Electronic origin of high superconducting critical temperature in trilayer cuprates

In high-temperature cuprate superconductors, the superconducting transition temperature (Tc) depends on the number of CuO2 planes in the structural unit and the maximum Tc is realized in the trilayer system. Trilayer superconductors also exhibit an unusual phase diagram where Tc is roughly constant in the overdoped region, which is in contrast to the decrease usually found in other cuprate superconductors. The mechanism for these two effects remains unclear. Here we report features in the electronic structure of Bi2Sr2Ca2Cu3O10+δ superconductor that helps to explain this issue. Our angle-resolved photoemission spectroscopy measurements show the splitting of bands from the three layers, and this allows us to parameterize a three-layer interaction model that effectively describes the data. This, in turn, demonstrates the electronic origin of the maximum Tc and its persistence in the overdoped region. These results are qualitatively consistent with a composite picture where a high Tc is realized in an array of coupled planes with different doping levels such that a high pairing strength is derived from the underdoped planes, whereas a large phase stiffness comes from the optimally or overdoped ones. Measurements of the electronic structure of a trilayer cuprate superconductor suggest that its high critical temperature is explained by the different doping levels of the layers. The combination of underdoped inner layer and overdoped outer layers supports superconductivity.