http://dspace.nbuv.gov.ua:80/handle/123456789/117377 Sat, 25 Apr 2026 01:44:01 GMT 2026-04-25T01:44:01Z http://dspace.nbuv.gov.ua:80/bitstream/id/349010/ http://dspace.nbuv.gov.ua:80/handle/123456789/117377 http://dspace.nbuv.gov.ua:80/handle/123456789/120876 Preface Sun, 01 Jan 2006 00:00:00 GMT http://dspace.nbuv.gov.ua:80/handle/123456789/120876 2006-01-01T00:00:00Z http://dspace.nbuv.gov.ua:80/handle/123456789/120201 Magnetic field induced finite size effect in type-II superconductors Schneider, T. We explore the occurrence of a magnetic field induced finite size effect on the specific heat and correlation lengths of anisotropic type-II superconductors near the zero field transition temperature Tc. Since near the zero field transition thermal fluctuations are expected to dominate and with increasing field strength these fluctuations become one dimensional, whereupon the effect of fluctuations increases, it appears unavoidable to account for thermal fluctuations. Invoking the scaling theory of critical phenomena it is shown that the specific heat data of nearly optimally doped YBa₂Cu₃O₇₋δ are inconsistent with the traditional mean-field and lowest Landau level predictions of a continuous superconductor to normal state transition along an upper critical field Hс₂(T). On the contrary, we observe agreement with a magnetic field induced finite size effect, whereupon even the correlation length longitudinal to the applied field H cannot grow beyond the limiting magnetic length LH ∝ 0/H. It arises because with increasing magnetic field the density of vortex lines becomes greater, but this cannot continue indefinitely. LH is then roughly set on the proximity of vortex lines by the overlapping of their cores. Thus, the shift and the rounding of the specific heat peak in an applied field is traced back to a magnetic field induced finite size effect in the correlation length longitudinal to the applied field. Sun, 01 Jan 2006 00:00:00 GMT http://dspace.nbuv.gov.ua:80/handle/123456789/120201 2006-01-01T00:00:00Z http://dspace.nbuv.gov.ua:80/handle/123456789/120200 Modern superconductive materials for electrical machines and devices working on the principle of levitation Prikhna, T.A. The peculiarities of high-pressure synthesis of highly dense nanostructural MgB₂-based superconductive materials, of thermobaric treatment of MT–YBCO (melt-textured YBa₂Cu₃O₇₋δ- based superconductor), high-pressure sintering of YBa₂Cu₃O₇₋δ, oxygenation of MT–YBCO under high isostatic pressure of oxygen and processes of formation of superconductive junctions between MT-YBCO blocks are considered. The attained level of superconductive and mechanical properties of such materials and junctions make them promising for application in cryogenic devices working on the principle of levitation: electricmotors, generators, pumps for liquid-gas transfer, magnetic bearings, flywheels, fault current limiters, maglev transport, etc. High-pressure synthesized MgB₂ (with Ti additions) blocks were for the first time tried in the superconductive electricmotor at 20 K and demonstrated an efficiency similar to MT–YBCO (at the same working temperature). Sun, 01 Jan 2006 00:00:00 GMT http://dspace.nbuv.gov.ua:80/handle/123456789/120200 2006-01-01T00:00:00Z http://dspace.nbuv.gov.ua:80/handle/123456789/120199 Microwave response of single crystal YBa₂Cu₃O₇–δ films as a probe for pairing symmetry Pan, V.M.; Kalenyuk, O.A.; Kasatkin, O.L.; Komashko, V.A.; Ivanyuta, O.M.; Melkov, G.A. Temperature dependences of the microwave surface impedance, Zs(T), are measured in the c-axis oriented single-crystal high-Tc superconducting cuprate YBa₂Cu₃O₇₋δ (YBCO) thin films deposited by the off-axis dc magnetron sputtering onto CeO₂-buffered single-crystal sapphire substrates (film thickness is d ≈ 150, 300, 480 nm). Measurements are performed by a use of the coplanar resonator as well as the end-plate cylindrical cavity resonator techniques at a number of several discrete frequencies within the range of 5–134 GHz. The measurements have revealed unexpected peculiarities on the Zs(T)-dependences for the most perfect films under study. The peculiarities appear to be most strongly pronounced on the temperature dependences of the film surface resistance Rs(T) = Re {Zs(T)}. The most important features of the unusual surface resistance behavior are: (i) the temperature dependence Rs(T) of YBCO films under study at low temperatures obeys the exponential law: Rs(T) = Rres + R₀ exp [–∆s/T] with the small energy gap ∆s (∆s 0.5Tc at f = 5 GHz); (ii) the most perfect films reveal a distinct two-peak structure of the Rs(T) dependence with peaks positioned at 27–30 K and 48–51 K, while such peaks are not observed in less perfect films. The peaks are mostly pronounced at moderate (e.g. 34 GHz) frequencies and gradually disappear both at higher and lower frequencies, while their temperature positions remain unchanged. These features of perfect single-crystalline YBCO films are believed to reveal their intrinsic electron properties. Taking into account the possibility of a mixed (s+id)-type pairing symmetry as well as a significant role of extended out-of-plane crystal defects (such as dislocation lines or twin planes) in Bogolyubov’s quasiparticle scattering within the most perfect YBCO films, one can suggest a consistent explanation for the anomalies observed in the Zs(T) behavior. Sun, 01 Jan 2006 00:00:00 GMT http://dspace.nbuv.gov.ua:80/handle/123456789/120199 2006-01-01T00:00:00Z