Dynamics of Steps on Vicinal Surfaces

This theoretic work deals with the dynamics of steps on vicinal surfaces, where the bunching instability on the vicinal Si(111) is the physically relevant phenomenology. Thereby two models are studied in detail. The first one is the standard Burton- Cabrera-Frank model from 1951 with the quasi-static approximation for the adatom concentrations on the terraces, considered for non-permeable steps and in the limiting case of fast attachment/detachment kinetics and slow diffusion. In our derivation of the discrete equations we take into account higher order non-linear terms, neglected in the previous studies. We found, that those terms are present in the case of sublimation, but not in the case of growth. Analytical and numerical methods are employed in order to study the impact of these terms on the step dynamics. For both asymmetry effects, the Ehrlich-Schwoebel effect and the effect of electromigration, there is a change in the dispersion relation obtained from the linear stability analysis, whereas there is no such change in the case of growth. Due to the non-linear terms, the dynamics changes from conservative to non-conservative with respect to the crystal volume. The continuum limits of the discrete equations for both asymmetry cases yield a hint of slope selection in the so called mechanical analog of the partial differential equation. As a consequence, the scaling relations of the bunching geometry in the case of sublimation differ strongly from those in the case of growth. The numerical simulations of the discrete equations confirm these analytic results. In the non-linear regime there is anti-coarsening or arrested coarsening of the step bunches and thus there are stationary solutions with bounded maximal slope. A sensitive dependence on the initial conditions is observed. The second model we analyze was recently introduced by Ranguelov and Stoyanov. It accounts for the case of strong transparency, fast diffusion and slow attachment/detachment kinetics. This model goes beyond the approximation of quasi-static concentration profiles of adatoms. Calculations in order to reproduce Ranguelov and Stoyanov’s results for the gradient of the adatom concentration, depending on the electromigration force as well as for the linear stability analysis were carried out. Quantitative deviations were found and the corrections are presented. Finally, the equations are simulated and the dependence of the maximal slope on the different input parameters in the bunching regime is illustrated

Anti-Coarsening and Complex Dynamics of Step Bunches on Vicinal Surfaces during Sublimation

A sublimating vicinal crystal surface can undergo a step bunching instability when the attachment-detachment kinetics is asymmetric, in the sense of a normal Ehrlich-Schwoebel effect. Here we investigate this instability in a model that takes into account the subtle interplay between sublimation and step-step interactions, which breaks the volume-conserving character of the dynamics assumed in previous work. On the basis of a systematically derived continuum equation for the surface profile, we argue that the non-conservative terms pose a limitation on the size of emerging step bunches. This conclusion is supported by extensive simulations of the discrete step dynamics, which show breakup of large bunches into smaller ones as well as arrested coarsening and periodic oscillations between states with different numbers of bunches.Comment: 26 pages, 11 figure

Probing colored glass via qqˉq\bar{q} photoproduction

In this paper, we calculate the cross-section for the photoproduction of quark-antiquark pairs in the peripheral collision of ultra-relativistic nuclei, by treating the color field of the nuclei within the Color Glass Condensate model. We find that this cross-section is sensitive to the saturation scale $Q_s^2$ that characterizes the model. In particular, the transverse momentum spectrum of the produced pairs could be used to measure the properties of the color glass condensate.Comment: 25 pages LaTeX document - A reference and some footnotes added (version to appear in Nucl. Phys. A

The dipole representation of vector meson electroproduction beyond leading twist

We link the recent computation beyond leading twist of the impact factor of the transition gamma*T -> rhoT performed in the light-cone collinear approach, to the dipole picture by expressing the hard part of the process through its Fourier transform in coordinate space. We show that in the Wandzura-Wilczek approximation the impact factor up to twist 3 factorises in the wave function of the photon combined with the distribution amplitudes of the rho-meson and the colour dipole scattering amplitude with the t-channel gluons. We show also that beyond the Wandzura-Wilczek approximation, the hard contribution of the amplitude still exhibits the signature of the interaction of a single colour dipole with the t-channel gluons. This result allows a phenomenological approach of the helicity amplitudes of the leptoproduction of vector meson, by combining our results to a dipole/target scattering amplitude model.Comment: 63 pages, 18 figure

High energy pA collisions in the color glass condensate approach I. Gluon production and the Cronin effect

We study gluon production in high energy proton-nucleus collisions in the semi-classical framework of the Color Glass Condensate. We develop a general formalism to compute gluon fields in covariant gauge to lowest order in the classical field of the proton and to all orders in the classical field of the nucleus. The use of the covariant gauge makes the diagrammatic interpretation of the solution more transparnt. k_t-factorization holds to this order for gluon production -- Our results for the gluon distribution are equivalent to the prior diagrammatic analysis of Kovchegov and Mueller. We also show that these results are equivalent to the computation of gluon production by Dumitru and McLerran in the Fock-Schwinger gauge. We demonstrate how the Cronin effect arises in this approach, and examine its behavior in the two extreme limits of a) no small-x quantum evolution, and b) fully saturated quantum evolution. In both cases, the formalism reduces to Glauber's formalism of multiple scatterings. We comment on the possible implications of this study for the interpretation of the recent results on Deuteron-Gold collisions at the Relativistic Heavy Ion Collider (RHIC).Comment: 46 pages, 12 postscript figure

