S. Piechota

Comparative study of the pressure effect on critical parameters of GdBa2Cu4O8 and YBa2Cu4O8

Abstract The influence of hydrostatic pressure up to 1.1 GPa on critical parameters (Tc, Hc1, jc) of YBa2Cu4O8 (Y124) and GdBa2Cu4O8 (Gd124) ceramics has been investigated using a CuBe pressure bomb. A linear of Tc with the growth of pressure was found, with dTc/dp = 5.5 K/GPa and 4.5 K/GPa for Y124 and Gd124, respectively. The difference in Tc increase on pressure, dTc/dp, between the two compounds is assumed to have the following possible mechanisms: first, the replacement of yttrium by gadolinium constrains the change of interionic distances within and between the CuO2 planes and thus the change of charge transfer integral having an effect on the density of electronic states at CuO2 planes; secondly, this replacement modifies the charge transfer into the CuO2 planes from rare-earth sites. It was found that the critical current density increased a little under pressure; nevertheless a different behaviour on temperature has been observed for the two compounds. Insignificant changes of Hc1 under pressure were found at temperatures up to 40 K. However, at higher temperatures a little growth of Hc1 was observed.

Temperature-induced change in the ESR spectrum of the Fe3+ ion in polyaniline

Research on polyaniline doped with K3[Fe(CN)6] has revealed an effect which is manifested in an unusual temperature-induced change in the ESR spectrum of the Fe3+ ion. At low temperature (T=4.2 K) one observes the first resonance line (g1=4.22±0.03), and at high temperature (T=295 K) the second line (g2=2.13±0.05). The transition from the low-to the high-temperature spectrum occurs gradually and is accompanied by a redistribution of the absorption intensity. The observed properties of the temperature dependence of the ESR spectrum are typical of systems with a multiple-minimum potential.

Magnetothermal instabilities in type II superconductors: The influence of magnetic irreversibility

Complete magnetization and magnetostriction loops for type II superconductors were calculated in terms of the following pinned critical state models which incorporate the flux jump instability criterion: the Kim–Anderson model, the exponential model, the linear model, and the model with peak effect. These results were used for constructing magnetic field–temperature (H–T) diagrams of instability of these critical state models. Magnetothermal instabilities of superconductors with peak effect were investigated experimentally and theoretically. Regions in the H–T diagram where flux jumps occur were built and compared with experimental M(H) hysteresis loops. We have explained the phenomenon of “island” jumps. The influence of flux trapping on the flux jump regions of magnetic fields was studied.

Observation of an odd longitudinal magnetoresistance in YBa2Cu3O7−δ single crystals with unidirected twins

Abstract Unidirected twin planes are the cause of anisotropic pinning of vortices in YBa 2 Cu 3 O 7− δ (below YBaCuO) single crystals. In contrast to the case of only isotropic pinning, two new effects may appear due to the pinning anisotropy: guiding of vortices along twin planes and anisotropy of the Hall resistivity. The former leads to the appearance of the even (with respect to the reversal of a magnetic field direction) transverse resistivity and influences the Hall angle, while the last causes new additional odd contribution to the longitudinal resistivity. The paper presents magnetoresistivity measurements in the vortex liquid phase of YBaCuO single crystals with different angles between unidirected twins and current direction in the magnetic fields up to 12 T. Field inversion was used for separation of the even and odd components of the measured transverse and longitudinal voltages in order to select new contributions to the resistivity in the case of anisotropic pinning. The results are consistent with the model of anisotropic pinning where the anisotropy of the Hall drag coefficient is caused by twins.

Guiding of vortices in YBa2Cu3O7−δ single crystals with unidirected twins

Abstract Guided motion (GM) of vortices in vortex liquid phase of YBa2Cu3O7−δ (YBaCuO) single crystal with unidirected twins is studied by measuring transverse voltages in geometry where the current is flowing in the a–b plane and the external magnetic field H is oriented along the c-axis. Field inversion was used for separation of the even component of the measured transverse and longitudinal voltages in order to select new even contribution to magnetoresistivity caused by GM of vortices. The results are discussed in terms of the phenomenological approach to the anisotropic pinning caused by unidirected twins.

H-T PHASE DIAGRAM FOR THE GIANT MAGNETIC FLUX JUMPS IN LOW TEMPERATURE SUPERCONDUCTORS AND HIGH TEMPERATURE SUPERCONDUCTORS

We have studied magnetothermal instabilities both theoretically and experimentally. Magnetostriction and magnetization hysteresis loops with flux jumps were calculated over a wide range of experimental parameters employing three critical state models: the original Bean model, the Kim–Anderson model, and the exponential model. The influence of the magnetic prehistory on the flux jumps, magnetostriction, and the magnetization was investigated for the LaSrCuO crystal. H–T diagrams of flux instabilities were constructed from calculation and from experimental results. One can see good qualitative agreement between these two diagrams.

ESR-study of the low-temperature phase transition in KDy(WO4)2

Abstract The low-temperature structural phase transition of the Jahn-Teller type in a KDy(WO4)2 single crystal has been studied by means of the ESR method. The ESR line width near the point of transition (Tspt≈7 K) is shown to increase from 0.17 T up to a maximum value of 0.24T and then it decreases to 0.19 T. The width of transition area is about 8 K. The symmetry of g-tensor does not change at the point of the phase transition while the values of the g-tensor components change considerably: gz=3.13; gx≈0; gy=0.82 at T > Tspt; gz=1.98; gx≈0; gy=1.19 at T

The Peculiarities of Magnetic Flux Dynamics at Magnetothermal Instability in Textured Bi2Sr2CaCu2O8+δ

The magnetic flux dynamics was investigated in a textured Bi2Sr2CaCu2O8+δ sample using a miniature Hall probe placed on the surface of the superconductor. The process of magnetic flux penetration into the superconductor was found to have a complex structure and consist of two processes with different time scales. The first one has a length of tens of microseconds and is considered as locally adiabatic. The second one has a length comparable with the time of thermal diffusion. The importance of the above two processes in the development of magnetic instability is analyzed.

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.