Tc and dTc/dP have been systematically measured for fully oxygenated RBa2Cu3O7-δ (R-123) with R=Yb, Tm, Ho, Dy, Gd, Sm, and Nd. Tc has been observed to increase from 88 to 94 K with increasing radius of the R ion (r) from 0.98 to 1.12. Based on the lattice strains and our dTc/dP data, we have calculated Tc for the series of R-123 and compared them with the measured Tc. Good agreement between the experimental and the calculated results strongly suggests that the R effect on Tc in R-123 originates from the strain-induced charge redistribution between the charge reservoir and the CuO2 plane.
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