(pyridine)(m)- clusters: absence of metal inserted structures

A synergistic approach combining the experimental photoelectron spectroscopy and theoretical electronic structure studies is used to probe the geometrical structure and the spin magnetic moment of Co(n)(pyridine)(m) (-) clusters. It is predicted that the ground state of Co(pyridine)(-) is a structure where the Co atom is inserted in a CH bond. However, the insertion is marked by a barrier of 0.33 eV that is not overcome under the existing experimental conditions resulting in the formation of a structure where Co occupies a site above the pyridine plane. For Co(2)(pyridine)(-), a ground-state structure is predicted in which the Co(2) diametric moiety is inserted in one of the CH bonds, but again because of a barrier, the structure which matches the photoelectron spectrum is a higher-energy isomer in which the Co(2) moiety is bonded directly to nitrogen on the pyridine ring. In all cases, the Co sites have finite magnetic moments suggesting that the complexes may provide ways of making cluster-based magnetic materials.     

Investigation of finite system-size effects in molecular dynamics simulations of lipid bilayers

In the absence of external stress, the surface tension of a lipid membrane vanishes at equilibrium, and the membrane exhibits long wavelength undulations that can be described as elastic (as opposed to tension-dominated) deformations. These long wavelength fluctuations are generally suppressed in molecular dynamics simulations of membranes, which have typically been carried out on membrane patches with areas <100 nm2 that are replicated by periodic boundary conditions. As a result, finite system-size effects in molecular dynamics simulations of lipid bilayers have been subject to much discussion in the membrane simulation community for several years, and it has been argued that it is necessary to simulate small membrane patches under tension to properly model the tension-free state of macroscopic membranes. Recent hardware and software advances have made it possible to simulate larger, all-atom systems allowing us to directly address the question of whether the relatively small size of current membrane simulations affects their physical characteristics compared to real macroscopic bilayer systems. In this work, system-size effects on the structure of a DOPC bilayer at 5.4 H2O/lipid are investigated by performing molecular dynamics simulations at constant temperature and isotropic pressure (i.e., vanishing surface tension) of small and large single bilayer patches (72 and 288 lipids, respectively), as well as an explicitly multilamellar system consisting of a stack of five 72-lipid bilayers, all replicated in three dimensions by using periodic boundary conditions. The simulation results are compared to X-ray and neutron diffraction data by using a model-free, reciprocal space approach developed recently in our laboratories. Our analysis demonstrates that finite-size effects are negligible in simulations of DOPC bilayers at low hydration, and suggests that refinements are needed in the simulation force fields.     

Beamformer performance with acoustic vector sensors in air

For some time, compact acoustic vector sensors (AVSs) capable of sensing particle velocity in three orthogonal directions have been used in underwater acoustic sensing applications. Potential advantages of using AVSs in air include substantial noise reduction with a very small aperture and few channels. For this study, a four-microphone array approximating a small (1 cm3) AVS in air was constructed using three gradient microphones and one omnidirectional microphone. This study evaluates the signal extraction performance of one nonadaptive and four adaptive beamforming algorithms. Test signals, consisting of two to five speech sources, were processed with each algorithm, and the signal extraction performance was quantified by calculating the signal-to-noise ratio (SNR) of the output. For a three-microphone array, robust and nonrobust versions of a frequency-domain minimum-variance (FMV) distortionless-response beamformer produced SNR improvements of 11 to 14 dB, and a generalized sidelobe canceller (GSC) produced improvements of 5.5 to 8.5 dB. In comparison, a two-microphone omnidirectional array with a spacing of 15 cm yielded slightly lower SNR improvements for similar multi-interferer speech signals.     

Monaural speech segregation using synthetic speech signals

When listening to natural speech, listeners are fairly adept at using cues such as pitch, vocal tract length, prosody, and level differences to extract a target speech signal from an interfering speech masker. However, little is known about the cues that listeners might use to segregate synthetic speech signals that retain the intelligibility characteristics of speech but lack many of the features that listeners normally use to segregate competing talkers. In this experiment, intelligibility was measured in a diotic listening task that required the segregation of two simultaneously presented synthetic sentences. Three types of synthetic signals were created: (1) sine-wave speech (SWS); (2) modulated noise-band speech (MNB); and (3) modulated sine-band speech (MSB). The listeners performed worse for all three types of synthetic signals than they did with natural speech signals, particularly at low signal-to-noise ratio (SNR) values. Of the three synthetic signals, the results indicate that SWS signals preserve more of the voice characteristics used for speech segregation than MNB and MSB signals. These findings have implications for cochlear implant users, who rely on signals very similar to MNB speech and thus are likely to have difficulty understanding speech in cocktail-party listening environments.     

