M. Nikolo

Flux dynamics in high-temperature superconductors

Magnetic flux penetration, flux pinning and depinning are examined. The induced electric field and the resistivity due to the thermally activated hopping of the flux lines are analysed. Depending on the relative magnitude of the transport or the induced current density, J, relative to the critical current density, Jc,, three flux regimes are identified: (i) the thermally activated flux flow (or TAFF) for J > Jc. The critical state and its relation to flux creep is defined. The flux creep, TAFF and flux flow are contrasted in terms of the current-voltage characteristics and in terms of the vortex lattice. A phase diagram, in the magnetic field versus temperature plane, is identified for a typical high-temperature superconductor. Vortex fluid (TAFF) and vortex solid (flux creep) phases are defined. Flux relaxation and the flux profile inside a superconducting sample are examined. The characteristic time frame for the flux hopping is contrasted with a characteristic time of measurement in an AC field situation and frequency effects are explained. Finally, a basic outline of pinning and the methods which have been attempted to increase pinning are discussed.

Induced voltage, magnetization waveforms and the spectral response at the grain boundaries of YBCO superconductors and the critical state model

Abstract Critical state model for granular superconductors is applied to study the nonlinear a.c. response of YBaCuO superconductors at the grain boundaries. The measured induced voltage waveforms are studied and explained at different temperatures and the corresponding calculated waveforms are shown to match the data. The intergranular hysteretic magnetization losses and the first 10 spectral components of the a.c. harmonic response are also measured and calculated at different temperatures.

Ac Susceptibility of High-Temperature Ceramic Superconductors: Critical State and Creep at Grain Boundaries and in Grains

The ac susceptibilities of different ceramic high-temperature superconductors were measured as a function of temperature, ac magnetic field amplitude and frequency. The susceptibility data are well described employing a critical state model for a granular superconductor where intergranular Josephson vortices and intragranular Abrikosov-like vortices sweep in and out of the superconductor during each ac cycle. By incorporating the Anderson flux creep model, the frequency dependence of the ac susceptibility can be predicted.

Irreversibility Line Measurement and Vortex Dynamics in High Magnetic Fields in Ni- and Co-Doped Iron Pnictide Bulk Superconductors

The de-pinning or irreversibility lines were determined by ac susceptibility, magnetization, radio-frequency proximity detector oscillator (PDO), and resistivity methods in Ba(Fe0.92Co0.08)2As2 ( Tc = 23.2 K), Ba(Fe0.95Ni0.05)2As2 ( Tc = 20.4 K), and Ba(Fe0.94Ni0.06)2As2 ( Tc = 18.5 K) bulk superconductors in ac, dc, and pulsed magnetic fields up to 65 T. A new method of extracting the irreversibility fields from the radio-frequency proximity detector oscillator induction technique is described. Wide temperature broadening of the irreversibility lines, for any given combination of ac and dc fields, is dependent on the time frame of measurement. Increasing the magnetic field sweep rate (dH/dt) shifts the irreversibility lines to higher temperatures up to about dH/d t = 40,000 Oe/s; for higher dH/dt, there is little impact on the irreversibility line. There is an excellent data match between the irreversibility fields obtained from magnetization hysteresis loops, PDO, and ac susceptibility measurements, but not from resistivity measurements in these materials. Lower critical field vs. temperature phase diagrams are measured. Their very low values near 0 T indicate that these materials are in mixed state in nonzero magnetic fields, and yet the strength of the vortex pinning enables very high irreversibility fields, as high as 51 T at 1.5 K for the Ba(Fe0.92Co0.08)2As2 polycrystalline sample, showing a promise for liquid helium temperature applications.

Intergranular activation energies in YBa2Cu3O7 bulk superconductors

Abstract Ac susceptibility is used to measure a small shift of the coupling peak of the imaginary part of the ac susceptibility (ρ″) to a higher temperature as the frequency is increased. Applying the Bean model one obtains the zero-field and zero-temperature intergranular pinning potential U(0,0) and critical, integranular current density Jc(0,0) from the data. Here it is shown how one can calculate the intergranular flux creep activation energy Ea(T,H) as a function of the applied magnetic field H and the temperature T if the average grain radius Rg and the zero temperature London penetration depth λL(0) are known. We arrive at Ea(T,H) values between 0 and 26 me V for a range of temperatures between 0 and Tc and a range of ac fields between 0 and 20 Oe.

Frequency dependent flux dynamics: Flux creep hopping rate via AC susceptibility

Abstract A critical state model for granular superconductors is applied to model frequency dependence of ac susceptibility, at the grain boundaries of a bulk YBa 2 Cu 3 O 7 superconductor. We obtain frequency dependent magnetic flux profiles and arrive at flux creep velocities and hopping rates as a function of temperature.