Zh. S. Gevorkian

Unzipping of DNA with correlated base sequence

We consider force-induced unzipping transition for a heterogeneous DNA model with a correlated base sequence. Both finite-range and long-range correlated situations are considered. It is shown that finite-range correlations increase stability of DNA with respect to the external unzipping force. Due to long-range correlations the number of unzipped base pairs displays two widely different scenarios depending on the details of the base sequence: either there is no unzipping phase transition at all, or the transition is realized via a sequence of jumps with magnitude comparable to the size of the system. Both scenarios are different from the behavior of the average number of unzipped base pairs (non-self-averaging). The results can be relevant for explaining the biological purpose of correlated structures in DNA.

How adsorption influences DNA denaturation

The thermally induced denaturation of DNA in the presence of an attractive solid surface is studied. The two strands of DNA are modeled via two coupled flexible chains without volume interactions. If the two strands are adsorbed on the surface, the denaturation phase transition disappears. Instead, there is a smooth crossover to a weakly naturated state. Our second conclusion is that even when the interstrand attraction alone is too weak for creating a naturated state at the given temperature and also when the surface-strand attraction alone is too weak for creating an adsorbed state, the combined effect of the two attractions can lead to a naturated and adsorbed state.

Adhesion-Induced DNA Naturation

DNA adsorption and naturation is modeled via two interacting flexible homopolymers coupled to a solid surface. DNA denatures if the entropy gain for unbinding the two strands overcomes the loss of binding energy. When adsorbed to a surface, the entropy gain is smaller than in the bulk, leading to a stronger binding and, upon neglecting self-avoidance, absence of a denatured phase. Now consider conditions where the binding potentials are too weak for naturation, and the surface potential too weak to adsorb single strands. In a variational approach it is shown that their combined action may lead to a naturated adsorbed phase. Conditions for the absence of naturation and adsorption are derived too. The phase diagram is constructed qualitatively.

Smith-Purcell Radiation from Rough Surfaces

gevork@yerphi.am(Dated: October 12, 2010)AbstractRadiation of a charged particle moving parallel to a inhomogeneous surface is considered. Withina single formalism periodic and random gratings are examined. For the periodically inhomogeneoussurface we derive new expressions for the dispersion relation and the spectral-angular intensity. Inparticular, for a given observation direction two wavelengths are emitted instead of one wavelengthof the standard Smith-Purcell effect. For a rough surface we show that the main contribution tothe radiation intensity is given by surface polaritons induced on the interface between two media.These polaritons are multiply scattered on the roughness of surface and convert into real photons.The spectral-angular intensity is calculated and its dependence on different parameters is revealed.

Observation of Resonant Diffusive Radiation in Random Multilayered Systems

Diffusive Radiation is a new type of radiation predicted to occur in randomly inhomogeneous media due to the multiple scattering of pseudophotons. This theoretical effect is now observed experimentally. The radiation is generated by the passage of electrons of energy 200KeV-2.2MeV through a random stack of films in the visible light region. The radiation intensity increases resonantly provided the Cherenkov condition is satisfied for the average dielectric constant of the medium. The observed angular dependence and electron resonance energy are in agreement with the theoretical predictions. These observations open a road to application of diffusive radiation in particle detection, astrophysics, soft X-ray generation and etc.. `

Radiation of a relativistic charged particle in a system of randomly spaced plates

Abstract Radiation of a charged particle moving in a system of randomly spaced plates is considered. It is shown that the diffusion mechanism of radiation may play an important role. The total intensity of radiation is investigated and its quadratic dependence on particle energy is revealed in the optical region. A comparison with the Cherenkov radiation is carried out.

