A.I. D'yachenko

Phonon spectra in the conductance of Andreev junctions on Bi2223 at high pressure

Abstract A softening of the high-frequency phonon modes in Bi2223 ( T c =110 K) at high pressure is found. These modes correspond to the oxygen breathing mode and also the Cu–O bending modes. Qualitatively, the result obtained explains the observed rise in R =2 Δ / kT c ( P ) ratio in the framework of the strong electron–phonon interaction model. However, the analysis of the R -ratio in the framework of the s+d model has revealed a possibility of explanation of the observed rise in R ( P ) also for the d-wave pairing.

Failure of textured YBaCuO samples in the strong magnetic field

Abstract It has been shown that the effect of strong magnetic field on the textured YBaCuO superconductors leads to a new phenomenon — magnetolamination. This phenomenon consists in mechanical lamination parallel to the ab -plane into the thin platelets under the high magnetic field cycling. A mechanism responsible for the textured sample cracking due to the high magnetic field is proposed and discussed in detail.

Reversible mechanism of magnetothermal instabilities in melt-textured YBaCuO

Abstract The anisotropy of the development of instabilities in the metl-textured YBaCuO samples with remanent magnetization of 600 emu/g has been studied in a magnetic field up to 12 T. Large 500 emu/g jumps of the magnetic moment of the sample due to the development of the magnetothermal instability have been observed. A giant (10 −4 ) magnetostriction caused by the magnetic flux pinning has been revealed. Stabilization of the critical state of the sample by reversal of magnetization has been found. However, when the sample is heated to temperatures higher than 100 K and subsequently cooled the magnetic jumps restore completely. A new reversible mechanism of instability generation based on the phenomenon reversibility of plastic deformations of high-temperature superconductors (HTS) is proposed to explain the regeneration of critical state instability.

Irreversible phenomena in RF electromagnetic field absorption in MTG YBaCuO plates

Abstract It was found that in YBaCuO textured MTG-ceramics RF (∼ 10 6 and ∼ 10 9 Hz) absorption is increased at reverse variation of the external static field. This result is contrary to the situation, observed in common metal ceramic samples, where RF absorption is decreased at decrease of the external field.

Role of the field dependence of the heat capacity for the flux jump process in HTSC materials

Abstract Magnetization and magnetostriction loops were calculated in terms of the Kim–Anderson model incorporating the flux jump instability criterion and taking into account magnetic field and temperature dependencies of the heat capacity of YBaCuO. Regions in the H – T diagram where flux jumps occur were determined. The stabilizing role of the specific heat for the critical state of the material was observed.

Analysis of H-T flux jump diagrams from different critical state models

The magnetic field - temperature ranges where the critical state for type II superconductors is unstable were calculated in terms of the following pinned critical state models: the Kim-Anderson model, the exponential model and the linear model. Comparative discussion of H-T diagrams of flux instabilities is presented.

Transport properties of doped (Sr,Ca)10Cu17O29 single crystals under high hydrostatic pressure

Abstract The influence of hydrostatic pressure up to 10 kbar on the superconducting transition temperature in doped single crystal of (Sr,Ca) 10 Cu 17 O 29 has been studied. A strong decrease of the normal state electrical resistivity and the pressure effect d T c /d P as high as 0.4 K/kbar were obtained. It is suggested that the superconductivity in the doped (Sr,Ca) 10 Cu 17 O 29 system is related to the inhomogeneous charge carrier distribution. The existence of quasi-one-dimensional (quasi-1D) metallic and spin-gapped regions in close electrical contact can explain the observed magnetic properties of (Sr,Ca) 10 Cu 17 O 29 and the effect of pressure on its transport properties.

Peak effect and giant flux jumps in hard superconductors: the problem of “islands” jumps on H-T diagram

Abstract Magnetic properties of superconductors with peak effect were investigated both experimentally and theoretically in frames of the critical state model which incorporates the flux jump instability criterion. Theoretical analyses show some “forbidden” band for flux jumps on the magnetic field axis. Features of H-T diagrams of instability of superconductors with peak effect are discussed.

Andreev reflection spectroscopy of high-Tc superconductors at high pressures

Abstract Andreev reflection effect has allowed one to determine the anisotropy of energy gap Δ ( ϕ ) in high- T c superconductors and its pressure response. The values of Δ ( ϕ ) in the ab plane are found to be Δ max =42 mV, Δ min =19.5 mV ((BiPb)2223 phase, T c =110 K, d T c /d P =0.16 K/kbar); Δ max =44 mV, Δ min =12.5 mV, d T c /d P =0.28 K/kbar (HgPb)2223 phase, T c =133 K). A rise in r =2 Δ max / kT c with pressure is found: for Bi2223 d r /d P ≈0.03 kbar −1 , and for Hg2223 d r /d P ≈0.06 kbar −1 .