Abnormal Behavior of the Angular Dependence of Resistivity in YBCO Thin Films

Abstract

Careful investigation of the angular dependence of resistivity ρ(θ) (θ is the angle between the magnetic field and the ab-planes) as a function of the temperature within the superconducting transition in an applied magnetic field B up to 1 T for a series of YBa₂Cu₃O_7−δ (YBCO) thin films revealed a large variation in the shape and width of the minimum displayed in the vicinity of θ=0°, from a flat to a very sharp behavior. The series of films studied included both optimally doped and underdoped samples of different T _c , critical current density J _c , film thickness, and preparation techniques. ρ(θ) measured for B close to θ=0° (B parallel to ab-planes) for both B‖J and B⊥J (J is the applied current density) showed two classes of samples; class of samples where ρ(θ) is independent of the direction of B relative to J and the other class where ρ(θ) depends on the orientation B relative to J. This unusual unique behavior motivated us to investigate its origin by looking at the scaling of ρ(θ) as a function of the reduced field. Scaling of ρ(θ)) with the reduced field B(γ ⁻²cos ² θ+sin ²)^1/2 allowed a quantitative determination of the value of γ (intrinsic anisotropy) which varies between 7 and 400, and is independent of film thickness and J _c . Analysis of the microstructure though XRD of the films studied showed that the anisotropy is related to microstrain of the films.