Abstract
Magnetoresistivity of YBa2Cu3O7−δ films with thicknesses between 7 and 100 nm deposited on CeO2 and PrBa2Cu3O7−δ buffer layers on sapphire substrate has been measured to analyze the temperature dependence of the second critical magnetic field Bc2. To define Bc2, the mean-field transition temperature Tc was evaluated by fitting the resistive transition in zero magnetic field with the fluctuation conductivity theory of Aslamazov and Larkin. At T → Tc the Bc2(T) dependence shows a crossover from downturn to upturn curvature with the increase in film thickness.