Angular dependence of resistivity and intrinsic properties of YBCO thin films

We have investigated systematically the angular dependence of resistivity ρ(&thetas;), where &thetas; is the angle between the magnetic field and the ab-planes, and the temperature dependence of resistivity ρ(T) over a temperature range between T_c0 and T_c-onset within the superconducting transition in an applied magnetic field B up to 1 T in a series of YBa ₂Cu₃O_7−δ (YBCO) thin films both optimally doped and underdoped of different T_c critical current density J_c film thickness, and preparation techniques. The angular dependence of ρ was measured using a high resolution angle-control system with the angular position of the sample accurately monitored by an optical encoder. Our studies were concentrated on four different important properties of ρ(&thetas;) and ρ(T): T_c dependence of the normal state resistivity, pinning by planar defects (twin boundaries) close to T_c, variation of anisotropy factor γ, and phase-slip resistivity. (1) The studies of the normal state resistivity revealed an upward deviation in ρ(T) from the high temperature T-linear dependence below a certain characteristic temperature T*. (2) For a few YBCO films, investigation of ρ(&thetas;) for the angular range 0–180° revealed a minimum in ρ(&thetas;) near &thetas; = 90° (B || c-axis) which was attributed to magnetic pinning by twin boundaries (TB's). However, the majority of the films display a broad maximum around &thetas; = 90° similar to that observed in untwinned YBCO single crystals. (3) ρ(&thetas;) measured for B close to &thetas; = 0° (B || ab-planes) for both B || J and B ⊥ J (where J is the current density) revealed a sample dependent variations in the shape and width of the minimum at &thetas; = 0°, from a flat to a very sharp behavior. The results of scaling of ρ(&thetas;) with the reduced field B(γ−2 cos2&thetas; + sin2&thetas;)1/2 allowed a quantitative determination of the value of the intrinsic anisotropy factor γ which varies between 7 and 400, and is independent of T_c, film thickness, or J_c. (4) For a 2D-like highly anisotropic (γ ∼ 400) film, the measurements of ρ revealed a resistive maximum in ρ(&thetas;) at &thetas; = 0° over a narrow temperature range below T_c-onset. The width of this maximum decreases with an increasing B. A corresponding maximum has been observed in ρ( T) at temperatures just below T_c onset. The resistivity of the maximum is higher than the normal state resistivity at T_c onset. The maximum in ρ(T) is almost independent of the magnitude of magnetic field applied along the ab-planes. (Abstract shortened by UMI.)...