Hc₂ of Y₁Ba₂Cu₃O_7-δ thin films from conductivity fluctuations

We have measured the resistive transitions of c-axis oriented YBCO thin films in applied magnetic fields for both H {norm of matrix} ab planes and H {norm of matrix} c axis up to 8T. We have analyzed the data using the 3D fluctuation theory of Ullah and Dorsey, and found a good fit to the predicted scaling function above and below the onset of the transition. The deduced Tc(H) is linear in all the field range, and the extracted dHc₂/dT values for both of the field orientations are very close to the accepted one, i.e. -(dHc₂/dT)=(1.4-1.8) T/K for H parallel to the c-axis (1.4 T/K is the slope derived for one sample and 1.8 T/K is the slope for two other samples) and 10(T/K) for H parallel to the ab planes. For H dfnc; c, the upper critical field slopes from the fluctuation conductivity agree very well with the slopes estimated from the midpoints of the resistive transitions.