No abstract

We have measured the temperature dependence of the resistivity ρ of an epitaxial (YBa 2 Cu 3 O 7−δ ) 24 /(PrBa 2 Cu 3 O 7−δ ) 2 multilayer nanometre film at several magnetic fields parallel to the c-axis and at a constant magnetic field H = 4 T. The measurements were taken for various angles α between the magnetic field and the surface of the film with H in the plane between the measuring current I and the c-axis, and for various angles θ between the magnetic field H and the surface of the film (ab-plane) with H ⊥ I . The resistive broadening depends mainly on the effective applied magnetic field H e = H × (sin 2 θ + cos 2 θ/γ 2 ) 1/2 with respect to the c-axis, and shows a three-dimensional anisotropic behaviour, where γ is the anisotropy parameter. The angular dependences of the peak temperature T p and the activation energy U e for angle θ and α > 5 • are in agreement with the effective mass model, namely T p (0) − T p (H, θ ) ∝ (sin 2 θ + cos 2 θ/γ 2 ) 1/2n and U e ∝ H −m e with γ = 10, n = 1.31 and m = 0.87. The dissipative resistivity below T p for various magnetic fields can be scaled onto a single curve by thermally activated flux motion. When the applied field tilts the CuO 2 plane, the angular dependences of T p and U e reveal a lock-in transition, showing that the flux lines were trapped in the non-superconducting PrBCO layers.

The behaviour of the electric field in the magnetization process for a superconducting slab of infinite length is studied by numerical simulation. On the basis of the results of our numerical calculations, a new treatment is suggested, the results of which are compared with those from the straight-line approximation. Both sets of results are further compared with the results of numerical calculations.

Within the framework of the thermally activated process of the flux line or flux line bundles, and by time integration of the 1D equation of motion of the circulating current density ( ,t ), which is suitable for thin superconducting films (R >>d , ), we present numerical calculations of the current profiles, magnetization hysteresis loops and ac susceptibility n = ´n +i ´´n for n = 1, 3 and 5 of a thin disc immersed in an axial time-dependent external magnetic field Ba (t ) = Bdc +Bac cos(2 t ). Our calculated results are compared with those of the critical state model (CSM) and found to prove the approximate validity of the CSM below the irreversibility field. The differences between our computed results and those of the CSM are also discussed.

Based on the Bean critical state model j 5 j , for both longitudinal and transverse geometry, the total dissipation power P along the virgin magnetization curves is calculated. The dissipation power obtained from the corresponding magnetic measurements for a disk-shaped YBa2 Cu307--s thin film in a perpendicular magnetic field is verified. The virgin magnetization curves for the sample a t different temperatures are calculated.