Topology of Vibro-Impact Systems in the Neighborhood of Grazing

The grazing bifurcation is considered for the Newtonian model of vibro-impact systems. A brief review on the conditions, sufficient for existence of a grazing family of periodic solutions, is given. The properties of these periodic solutions are discussed. A plenty of results on the topological structure of attractors of vibro-impact systems is known. However, since the considered system is strongly nonlinear, these attractors may be invisible or, at least, very sensitive to changes of parameters of the system. On the other hand, they are observed in experiments and numerical simulations. We offer (Theorem 2) an approach which allows to explain this contradiction and give a new robust mathematical model of the non-hyperbolic dynamics in the neighborhood of grazing.Comment: Submitted to Physica

Panoptix multifocal crystal implant – the ideal treatment to get rid of your cataract and glasses

Summary. Trifocal lenses are the latest artificial lenses available today for cataract surgery. The ACRYSOF® IQ PANOPTIX® Artificial Trifocal Lens is specially designed to see clearly after cataract surgery based on experience with over 100 million lenses implanted worldwide. A study found that 94.8% of patients who chose these lenses did not wear glasses for distance vision. Compared to other trifocal lenses, it allows a clear view at a more appropriate and natural intermediate distance, 60 centimeters. It`s a good alternative for correcting visual acuity in cataract and refractive surgery. Requires rigorous and precise surgical technique as it treats both ametropia and presbyopia.Cristalinele trifocale sunt cele mai recente cristaline artificiale disponibile astăzi pentru operațiile de cataractă. Cu ele puteți vedea clar atât de aproape, cât și la distanțe intermediare și la depărtare. Sunt proiectate astfel încât pacienții care duc o viață activă să nu mai aibă nevoie de ochelari.6-13 Cristalinul artificial trifocal ACRYSOF® IQ PANOPTIX ® este conceput special ca să puteți vedea clar după operația de cataractă6,8-13 pe baza experienței cu peste 100 de milioane de cristaline AcrySof® IQ PanOptix® implantate la nivel mondial.1-5 Avantajele acestui cristalin artificial sunt următoarele: Un studiu a arătat că 94.8% dintre pacienții care au ales AcrySof® IQ PanOptix® nu au mai folosit ochelari pentru vederea la distanță.8 Datorită stabilității sale dovedite pe termen lung, nevoia de ajustare după implantarea acestui cristalin artificial este redusă.3 Comparativ cu alte cristaline trifocale, AcrySof® IQ PanOptix® permite o vedere clară la o distanță intermediară mai potrivită și mai naturală, adică 60 de centimetri, care este lungimea brațului unei persoane de înălțime medie.10,13,16 AcrySof® IQ PanOptix™ este o bună alternativă de corectare a acuității vizuale în chirurgia cataractei și chirurgia refractivă. Asigură rezultate predictibile (o remarcă deosebită pentru pacienții cu CLE), vedere îmbunătățită la toate distanțele, vedere foarte bună la distanță intermediară. Necesită tehnică chirurgicală riguroasă și precisă, întrucât tratează atât ametropia cât si presbiopia

Quarkonia Photoproduction at Nucleus Colliders

Exclusive photoproduction of heavy quarkonia in high-energy ultraperipheral ion-ion interactions (gamma A --> V A, where V = J/Psi,Upsilon and the nucleus A remains intact) offers a useful means to constrain the small-x nuclear gluon density. We discuss preliminary results on J/Psi photoproduction in Au-Au collisions at RHIC energies, as well as full simulation-reconstruction studies of photo-produced Upsilon's in Pb-Pb interactions at the LHC.Comment: 4 pages, invited talk PHOTON'07 (Paris), Proceeds. to appear in Nucl. Phys.

Towards a symmetric approach to high energy evolution: generating functional with Pomeron loops

We derive an evolution equation for the generating functional which accounts for processes for both gluon emission and recombination. In terms of color dipoles, the kernel of this equation describes evolution as a classical branching process with conserved probabilities. The introduction of dipole recombination allows one to obtained closed loops during the evolution, which should be interpreted as Pomeron loops of the BFKL Pomerons. In comparison with the emission, the dipole recombination is formally $1/N_c^2$ suppressed. This suppression, nevertheless, is compensated at very high energies when the scattering amplitude tends to its unitarity bound.Comment: 27 pages and 5 figures. Significantly revised version. To appear in Nuclear Physics

Dipole model for double meson production in two-photon interactions at high energies

In this work the double vector meson production in two-photon interactions at high energies is investigated considering saturation physics. We extend the color dipole picture for this process and study the energy and virtuality dependence of the forward differential cross section. Comparison with previous results is presented and the contribution of the different photon polarizations is estimated.Comment: 20 pages, 8 figures, 1 table. Version to be published in European Physical Jornal