Effect of Hole-Doping in High-Tc Ln1Ba2Cu3O7-δ (Ln=Nd, La, Pr)

Systems of solid solution Ln1-xCaxBa2Cu3O7-δ (Ln=Nd, La, Pr) have been investigated in order to clarify the effect of hole-doping on the superconducting state. The transition temperature Tc decreases with increasing Ca-ion substitution in either Nd1-xCaxBa2Cu3O7-δ or La1-xCaxBa2Cu3O7-δ. Combined with the fact that the electron-doping in the system Ln1+xBa2-xCu3O7-δ (Ln=La, Nd, Eu) decreases Tc, the experiment indicates that the nondoped LnBa2Cu3O7-δ (Ln=Nd, La) has an optimum carrier concentration for Tc. For the nonsuper-conducting Pr1-xCaxBa2Cu3O7-δ, the conductivity at 77 or 280 K exhibits a maximum at the formal valence of ca. 3 for Pr1-xCax.