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Sumy, Ukraine

The Sumy State University is located in Sumy Oblast, in the historical region of Sloboda Ukraine. According to the Webometrics Rating, Sumy State University ranks 6th in worldwide Internet rating among Ukrainian institutes of higher education as of July 2012. Wikipedia.

Denisov S.,Sumy State University | Denisov S.,University of Augsburg | Denisov S.,Nizhny Novgorod State Technical University | Flach S.,Massey University | And 2 more authors.
Physics Reports | Year: 2014

Transport properties of particles and waves in spatially periodic structures that are driven by external time-dependent forces manifestly depend on the space-time symmetries of the corresponding equations of motion. A systematic analysis of these symmetries uncovers the conditions necessary for obtaining directed transport. In this work we give a unified introduction into the symmetry analysis and demonstrate its action on the motion in one-dimensional periodic, both in time and space, potentials. We further generalize the analysis to quasi-periodic drives, higher space dimensions, and quantum dynamics. Recent experimental results on the transport of cold and ultracold atomic ensembles in ac-driven optical potentials are reviewed as illustrations of theoretical considerations. © 2014 Elsevier B.V. Source

Pogrebnjak A.D.,Sumy State University
Journal of Nanomaterials | Year: 2013

Nanostructured coatings of (Ti-Zr-Hf-V-Nb)N were obtained by the Cathodic Arc Vapor Deposition (CAVD) method. To investigate these coatings, a number of complementary methods of elemental and structural analysis were used, namely, slow positron beam (SPB), proton microbeam (μ-PIXE), micro- and nanoelectron beam (EDS and SEM analyses), and X-ray diffraction method (XRD), including method of " α- sin 2 φ " measurement of a stress-strain state (X-ray strain measurement). Additionally, the texture of coatings before and after annealing up to 873 K (for time of annealing τ = 30 min) was also investigated. It was shown that increasing of stress-strain state in the coating during deposition increases the resistance to oxidation at high temperatures of annealing. It was also found that the redistribution of elements and defects, as well as their alignment (segregation), appeared due to thermally stimulated diffusion. It is also connected with the process of spinodal segregation near grain boundaries and interfaces around the grains and subgrains. © 2013 Alexander D. Pogrebnjak. Source

Zaburdaev V.,Max Planck Institute for the Physics of Complex Systems | Denisov S.,University of Augsburg | Denisov S.,Sumy State University | Klafter J.,Tel Aviv University
Reviews of Modern Physics | Year: 2015

Random walk is a fundamental concept with applications ranging from quantum physics to econometrics. Remarkably, one specific model of random walks appears to be ubiquitous across many fields as a tool to analyze transport phenomena in which the dispersal process is faster than dictated by Brownian diffusion. The Lévy-walk model combines two key features, the ability to generate anomalously fast diffusion and a finite velocity of a random walker. Recent results in optics, Hamiltonian chaos, cold atom dynamics, biophysics, and behavioral science demonstrate that this particular type of random walk provides significant insight into complex transport phenomena. This review gives a self-consistent introduction to Lévy walks, surveys their existing applications, including latest advances, and outlines further perspectives. © 2015 American Physical Society. © 2015 American Physical Society. Source

The boundary friction regime appearing between two atomically smooth solid surfaces with an ultrathin lubricating layer between them is considered. The interrupted (stick-slip) regime of motion typical of the boundary lubrication is represented as a first-order phase transition between the structural states of the lubricant. The thermodynamic and shear melting is described. The universal dependence of the viscosity of high-molecular alkanes (lubricants) on the temperature and velocity gradient is taken into account. The dependence of the friction force on the lubricant temperature and the relative shear velocity of the interacting surfaces are analyzed. It is shown that the temperature dependence of the viscosity makes it possible to describe some experimentally observed effects. The possibility of prolonged damped oscillations after lubricant melting prior to the stabilization of the steady-state sliding mode is predicted. In the stick-slip regime in a wide range of parameters, a reversive motion is observed when the upper block moves in both directions after melting. © 2013 Pleiades Publishing, Ltd. Source

Lyashenko I.A.,Sumy State University
Technical Physics | Year: 2011

A mechanical analog of a tribological system in the boundary friction mode is studied. A thermodynamic model is used to analyze the first-order phase transition between liquidlike and solidlike structures of a lubricant. The time dependences of the friction force, the relative velocity of the interacting surfaces, and the elastic component of the shear stresses appearing in the lubricant are obtained. It is shown that, in the liquidlike state, the shear modulus of the lubricant and the elastic stresses become zero. The intermittent (stick-slip) friction mode detected experimentally is described. It is shown that, as the lubricant temperature increases, the frequency of phase transitions between the lubricant structural states decreases and the total friction force and elastic stress amplitudes lower. When the temperature or the elastic strain exceeds the corresponding critical value, the lubricant melts and a kinetic slip mode in which the elastic component of the friction force is zero takes place. © 2011 Pleiades Publishing, Ltd. Source

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