Jeannot Schroeder

Thermally activated flux motion in YBa2(Cu1−xZnx)3O7−δ epitaxial thin films: influence of magnetic field and Zn doping

Abstract High quality c -axis oriented epitaxial YBa 2 (Cu 1− x ) 3 O 7− δ ( x = 0, 0.02, 0.04) thin films were deposited by DC magnetron sputtering on MgO (100) substrates. The broadening of the resistive transition has been investigated in a magnetic field range from 0.03 to 20 T for H ⊥ c -axis in and H ∥ c -axis in order to study the influence of different microstructures on the pinning properties. The deduced field dependence of the activation energy follows a power law: U 0 ( H ) ∼ H − α with a pronounced anisotropic behaviour. We observe crossovers of the α-values from α 1 = 0.13 to α 2 = 0.5 (planar pinning) for H ⊥ c -axis and from α 1 = 0.3 to α 2 = 1 (point defect pinning) for H ∥ c -axis. For both orientations the α-values obtained in the high field region can be explained in the framework of collective pinning. The results are discussed taking account of the influence of the Zn substitution on the flux pinning in epitaxial YBa 2 Cu 3 O 7−δ thin films.

Magnetic-field dependence of the thermal activation energy in epitaxial YBa2Cu3O7−δ thin films

Abstract Thermally activated flux motion has been studied in c -axis oriented epitaxial YBa 2 Cu 3 O 7−δ thin films with a critical current density J c of 4 × 10 6 A/cm 2 at 77 K in the absence of an external magnetic field. The magnetoresistivity and the critical current density were measured in applied magnetic fields H , up to 20 T, for H perpendicular and parallel to the c -axis of the films. Critical current densities at 77 K are still as high as 7 ×10 4 10 A/cm 2 in 20 T field for H ⊥ c -axis and 2 × 10 A/cm in 10 3 A/cm 2 10 T field for H ∥ c -axis. It was found that the magnetic-field dependence of the activation energy follows a power law, U 0 ( H ) ∼ H −α Crossovers of the α values were observed from α 1 -0.22 to α 2 − 0.5 at 0.3 T for H ⊥ c -axis and from α 1 − 0.33 to α 2 − 1 at 6 T for H ∥ c -axis. These crossovers were interpreted as transitions from a low-field single-vortex pinning to a collective-vortex pinning in high field as predicted by Yeshurun and Malozemoff [Phys. Rev. Lett, 60 (1988) 2202]. In the collective-pinning regime, the observed anisotropic dependences of U 0 H ) are assumed to be due to the intrinsic planar pinning for H ∥ c -axis and extrinsic point pinning for H ∥ c -axis.

The effect of Sn substitution in the superconducting system

We studied the effect of Sn substitution in a new mercury-based superconductor with nominal composition . At this stage of investigation the material is multiphasic with dominating superconducting phases up to x = 0.3. It was established that Sn stimulates the formation of the Hg-1212 phase and enhances the diamagnetic volume fraction but also the weak-link behaviour. Together with the SEM - EDS analysis, the shift of some of the XRD peaks, as well as the gradual decrease of with the dopant content x, suggests that the substitution is effective and some of the Sn is incorporated into the structure of the superconducting phases.

Structural properties of Zn-substituted epitaxial thin-films

We optimized the deposition of thin-films using inverted cylindrical magnetron sputtering and report here a detailed structural study, especially in relation to crystal growth, associated surface morphology, precipitation and other secondary phases important for flux pinning. We find that the epitaxial quality of the Zn-substituted films is decreased compared with high-quality pure films prepared under identical conditions. The pure films have smoother surfaces, while those of Zn-substituted films contain pinholes and outgrowths. Secondary phases and a-axis grains were observed in the Zn-substituted films. precipitates with typical dimensions of 50 - 100 A have been found in both pure and Zn-substituted samples. However, their density of about , observed in the pure films, is significantly reduced in the Zn-substituted films when increasing the Zn concentration up to 4%.

Intergranular critical currents and excess conductivity in the Fe-substituted Bi-2223 high Tc superconductor

In this paper me present the effect of iron substitution in the (Bi1.6Pb0,4)(Sr1.88Ba0.2)Ca2(Cu1−xFex)3.1Oy system. From the resistivity and AC susceptibility data we observed that a monotonie decrease of Tc occurs with increasing Fe concentration. A strong reduction of Jc (calculated from AC susceptibility data performed at low magnetic fields in the range 0.4A/m-800A/m) is observed in the doped samples. The crossover temperature from the Anderson-Kim thermally activated flux creep to the thermally activated phase slippage behaviour is depending on the doping concentration. The excess conductivity and 2D-3D crossover is well described by considering Δσ/σ0 as a product of a Gaussian centered at Tc with a Student distribution centered at 2Tc. The alteration of 2D dimensionality above Tc by doping results in the decrease of the number of freedom degrees of the Student distribution.

AC susceptibility study of epitaxial YBa2(Cu1−xZnx)3O7 thin films

Abstract AC susceptibility measurements were performed on epitaxial YBa 2 (Cu 1−x Zn x ) 3 O 7 thin films. Both the real part λ′ and the imaginary λ″ of the complex Ac susceptibility were investigated as function of the temperature and the AC magnetic field. Critical current densities are determined from these measurements and are discussed in the framework of a thermally activated flux creep model.

Low angle deviation from the anisotropic effective mass model in epitaxial YBa2(Cu1−xZnx)3O7−δ thin films

Abstract The magnetic field effect on the electrical in-plane resistivity ϱ ab has been investigated for high quality c-axis oriented YBa 2 (Cu 1− x Zn x ) 3 O 7−δ thin films (x=0.02 and 0.04) by magnetoresistance measurements up to 20 Teslas for different orientations of the magnetic field with respect to the a-b plane, at different temperatures below T c . Deviation in the scaling behaviour expected from the anisotropic effective mass model is observed for small misalignement between the magnetic field direction and the superconducting CuO 2 planes, which is attributed to the intrinsic pinning effect. The Zn doping influence on the anisotropic ratio γ = ξ ab /ξ c is studied from the high angle scaling properties of the film.

Anisotropy of the magnetoresistance in pure and iron-doped YBaCuO thin films

Abstract Transport measurements were performed on highly c-axis oriented Y 1 Ba 2 (Cu (1−x) Fe x ) 3 O 7 (x=0 and 0.04) thin films below T c in external magnetic fields up to 4T. By rotating the sample in the magnetic field we determined the angular dependence of the in-plane magnetoresistivity ϱ (ab) . The dissipation has been studied in relation to the transverse field component along the c-axis. The influence of the substitution of copper by iron on the dissipation mechanism is studied through a scaling law behavior for the field and angular dependence of the magnetoresistivity.