Influence of confinement on the fragility of antiplasticized and pure polymer films

We investigate by molecular dynamics simulation how thin film confinement modifies the fragility of a model glass-forming liquid characterized previously in the bulk. Film confinement is found to reduce the relative fragility of the polymer fluid, leading to effects similar to simulations of the addition of an antiplasticizer additive. A reduction of fragility is not observed for the antiplasticized polymer film. These effects are interpreted in terms of variation in the string (cooperatively moving segments) concentration with film confinement and the addition of antiplasticizing additives.     

Multimodal imaging of the sonic organ of Porichthys notatus, the singing midshipman fish

The sonic midshipman fish, Porichthys notatus, is a bottom-dwelling species whose swim bladder has evolved into a highly specialized, sound-producing organ. The males of this species exist in two distinct morphs with different physical characteristics and sexual strategies. The Type I males have a much larger sound organ and are capable of generating a loud approximately 100 Hz tone continuously for over an hour to attract females. This sound is produced by sonic muscle and represents one of the most superfast and super-enduring striated muscles found in nature. Each fiber contains a hollow, tubular contractile apparatus composed of radially arranged myofibrils with extremely broad Z-bands that are supported by a desmin-rich cytoskeleton. We have used micro computed tomography (CT) imaging and magnetic resonance (MR) imaging to visualize the location of the sonic organ in an intact male fish. We have also obtained high-resolution MR images of the excised swim bladders from both male types. The images of the Type I sonic organ are strikingly detailed and high-contrast, revealing both the internal organization of the bladder and the crisscrossing muscle fibers and their mode of attachment to the underlying bladder. The high-contrast variation in these images is due to different T(2) values for fiber bundles and the spaces between the bundles. Direct MR imaging of intact Type I sonic organ in Type I midshipman fish is a powerful approach to understanding the contraction of this superfast muscle and the oscillation of its bladder to produce mating calls, and how placement of the sonic organ in the body of the fish sheds light on its prodigious ability to produce and transmit its loud mating call.     

The Maximum Flow Network Interdiction Problem: Valid inequalities, integrality gaps, and approximability

We present two classes of polynomially separable valid inequalities for the Maximum Flow Network Interdiction Problem. We prove that the integrality gap of the standard integer program is not bounded by a constant, even when strengthened by our valid inequalities. Finally, we provide an approximation-factor-preserving reduction from a simpler interdiction problem.     

Online sum-paintability: The slow-coloring game

The slow-coloring game is played by Lister and Painter on a graph $G$. On each round, Lister marks a nonempty subset $M$ of the uncolored vertices, scoring $\left|M\right|$ points. Painter then gives a color to a subset of $M$ that is independent in $G$. The game ends when all vertices are colored. Painter and Lister want to minimize and maximize the total score, respectively. The best score that each player can guarantee is the sum-color cost of $G$, written $\stackrel{?}{s}\left(G\right)$. The game is an online variant of online sum list coloring.We prove $\frac{\left|V\left(G\right)\right|}{2\alpha \left(G\right)}+\frac{1}{2}\le \frac{\stackrel{?}{s}\left(G\right)}{\left|V\left(G\right)\right|}\le max\left\{\frac{\left|V\left(H\right)\right|}{\alpha \left(H\right)}:H?G\right\}$, where $\alpha \left(G\right)$ is the independence number, and we study when equality holds in the bounds. We compute $\stackrel{?}{s}\left(G\right)$ for graphs with $\alpha \left(G\right)=2$. Among $n$-vertex trees, we prove that $\stackrel{?}{s}$ is minimized by the star and maximized by the path. We also study $\stackrel{?}{s}\left(K_{r,s}\right)$.