Modulational instability in nonlocal Kerr-type media with random parameters

Modulational instability of continuous waves in nonlocal focusing and defocusing Kerr media with stochastically varying diffraction (dispersion) and nonlinearity coefficients is studied both analytically and numerically. It is shown that nonlocality with the sign-definite Fourier images of the medium response functions suppresses considerably the growth rate peak and bandwidth of instability caused by stochasticity. Contrary, nonlocality can enhance modulational instability growth for a response function with negative-sign bands.Comment: 6 pages, 12 figures, revTeX, to appear in Phys. Rev.

The Australia Telescope campaign to study southern class I methanol masers

The Australia Telescope Compact Array (ATCA) and the Mopra facility have been used to search for new southern class I methanol masers at 9.9, 25 (J=5) and 104 GHz, which are thought to trace more energetic conditions in the interface regions of molecular outflows, than the widespread class I masers at 44 and 95 GHz. One source shows a clear outflow association.Comment: 2 pages, 1 figure (composed from 3 files), to appear in proceedings of IAU Symposium 242 "Astrophysical masers and their environment" (eds. J. Chapman and W. Baan

Modulational instability and nonlocality management in coupled NLS system

The modulational instability of two interacting waves in a nonlocal Kerr-type medium is considered analytically and numerically. For a generic choice of wave amplitudes, we give a complete description of stable/unstable regimes for zero group-velocity mismatch. It is shown that nonlocality suppresses considerably the growth rate and bandwidth of instability. For nonzero group-velocity mismatch we perform a geometrical analysis of a nonlocality management which can provide stability of waves otherwise unstable in a local medium.Comment: 15 pages, 12 figures, to be published in Physica Script

Marangoni instability in oblate droplets suspended on a circular frame

We study theoretically internal flows in a small oblate droplet suspended on the circular frame. Marangoni convection arises due to a vertical temperature gradient across the drop and is driven by the surface tension variations at the free drop interface. Using the analytical basis for the solutions of Stokes equation in coordinates of oblate spheroid we have derived the linearly independent stationary solutions for Marangoni convection in terms of Stokes stream functions. The numerical simulations of the thermocapillary motion in the drops are used to study the onset of the stationary regime. Both analytical and numerical calculations predict the axially-symmetric circulatory convection motion in the drop, the dynamics of which is determined by the magnitude of the temperature gradient across the drop. The analytical solutions for the critical temperature distribution and velocity fields are obtained for the large temperature gradients across the oblate drop. These solutions reveal the lateral separation of the critical and stationary motions within the drops. The critical vortices are localized near the central part of a drop, while the intensive stationary flow is located closer to its butt end. A crossover to the limit of the plane film is studied within the formalism of the stream functions by reducing the droplet ellipticity ratio to zero value. The initial stationary regime for the strongly oblate drops becomes unstable relative to the many-vortex perturbations in analogy with the plane fluid films with free boundaries

Circulating Marangoni flows within droplets in smectic films

We present theoretical study and numerical simulation of Marangoni convection within ellipsoidal isotropic droplets embedded in free standing smectic films (FSSF). The thermocapillary flows are analyzed for both isotropic droplets spontaneously formed in FSSF overheated above the bulk smectic-isotropic transition, and oil lenses deposited on the surface of the smectic film. The realistic model, for which the upper drop interface is free from the smectic layers, while at the lower drop surface the smectic layering still persists is considered in detail. For isotropic droplets and oil lenses this leads effectively to a sticking of fluid motion at the border with a smectic shell. The above mentioned asymmetric configuration is realized experimentally when the temperature of the upper side of the film is higher than at the lower one. The full set of stationary solutions for Stokes stream functions describing the Marangoni convection flows within the ellipsoidal drops were derived analytically. The temperature distribution in the ellipsoidal drop and the surrounding air was determined in the frames of the perturbation theory. As a result the analytical solutions for the stationary thermocapillary convection were derived for different droplet ellipticity ratios and the heat conductivity of the liquid crystal and air. In parallel, the numerical hydrodynamic calculations of the thermocapillary motion in the drops were performed. Both the analytical and numerical simulations predict the axially-symmetric circulatory convection motion determined by the Marangoni effect at the droplet free surface. Due to a curvature of the drop interface a temperature gradient along its free surface always persists. Thus, the thermocapillary convection within the ellipsoidal droplets in overheated FSSF is possible for the arbitrarily small Marangoni numbers

