Radio-wave propagation in the non-Gaussian interstellar medium

Radio waves propagating from distant pulsars in the interstellar medium (ISM), are refracted by electron density inhomogeneities, so that the intensity of observed pulses fluctuates with time. The theory relating the observed pulse time-shapes to the electron-density correlation function has developed for 30 years, however, two puzzles have remained. First, observational scaling of pulse broadening with the pulsar distance is anomalously strong; it is consistent with the standard model only when non-uniform statistics of electron fluctuations along the line of sight are assumed. Second, the observed pulse shapes are consistent with the standard model only when the scattering material is concentrated in a narrow slab between the pulsar and the Earth. We propose that both paradoxes are resolved at once if one assumes stationary and uniform, but non-Gaussian statistics of the electron-density distribution. Such statistics must be of Levy type, and the propagating ray should exhibit a Levy flight. We propose that a natural realization of such statistics may be provided by the interstellar medium with random electron-density discontinuities. We develop a theory of wave propagation in such a non-Gaussian random medium, and demonstrate its good agreement with observations. The qualitative introduction of the approach and the resolution of the anomalous-scaling paradox was presented earlier in [PRL 91, 131101 (2003); ApJ 584, 791 (2003)].Comment: 27 pages, changes to match published versio

Non-Gaussian Radio-Wave Scattering in the Interstellar Medium

It was recently suggested by Boldyrev & Gwinn that the characteristics of radio scintillations from distant pulsars are best understood if the interstellar electron-density fluctuations that cause the time broadening of the radio pulses obey non-Gaussian statistics. In this picture the density fluctuations are inferred to be strong on very small scales ($\sim 10^8-10^{10} {cm}$). We argue that such density structures could correspond to the ionized boundaries of molecular regions (clouds) and demonstrate that the power-law distribution of scattering angles that is required to match the observations arises naturally from the expected intersections of our line of sight with randomly distributed, thin, approximately spherical ionized shells of this type. We show that the observed change in the time-broadening behavior for pulsar dispersion measures $\lesssim 30 {\rm pc} {\rm cm}^{-3}$ is consistent with the expected effect of the general ISM turbulence, which should dominate the scattering for nearby pulsars. We also point out that if the clouds are ionized by nearby stars, then their boundaries may become turbulent on account of an ionization front instability. This turbulence could be an alternative cause of the inferred density structures. An additional effect that might contribute to the strength of the small-scale fluctuations in this case is the expected flattening of the turbulent density spectrum when the eddy sizes approach the proton gyroscale.Comment: 15 pages, 3 figures, accepted to Ap

Multiple Local σ-aromaticity of Nonagermanide Clusters

Nonagermanide clusters are widely used in inorganic synthesis and are actively studied by experimentalists and theoreticians. However, chemical bonding of such versatile species is still not completely understood. In our work we deciphered a bonding pattern for various experimentally obtained nonagermanide species. We localized the electron density via the AdNDP algorithm for the model structures ([Ge9]4−, [Ge9{P(NH2)2}3]−, Cu[Ge9{P(NH2)2}3] and Cu(NHC)[Ge9{P(NH2)2}3]) and obtained a simple and chemically intuitive bonding pattern which can explain the variety of active sites and the existence of both D3h and C4v geometries for such clusters. Moreover, the [Ge9]4− core is found to be a unique example of an inorganic Zintl cluster with multiple local σ-aromaticity

Analog of Astrophysical Magnetorotational Instability in a Couette-Taylor Flow of Polymer Fluids

We report experimental observation of an instability in a Couette-Taylor flow of a polymer fluid in a thin gap between two coaxially rotating cylinders in a regime where their angular velocity decreases with the radius while the specific angular momentum increases with the radius. In the considered regime, neither the inertial Rayleigh instability nor the purely elastic instability are possible. We propose that the observed "elasto-rotational" instability is an analog of the magnetorotational instability which plays a fundamental role in astrophysical Keplerian accretion disks.Comment: 4 pages, 1 figur

On the ground electronic states of copper silicide and its ions

The low-lying electronic states of SiCu, SiCu^+, and SiCu^− have been studied using a variety of high-level ab initio techniques. As expected on the basis of simple orbital occupancy and bond forming for Si(s^2p^2)+Cu(s^1) species, ^2Π_r, ^1Σ^+, and ^3Σ^− states were found to be the ground electronic states for SiCu, SiCu^+, and SiCu^−, respectively; the ^2Π_r state is not that suggested in most recent experimental studies. All of these molecules were found to be quite strongly bound although the bond lengths, bond energies, and harmonic frequencies vary slightly among them, as a result of the nonbonding character of the 2π-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the ^1Σ^+, ^2Π_r, and ^3Σ^− series. The neutral SiCu is found to have bound excited electronic states of ^4Σ^−, ^2Δ, ^2Σ^+, and ^2Π_i symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the ^2Π_r ground state. It is possible but not yet certain that the ^2Π_i state is, in fact, the “B state” observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally

Integer Quantum Hall Transition and Random SU(N) Rotation

We reduce the problem of integer quantum Hall transition to a random rotation of an N-dimensional vector by an su(N) algebra, where only N specially selected generators of the algebra are nonzero. The group-theoretical structure revealed in this way allows us to obtain a new series of conservation laws for the equation describing the electron density evolution in the lowest Landau level. The resulting formalism is particularly well suited to numerical simulations, allowing us to obtain the critical exponent \nu numerically in a very simple way. We also suggest that if the number of nonzero generators is much less than N, the same model, in a certain intermediate time interval, describes percolating properties of a random incompressible steady two-dimensional flow. In other words, quantum Hall transition in a very smooth random potential inherits certain properties of percolation.Comment: 4 pages, 1 figur

Aromatic Character of [Au13]5+ and [MAu12]4+/6+ (M = Pd, Pt) Cores in Ligand Protected Gold Nanoclusters – Interplay between Spherical and Planar σ-Aromatics

The most characteristic feature of planar π-aromatics is the ability to sustain a long-range shielding cone under a magnetic field oriented in a specific direction. In this article, we showed that similar magnetic responses can be found in σ-aromatic and spherical aromatic systems. For [Au13]5+, long-range characteristics of the induced magnetic field in the bare icosahedral core are revealed, which are also found in the ligand protected [Au25(SH)18]− model, proving its spherical aromatic properties, also supported by the AdNDP analysis. Such properties are given by the 8-ve of the structural core satisfying the Hirsch 2(N + 1)2 rule, which is also found in the isoelectronic [M@Au12]4+ core, a part of the [MAu24(SR)18]2− (M = Pd, Pt) cluster. This contrasts with the [M@Au12]6+ core in [MAu24(SR)18]0 (M = Pd, Pt), representing 6-ve superatoms, which exhibit characteristics of planar σ-aromatics. Our results support the spherical aromatic character of stable superatoms, whereas the 6-ve intermediate electron counts satisfy the 4N + 2 rule (applicable for both π- and σ-aromatics), showing the reversable and controlled interplay between 3D spherical and 2D σ-aromatic clusters

Can Aromaticity be a Kinetic Trap? Example of Mechanically Interlocked Aromatic [2-5]catenanes Built from cyclo[18]carbon

The unusual stability of cyclo[18]carbon arising from its aromaticity might be used to provide the kinetic trapping needed in the design of interlocked systems. The kinetic barrier separating the interlocked rings and the chemically bonded complex is about 30 kcal mol−1. In addition, the rings can slide freely, which is a promising property for the design of molecular gears and motors