UPPER LIMIT OF T-C FOR THE COPPER OXIDES

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing fac tors: (1) the increase in carrier concentration, and (2) the pair-brea king effect of magnetic impurities. An analysis of the temperature dep endence of the critical field leads to the conclusion that magnetic im purities are present even in a sample with the maximum observed value of T-c. A new parameter, ''intrinsic'' T-c (T-c intr), which is its va lue in the absence of magnetic impurities, is introduced. The maximum value of T-c intr, which corresponds to the maximum doping level, appe ars to be similar for different cuprates and to be equal to 160-170 K. This is the upper limit of T-c in the cuprates.