M. M. Abdelhadi

Electrical Transport and Oxygen Disorder in YBCO

We investigated the origin of a non-linear temperature dependence of resistivity ρab(T) in the a-b planes of YBCO, frequently observed in optimally doped twinned YBCO single crystals and thin films and in untwinned YBCO crystals along the chain direction. We performed the measurements of ρab(T) in c-axis oriented YBCO thin films that were subjected to low temperature annealing at 120–140°C (400–420 K) in argon. Our results have shown that redistribution of oxygen at a fixed oxygen content in the chain layers of YBCO with a non-linear ρab(T), leads to an increase of Tc, an increase of resistivity and a reduction of the non-linear component in ρab(T).

PHASE-SLIPS AND FILAMENTS IN HTSC

The presence of nano-structures, i.e. static or dynamic stripe-like features or nano-domains in HTSC, can cause a quasi-1D flow of a transport current through these materials. We present the experimental results, which reveal the effect of a filamentary flow of the current on the normal and superconducting transport properties of HTSC.

Persistent supercurrents in MgB2

Abstract Persistent currents were induced in rings of superconducting granular MgB 2 . We measured the magnetic self-fields generated by these currents using a scanning Hall probe. We obtained both the temperature dependence of the persistent current at the critical level between 15 K and T c (39 K) and the time decay of the persistent current for up to 10 5 s. The time decay for this time interval is less than 1% at temperatures between 15 and 35 K. This rate is much smaller than that observed in high critical current rings of YBCO thin films, which is about 10% at 15 K.

Nanoscopic phase separation and the universal transport and magnetic properties of high {Tc} superconductors

We have found the correlation between nanoscopic phase separation in the copper-oxygen planes of YBCO and TlBCCO and the transport and magnetic properties of these materials in the a-b planes such as: the temperature dependence of the critical current density Jc(T), the temperature dependence of the superfluid density ns(T)∝1/λ2(T) at low temperatures, the temperature dependence of the normalized logarithmic relaxation rate S(T), and the dependence of the effective energy barrier against vortex motion on the current density Ueff(J). These properties are controlled by the ratio of the amount of an underdoped filamentary phase to that of an optimally doped one.