Ahmed A.A. Youssef

Magnetic flux creep properties in Bi2Sr2CaCu2O8 films made by the laser evaporation method

We measured the temperature dependence of resistivity, ρ( T ), of Bi 2 Sr 2 CaCu 2 O 8 thin films in magnetic fields. These films with a preferential orientation of the c -axis perpendicular to a surface of the substrate show an anisotropy of ρ( T ) versus T curves depending on the field direction and an Arrhenius type dependence, ρ( T )=ρ 0 exp(− U(T, H)/T ). Using the activation energy U(H, T) deduced from the flux creep model, log ρ( T ) versus U(H, T)/T plots are calculated. In low ρ( T ) region the Arrhenius type formula can well explain the ρ( T ) versus T curve, but the magnetic field dependence of U(T, H) is rather different from that expected from the conventional magnetic flux creep model.

A mechanism of magnetoresistance below Tc in C-axis-oriented high-Tc phase (Bi1-xPbx)2Sr2Ca2Cu3O10-y

Abstract Experimental results of peculiar temperature and magnetic field dependences of electrical resistivity below T c in c-axis-oriented high T c phase BiPbSrCaCuO thin films are presented and these are interpreted in terms of a new mechanism. Contrary to the conventional flux flow resistivity model, we consider that because of very short coherence length in this substance the flux lattice is largely destroyed in the higher temperature and magnetic field region beyond the occurrence of flux creep. However, owing to the short coherence length the superconducting nuclei in a small size can be locked in a multiconnected normal part. From this viewpoint we propose an empirical formula for the resistivity below T c , with which the anomalous properties in magneto-resistance can be well understood.

Magnetoresistance and the coherence length in c-axis-oriented high-Tc phase (Bi1−xPbx)2Sr2Ca2Cu3O10+y

Abstract Electrical resistance of c-axis-oriented high Tc phase Bi-Pb-Sr-Ca-Cu-O thin films was measured under high magnetic fields up to 10 T. We have shown that the coherence length derived from the fluctuation conductivity above Tc is nearly equal to that from -dHc2(T)/dT at T

Flux-Pinning Properties of YBa2Cu3O7

The flux-pinning properties of YBa2Cu3O7 semisingle crystalline films are investigated. The flux-pinning force density Fp versus a reduced field b=H/Hc2* approximately obeys the formula, Fp∝Hc2*nbp(1-b)q (n=2.6 –3.8, p~0.7, q~3.0), similarly to the case of the so-called weak-pinning type. Here, Hc2* is the upper critical field in a limit of zero current. The origin of Fp is briefly discussed.

The electronic and pinning properties of Bi2SrCaCu2O8 films by the laser evaporation method

We prepare semi-single crystalline films of oxide superconductor Bi 2 Sr 2 CaCu 2 O 8 using the laser evaporation method and investigate the temperature dependence of the Hall coefficient and the pinning properties of quantum magnetic flux lines. The flux pinning force density F p versus the reduced field b = H/H * c2 approximately obeys a formula, F p ∝ H * n c2 b p (1 − b ) q ( n =0.8−0.9, p =0.67−0.86, q =2.5−2.7), similarly to the case of the weak pinning type. Here, H * c2 is the upper critical field in the limit of zero current. The linear temperature dependence of the reciprocal of the Hall coefficient and the origin of the the F p are briefly discussed.

A hysteresis on temperature dependence of ultrasonic quantities in (Bi0.8Pb0.2)2Sr2Ca2Cu3Oy by a sintering method

Abstract We measured the temperature dependence of the resonance frequency and the amplitude at mechanical resonances of longitudinal continuous sound waves in (Bi 0.8 Pb 0.2 ) 2 Sr 2 Ca 2 Cu 3 O y prepared by a sintering method. A hysteresis on curves of acoustic quantities versus temperatures was observed between 200 and 240 K. The width of the hysteresis loop seems dependent on the rate of changing temperatures. This may be ascribed to a structural phase transition.

Preparation of Bi2Sr2CaCu2Oy films with preferential c-axis orientation by the laser evaporation method and the electronic properties

We have prepared semi-single crystalline films of oxide superconductor Bi 2 Sr 2 CaCu 2 0 8 using the laser evaporation method and investigated the quantum magnetic flux pinning properties of these films. The flux pinning force density F p versus the reduced field b = H/H c2 * approximately obeys the formula, F p ∝ H c2 * n b p (1- b ) q ( n = 0.8–0.9, p = 0.67–0.85, q = 2.5–2.7), similarly to the case of the so-called weak pinning. Here, H c2 * is the upper critical field in the limit of zero current. The origin of the F p is briefly discussed.

Preparation of Single Phase (Bi0.8Pb0.2)2Sr2Ca2Cu3Oy Films with Preferential Orientation of C-Axis by Laser Ablation Method

By a laser ablation method using an Ar-F excimer pulsed laser, (Bi0.8Pb0.2)2Sr2Ca2Cu3Oy (H-BSCC) films have been prepared in almost single phase on MgO(100) substrates. As-deposited films on substrates at room temperatures did not show any diffraction peaks of X-ray powder analysis. As-deposited films were treated thermally and the optimum conditions to obtain H-BSCC films in almost single phase were confirmed. Essential conditions were the temperature just below the melting point of H-BSCC, and heating for a prolonged duration together with a bulk sample of the same composition as that of the target.

Temperature Dependence of Some Fundamental Physical Quantities in YBa2Cu3O7

The resistivity and Hall coefficient of YBa2Cu3O7 were measured as a function of temperature. The data obtained were analyzed with a conductivity model of a diffusion type: σxx=ne2D/2kBT. It is found that the carrier concentration n of hole type varies as n=n0[1+αT] with the magnitude ratio αT>1 at T>Tc, and consequently the diffusion constant D varies as D∝1/n. Therefore, in terms of the relaxation time τ, it becomes 1/τ=AT+BT2. The origin of two terms is discussed.

A study of magnetoresistive transition in high-Tc phase Bi-Pb-Sr-Ca-Cu-O thin film.

Single high-Tc phase Bi-Pb-Sr-Ca-Cu-O superconductor thin films were prepared on (100) MgO by laser ablation method. Using these films we evaluated the coherence lengths from the fluctuation conductivity above Tc. These coherence lengths agreed with the ones derived from the highest threshold criterion in Hc2(T) versus T. We also proposed a modified mechanism of the magneto resistivity below Tc, by which the anomalous T- and H-dependences are well understandable.