Angular distribution of positrons in coherent pair production in deformed crystals

We investigate the angular distribution of positrons in the coherent process electronpositron pair creation process by high-energy photons in a periodically deformed single crystal with a complex base. The formula for the corresponding differential cross-section is derived for an arbitrary deformation field. The case is considered in detail when the photon enters into the crystal at small angles with respect to a crystallographic axis. The results of the numerical calculations are presented for${\mathrm{SiO}}_{2}$ and diamond single crystals and Moliere parameterization of the screened atomic potentials in the case of the deformation field generated by the acoustic wave of S-type.Comment: 11 pages, 4 figures, figures and references adde

Flux pinning properties of superconductors with an array of blind holes

We performed ac-susceptibility measurements to explore the vortex dynamics and the flux pinning properties of superconducting Pb films with an array of micro-holes (antidots) and non-fully perforated holes (blind holes). A lower ac-shielding together with a smaller extension of the linear regime for the lattice of blind holes indicates that these centers provide a weaker pinning potential than antidots. Moreover, we found that the maximum number of flux quanta trapped by a pinning site, i.e. the saturation number ns, is lower for the blind hole array.Comment: 6 figures, 6 page

Effective penetration length and interstitial vortex pinning in superconducting films with regular arrays of defects

In order to compare magnetic and non-magnetic pinning we have nanostructured two superconducting films with regular arrays of pinning centers: Cu (non-magnetic) dots in one case, and Py (magnetic) dots in the other. For low applied magnetic fields, when all the vortices are pinned in the artificial inclusions, magnetic dots prove to be better pinning centers, as has been generally accepted. Unexpectedly, when the magnetic field is increased and interstitial vortices appear, the results are very different: we show how the stray field generated by the magnetic dots can produce an effective reduction of the penetration length. This results in strong consequences in the transport properties, which, depending on the dot separation, can lead to an enhancement or worsening of the transport characteristics. Therefore, the election of the magnetic or non-magnetic character of the pinning sites for an effective reduction of dissipation will depend on the range of the applied magnetic field.Comment: 10 pages, 3 figure

Radiation from a charged particle-in-flight from a laminated medium to vacuum

The radiation from a charged particle-in-flight from a semi-infinite laminated medium to vacuum and back,- from vacuum to the laminated medium, has been investigated. Expressions for the spectral-angular distribution of radiation energy in vacuum (at large distances from the boundary of laminated medium) were obtained for both the cases with no limitations on the amplitude and variation profile of the laminated medium permittivity. The results of appropriate numerical calculations are presented and possible applications of the obtained results are discussed.Comment: 8 pages, 6 figures, contribution to Proceedings of International Symposium RREPS-2009, 07-11 September, 2009, Zvenigorod, Russi

Superconducting properties of Nb thin films deposited on porous silicon templates

Porous silicon, obtained by electrochemical etching, has been used as a substrate for the growth of nanoperforated Nb thin films. The films, deposited by UHV magnetron sputtering on the porous Si substrates, inherited their structure made of holes of 5 or 10 nm diameter and of 10 to 40 nm spacing, which provide an artificial pinning structure. The superconducting properties were investigated by transport measurements performed in the presence of magnetic field for different film thickness and substrates with different interpore spacing. Perpendicular upper critical fields measurements present peculiar features such as a change in the H_c2(T) curvature and oscillations in the field dependence of the superconducting resistive transition width at H=1 Tesla. This field value is much higher than typical matching fields in perforated superconductors, as a consequence of the small interpore distance.Comment: accepted for publication on Journal of Applied Physic

Coherent pair production in deformed crystals with a complex base

We investigate the coherent electron-positron pair creation by high-energy photons in a periodically deformed single crystal with a complex base. The formula for the corresponding differential cross-section is derived for an arbitrary deformation field. The conditions are specified under which the influence of the deformation is considerable. The case is considered in detail when the photon enters into the crystal at small angles with respect to a crystallographic axis. The results of the numerical calculations are presented for $\mathrm{SiO}_{2}$ single crystal and Moliere parametrization of the screened atomic potentials in the case of the deformation field generated by the acoustic wave of $S$ type. In dependence of the parameters, the presence of deformation can either enhance or reduce the pair creation cross-section. This can be used to control the parameters of the positron sources for storage rings and colliders.Comment: 10 pages, 4 figures, misprint in the numerical coefficients in figure captions is correcte

Langevin Dynamics of the vortex matter two-stage melting transition in Bi_2Sr_2CaCu_2O8+δ_{8+\delta} in the presence of straight and of tilted columnar defects

In this paper we use London Langevin molecular dynamics simulations to investigate the vortex matter melting transition in the highly anisotropic high-temperature superconductor material Bi_2Sr_2CaCu_2O$_{8+\delta}$ in the presence of low concentration of columnar defects (CDs). We reproduce with further details our previous results obtained by using Multilevel Monte Carlo simulations that showed that the melting of the nanocrystalline vortex matter occurs in two stages: a first stage melting into nanoliquid vortex matter and a second stage delocalization transition into a homogeneous liquid. Furthermore, we report on new dynamical measurements in the presence of a current that identifies clearly the irreversibility line and the second stage delocalization transition. In addition to CDs aligned along the c-axis we also simulate the case of tilted CDs which are aligned at an angle with respect to the applied magnetic field. Results for CDs tilted by $45^{\circ}$ with respect to c-axis show that the locations of the melting and delocalization transitions are not affected by the tilt when the ratio of flux lines to CDs remains constant. On the other hand we argue that some dynamical properties and in particular the position of the irreversibility line should be affected.Comment: 13 pages, 11 figure

Reform of History Education in Post-Soviet Armenia

The paper is devoted to education and to its role in Armenia. The education has always been a priority in Armenia - in a country, which has 1600 years old history of literacy. From the very beginning, the school has been the basis of the nation's political and cultural survival and the incentive for national progress. The current education network wasestablished during the short existence of the first Republic of Armenia in /1918 –1920/. It has further developed during the years of Soviet Union /1920 – 1990/. Traditionally, education in Armenia has been highly appreciated. Today as well, the most important national issue is the maintenance and development of the education system, ensuring its compatibility in the international environment. This can be proved by the laws and decrees proclaimed after the independence.Статья посвящена тому, что образование всегда было приоритетным в Армении - в стране, которая имеет 1600-летнюю историю грамотности. С самого начала школа была основой политического и культурного выживания страны и стимулом для национального прогресса.Нынешняя образовательная сеть была создана в течение короткого периода существования первой Республики Армения (1918-1920 гг.). Он получил дальнейшее развитие в годы Советской власти (1920-1990 годы).Традиционно образование в Армении было высоко оценено. Сегодня также важнейшим национальным вопросом считается поддержание и развитие системы образования, обеспечение ее совместимости в международной среде. Это можно доказать законами и декретами, провозглашенными после провозглашения независимости

Laboratory of Memory

Laboratory of memory as a methodological project intends to share, adapt and experiment at the European level the "Laboratory of memory", a laboratory model of intergenerational activities (involving teachers/tutors, students and seniors), aimed at updating the skills of teachers/trainers and improving alternative and informal learning of history in schools. Each "Laboratory of memory" will focus on the theme of "conflict".Лаборатория памяти как методологический проект намерена поделиться, адаптировать и экспериментировать на европейском уровне «Лаборатория памяти», лабораторная модель межпоколенной деятельности (с участием учителей / преподавателей, студентов и старших), направленная на обновление навыков учителей / Тренеров и улучшение альтернативного и неформального обучения истории в школах. Каждая «Лаборатория памяти» сосредоточится на теме «конфликт»