ON THE EFFECT OF ATOM SUBSTITUTIONS AND PRESSURE ON THE SUPERCONDUCTIVITY OF YTTRIUM CUPRATES

The effect of atom substitutions on T,variation under pressure is inve stigated for YBa2Cu3-xAgxO7-delta(1-2-3), 0 < x < 0.5, and Y1-xCaxBa2C u4O(8-delta)(1-2-4), 0 < x < 0.3. A non-linear concentration dependenc e of two types is observed for the dT(c)/dP derivative. In the 1-2-3 s ystem this variable reaches saturation at x=0.3, whereas in the 1-2-4 phase it has its minimum at x=0.1. The non-linear dependence of dT(c)/ dP(x) with extremum in 1-2-4 is apparently due to the electron-topolog y phase transition caused by doping. A weaker dependence of T-c and dT (c)/dP in 1-2-3 under substitution for Cu atoms may be due to the infl uence of the processes of carrier scattering on the anisotropy of the order parameter of the superconductor. From the function T-c(x) for th e 1-2-3 phase we have calculated the ratio of the coherence length xi 0 to the free length l in YBa2Cu3-xAgxO7-delta. The temperature depend ence of the contribution of superconducting fluctuations in YBa2Cu3-xA gxO7-delta is found to agree with the Aslamazov-Larkin theory.