Spectrum of radiation from rough surfaces

Radiation from a charged particle travelling parallel to a rough surface has been considered. Spectral-angular intensity is calculated in the weak scattering regime. It is shown that the main contribution to the radiation intensity is determined by the multiple scattering of polaritons induced by a charge on the surface. Multiple scattering effects lead to strong frequency dependence of radiation intensity. Possible applications in beam and surface diagnostics are discussed. Introduction. – Light scattering from rough surfaces attracted much interest [1]. In particular this interest is caused by the determination of microtopographic properties of rough metallic surfaces from the light scattering measurements [2]. The enhancement of intensity of scattered light on a rough surface is due to a resonant excitation of surface polaritons induced by incident light. Surface polaritons are multiply scattered on the roughness resulting in their diffusion and localisation [3] that lead to peculiarities in the light scattering from rough surfaces. Polaritons can be induced not only by an incident light but also by a charged particle. It is interesting to reveal the manifestation of polaritons multiple scattering on the charged particle radiation from rough surfaces. Origination of this radiation is due to the scattering of polaritons induced by the charged particle on the inhomogeneites of dielectric constant associated with the roughness of the surface. Earlier in this geometry main attention was paid to the periodical grating case when Smith-Purcell radiation(SPR) [4] is originated. Recently we have considered [5] the radiation from an uncorrelated rough surface. In the present paper we consider the influence of multiple scattering effects, including the localisation of polaritons, on the radiation of a charged particle travelling over a correlated rough surface.We will see that they lead to strong frequency dependence of intensity. Strong frequency dependence allows to separate the diffusional mechanism of radiation from other radiation mechanisms. Formulation of the Problem. – A charged particle moves uniformly in the vacuum at the distance d from the plane z = 0 separating vacuum and the isotropic medium distorted by roughness. We are interested in radiation field far away from the charge and interface. Maxwell equation for the electric field has the form ∇ ~ E(~r, ω)− graddivẼ(r̃, ω) + ω 2 c2 ε(r̃, ω)Ẽ(r̃, ω) = j̃(r̃, ω) (1) where ~j(~r, ω) = − 4πieω~v vc2 δ(z − d)δ(y)e is the current density associated with the charge. Here ~v is the velocity of the particle moving on 0x direction and ε(~r, ω) is the inhomogeneous dielectric permittivity of the system. For a rough surface it can be chosen as ε(~r, ω) = ε0(z, ω) + εr(~r, ω), where ε0(z, ω) = Θ(z) + Θ(−z)ε(ω) descibes the flat interface vacuum-metal and εr(~r, ω) = [ε(ω)−1]δ(z)h(x, y) is the contribution of small roughness. h(x, y) is the amplitude of surface roughness. To separate the radiation field one should decompose the electric field as ~ E = ~ E0+ ~ Er, analogous to the decomposition of dielectric constant. ~ E0 and ~ Er are the background field created by the charge and the radiation field,respectively. They obey the following equations ∇ ~ E0(~r, ω)− graddivẼ0(r̃, ω) + + ω c2 ε0(z, ω) ~ E0(~r, ω) = ~j(~r, ω) (2) ∇ ~ Er(~r, ω)− graddivẼr(r̃, ω) + ω c2 ε(z, ω)Ẽr(r̃, ω) + + ω c2 εr(~r, ω) ~ Er(~r, ω) = − ω c2 εr(~r, ω) ~ E0(~r, ω) (3) Radiation intensity at the frequencies ω, ω + dω and at the angles Ω, Ω + dΩ is determined as dI(ω, ~n) =Radiation from a charged particle travelling parallel to a rough surface has been considered. The spectral-angular intensity is calculated in the weak scattering regime. It is shown that the main contribution to the radiation intensity is determined by the multiple scattering of polaritons induced by a charge on the surface. Multiple scattering effects lead to a strong frequency dependence of the radiation intensity. Possible applications in beam and surface diagnostics are discussed.

LIGHT BACKSCATTERING IN A TWO-DIMENSIONAL RANDOM SYSTEMS

Light backscattering in random quasi-two-dimensional media is considered. Surface polariton backscattering on semi-infinite rough surfaces or light scattering in thin films with mirror surfaces are examples of the systems considered. The mean intensity of backscattered light is obtained, inelastic dephasing processes being taken into account. Angular correlations and fluctuations of backscattered intensity are analyzed.

Diffusive radiation in the infrared region

Diffusive radiation generated when a charged particle crosses a disordered stack of plates in the infrared region is considered. The main mechanism causing radiation is multiple scattering of electromagnetic field that is more effective in a medium with near to zero average dielectric permittivity. To obtain such a system we suggest using a stack of plates with a negative dielectric constant that, with a positive vacuum value, makes the average dielectric constant near to zero. Numerical estimates for realistic systems are presented.

Faraday rotation in a disordered medium

The Faraday rotation angle Θ is calculated in a diffusive regime in a three dimensional disordered slab. It is shown that tanΘ is (i) an oscillating function of the magnetic field or the medium’s internal properties, and (ii) proportional to the ratio of the inelastic mean free path lin to the mean free path l, that is to the average number of photon scatterings. The maximum rotation is achieved at frequencies when the photon’s elastic mean free path is minimal. We have obtained the rotation angle of polar backscattered light taking into account the maximally crossed diagrams. The latter leads to an ellipticity in the backscattered wave that can serve as precursor of weak localization. The critical strength of magnetic field Bc ∼ gc ∼ λ/l beyond which rotation in backscattered wave disappears. The traversal time of an electromagnetic wave through the slab is estimated in a diffusive regime. The disorder enhanced the traversal time by an additional factor lin/l, in comparison with a free light propagation time. Comparison with the experimental data is carried out. PACS numbers: 41.20Jb,71.55Jv,71.45Gm