ANGULAR-DEPENDENCE OF THE RESISTIVELY-DETERMINED IRREVERSIBILITY LINEIN YBCO

We present a wide set of resistive measurements taken in a YBCO thin f ilm, as a function of the temperature, magnetic field and field orient ation theta with respect to the (a,b) planes. We focus the attention o n the low portion of the resistance curves. Contrary to what is found close to T-c, no angular scaling is possible. We examine the angular b ehavior of the irreversibility line, conventionally taken as the field and angle dependent temperature T-i (H, theta) at which the resistivi ty exceeds some sensitivity threshold. It is found that T-i does not f ollow a simple 2D or 3D behavior, and that its angular dependence is a ffected by the extended defects present in the film (twin planes) and by the layered structure. We also show that the irreversibility line a s a function of the angle, taken at constant reduced field h = H/H-c2 (H-c2 is the temperature-and angle-dependent upper critical field, as determined from the fluctuation conductivity), presents a noticeable a ngular dependence.