Class I methanol masers in the outflow of IRAS 16547-4247

The Australia Telescope Compact Array (ATCA) has been used to image class I methanol masers at 9.9, 25 (a series from J=2 to J=9), 84, 95 and 104 GHz located in the vicinity of IRAS 16547-4247 (G343.12-0.06), a luminous young stellar object known to harbour a radio jet. The detected maser emission consists of a cluster of 6 spots spread over an area of 30 arcsec. Five spots were detected in only the 84- and 95-GHz transitions (for two spots the 84-GHz detection is marginal), while the sixth spot shows activity in all 12 observed transitions. We report the first interferometric observations of the rare 9.9- and 104-GHz masers. It is shown that the spectra contain a very narrow spike (<0.03 km/s) and the brightness temperature in these two transitions exceeds 5.3x10^7 and 2.0x10^4 K, respectively. The three most southern maser spots show a clear association with the shocked gas traced by the H_2 2.12 micron emission associated with the radio jet and their velocities are close to that of the molecular core within which the jet is embedded. This fact supports the idea that the class I masers reside in the interface regions of outflows. Comparison with OH masers and infrared data reveals a potential discrepancy in the expected evolutionary state. The presence of the OH masers usually means that the source is evolved, but the infrared data suggest otherwise. The lack of any class II methanol maser emission at 6.7 GHz in the source raises an additional question, is this source too young or too old to have a 6.7 GHz maser? We argue that both cases are possible and suggest that the evolutionary stage where the class I masers are active, may last longer and start earlier than when the class II masers are active. However, it is currently not possible to reveal the exact evolutionary status of IRAS 16547-4247.Comment: 14 pages, 6 figures, 4 tables, accepted by MNRA

1-(4-Methyl­phen­yl)-1H-1,2,3,4-tetra­zole

In the title compound, C8H8N4, the dihedral angle between the tetra­zole and benzene rings is 21.6 (1)°. An inter­molecular C—H⋯π inter­action is observed

Desorption of n-alkanes from graphene: a van der Waals density functional study

A recent study of temperature programmed desorption (TPD) measurements of small n-alkanes (CNH2N+2) from C(0001) deposited on Pt(111) shows a linear relationship of the desorption energy with increasing n-alkane chain length. We here present a van der Waals density functional study of the desorption barrier energy of the ten smallest n-alkanes (N = 1 to 10) from graphene. We find linear scaling with N, including a nonzero intercept with the energy axis, i.e., an offset at the extrapolation to N = 0. This calculated offset is quantitatively similar to the results of the TPD measurements. From further calculations of the polyethylene polymer we offer a suggestion for the origin of the offset.Comment: 3 pictures, 1 tabl

Superconductivity and Kondo Effect of PdxBi2Se3 Whiskers at Low Temperatures

Temperature dependencies of Bi2Se3 whiskers’ resistance with Pd doping concentration of (1-2) × 1019 cm − 3 where measured in temperature range 1.5-77 K. At temperature 5.3 K a sharp drop in the whisker resistance was found. The effect observed is likely resulted from the whiskers partial transition in superconducting state at temperature 5.3 K, which is likely connected with β-PdBi2 inclusions in the whiskers. Transverse magnetoresistance in n-type Bi2Se3 whiskers with different doping concentration in the vicinity to the metal-insulator transition from metal side of the transition was studied in magnetic field 0-10 T. The magnetic field suppression of superconductivity allows to determine the main parameters: upper critical magnetic field Bc2 = 1.5 T, superconductor coherence length ξ(0) = 15 nm, superconductive gap ∆~ 0.8 meV. Besides on the temperature dependence of the whisker resistance and magnetoresistance a minimum was observed in the temperature range 20-25 K that is connected with appearance of Kondo effect

Beat-wave generation of plasmons in semiconductor plasmas

It is shown that in semiconductor plasmas, it is possible to generate large amplitude plasma waves by the beating of two laser beams with frequency difference close to the plasma frequency. For narrow gap semiconductors (for example n-type InSb), the system can simulate the physics underlying beat wave generation in relativistic gaseous plasmas.Comment: 11 pages, LaTex, no figures, no macro