Manifestation of two-dimensional behavior of YBCO films in a study of their complex susceptibility

The temperature dependence of the complex magnetic susceptibility χ ′ +χ ″ of various YBCO films is investigated at different amplitudes of the exciting field H ac . It is found that when the temperature T m at which the maximum is observed on the χ ″ (T) curve is plotted as a function of H ac α (the parameter α=1, 1/2, or 2/3, depending on the character of the coupling between crystallites), a kink appears at a certain field H ac =H 1 * . It is suggested that the temperature T c1 obtained by extrapolating the T m (H ac 2/3 ) curve from the high-field region to zero field is the Berezinskii–Kosterlitz–Thouless transition temperature T BKT . This suggestion is based on a comparison of the present results with published data from a study of the complex susceptibility of a GdBa 2 Cu 3 O 6.75 single crystal for directions of H ac parallel to the c axis of the single crystal and in its ab plane, and also on the coincidence of the calculated values of certain characteristic temperatures near the BKT transition with our experimental values.