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

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

Transition to a Superconductor with Insulating Cavities

An extreme type II superconductor with internal insulating regions, namely cavities, is studied here. We find that the cavity-bearing superconductor has lower energy than the defect-free superconductor above a critical magnetic induction $B^*$ for insulating cavities but not for metallic ones. Using a numerical approach for the Ginzburg-Landau theory we compute and compare free energy densities for several cavity radii and at least for two cavity densities, assuming a cubic lattice of spherical cavities.Comment: 7 pages, 4 figures, to be published in Europhysics Letter

Effect of acoustic wave on the parametric X-ray generation by relativistic electrons in a crystal

An exhaustive description of the parametric X-ray (PXR) generation by relativistic electrons in a crystal excited by an acoustic wave is proposed. A principal question as to whether it is possible to enhance the PXR yield under these conditions is consideredyesBelgorod State Universit

Influence of temperature gradient on diffracted X-ray spectrum in quartz crystal

In this work characteristics of hard X-ray (with energy higher than 30 keV) were investigated. In the experiment we measured spectra of X-ray reflected by a quartz monocrystal in Laue geometry under influence of the temperature gradient. The measurements were made by the spectrometer BDER-KI-11K with 300 eV resolution on the 17.74 keV spectral line of Am241 and the spectrometer XR-100CR with 270 eV resolution on the same spectral line. An existence of temperature gradient leads to increasing of the diffracted beam intensity. The intensity was measured dependently on the temperature of one of the edge of the crystal

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