Correlating the charge-transfer gap to the maximum transition temperature in Bi2Sr2Can-1CunO2n+4+δ

As the number of CuO2 layers, n, in each unit cell of a cuprate family increases, the maximum transition temperature (T-c,T-max) exhibits a universal bell-shaped curve with a peak at n = 3. The microscopic mechanism of this trend remains elusive. In this study, we used advanced electron microscopy to image the atomic structure of cuprates in the Bi2Sr2Can-1CunO2n+4+delta family with 1 <= n <= 9; the evolution of the charge-transfer gap size (Delta) with n can be measured simultaneously. We determined that the n dependence of Delta follows an inverted bell-shaped curve with the minimum Delta value at n = 3. The correlation between Delta, n, and T-c,T-max may clarify the origin of superconductivity in cuprates.