Biodiversity, extinctions and evolution of ecosystems with shared resources

We investigate the formation of stable ecological networks where many species share the same resource. We show that such stable ecosystem naturally occurs as a result of extinctions. We obtain an analytical relation for the number of coexisting species and find a relation describing how many species that may go extinct as a result of a sharp environmental change. We introduce a special parameter that is a combination of species traits and resource characteristics used in the model formulation. This parameter describes the pressure on system to converge, by extinctions. When that stress parameter is large we obtain that the species traits concentrate at some values. This stress parameter is thereby a parameter that determines the level of final biodiversity of the system. Moreover, we show that dynamics of this limit system can be described by simple differential equations

Generation and development of small-amplitude disturbances in a laminar boundary layer in the presence of an acoustic field

A low-turbulence subsonic wind tunnel was used to study the influence of acoustic disturbances on the development of small sinusoidal oscillations (Tollmien-Schlichting waves) which constitute the initial phase of turbulent transition. It is found that acoustic waves propagating opposite to the flow generate vibrations of the model (plate) in the flow. Neither the plate vibrations nor the acoustic field itself have any appreciable influence on the stability of the laminar boundary layer. The influence of an acoustic field on laminar boundary layer disturbances is limited to the generation of Tollmien-Schlichting waves at the leading-edge of the plate

Trapped modes in zigzag graphene nanoribbons

We study a scattering on an ultra-low potential in zigzag graphene nanoribbon. Using mathematical framework based on the continuous Dirac model and augumented scattering matrix, we derive a condition for the existence of a trapped mode. We consider the threshold energies where the continuous spectrum changes its multiplicity and show that the trapped modes may appear for energies slightly less than a threshold and its multiplicity does not exceeds one. We prove that trapped modes do not appear outside the threshold, provided the potential is sufficiently small

Bragg Reflection Waveguide: Anti-Mirror Reflection and Light Slowdown

The effect of the light group velocity reduction in dielectric Bragg reflection waveguide structures (SiO$_2$/TiO$_2$) in the vicinity of the cutoff frequency is studied experimentally. The effect of anti-mirror reflection, specific for the Bragg reflection waveguides, is described and employed for detection of "slow light". The experiments were performed with the use of the Ti:sapphire laser pulses ~ 100 fs in length. The group index $n_g \sim$ 30 with a fractional pulse delay (normalized to the pulse width) of $\sim$ 10 is demonstrated. The problems and prospects of implementation of the slow-light devices based on the Bragg reflection waveguide structures are discussed.Comment: 11 pages, in the previous version, we failed to insert figure

Electric dipole moment of the electron in YbF molecule

Ab initio calculation of the hyperfine, P-odd, and P,T-odd constants for the YbF molecule was performed with the help of the recently developed technique, which allows to take into account correlations and polarization in the outercore region. The ground state electronic wave function of the YbF molecule is found with the help of the Relativistic Effective Core Potential method followed by the restoration of molecular four-component spinors in the core region of ytterbium in the framework of a non-variational procedure. Core polarization effects are included with the help of the atomic Many Body Perturbation Theory for Yb atom. For the isotropic hyperfine constant A, accuracy of our calculation is about 3% as compared to the experimental datum. The dipole constant Ad (which is much smaller in magnitude), though better than in all previous calculations, is still underestimated by almost 23%. Being corrected within a semiempirical approach for a perturbation of 4f-shell in the core of Yb due to the bond making, this error is reduced to 8%. Our value for the effective electric field on the unpaired electron is 4.9 a.u.=2.5E+10 V/cm.Comment: 7 pages, REVTE